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Holding the Line: Controlling
Unwanted Moisture in Historic Buildings
Sharon C. Park, AIA
Uncontrolled moisture is
the most prevalent cause of deterioration in older and
historic buildings. It leads to erosion, corrosion, rot, and
ultimately the destruction of materials, finishes, and
eventually structural components. Ever-present in our
environment, moisture can be controlled to provide the
differing levels of moisture necessary for human
comfort as well as the longevity of historic building
materials, furnishings, and museum collections. The challenge
to building owners and preservation professionals alike is to
understand the patterns of moisture movement in order to
better manage it-not to try to eliminate it. There is never a
single answer to a moisture problem. Diagnosis and treatment
will always differ depending on where the building is
located, climatic and soil conditions, ground water effects,
and local traditions in building construction.
Remedial
Actions within an Historic Preservation Context
In this Brief, advice
about controlling the sources of unwanted moisture is
provided within a preservation context based on philosophical
principles contained in the Secretary of the Interior's
Standards for the Treatment of Historic Properties.
Following the Standards means significant materials and
features that contribute to the historic character of the
building should be preserved, not damaged during remedial
treatment (see fig.1). It also means that physical treatments
should be reversible, whenever possible. The majority of
treatments for moisture management in this Brief stress
preservation maintenance for materials, effective drainage of
troublesome ground moisture, and improved interior
ventilation.
The Brief encourages a
systematic approach for evaluating moisture problems which,
in some cases, can be undertaken by a building owner. Because
the source of moisture can be elusive, it may be necessary to
consult with historic preservation professionals prior to
starting work that would affect historic materials.
Architects, engineers, conservators, preservation
contractors, and staff of State Historic Preservation Offices
(SHPOs) can provide such advice.
Regardless of who does
the work, however, these are the principles that should guide
treatment decisions:
-
Avoid remedial
treatments without prior careful diagnosis.
-
Undertake treatments
that protect the historical significance of the
resource.
-
Address issues of
ground-related moisture and rain run-off
thoroughly.
-
Manage existing
moisture conditions before introducing
humidified/dehumidified mechanical systems.
-
Implement a program of
ongoing monitoring and maintenance once moisture is
controlled or managed.
-
Be aware of
significant landscape and archeological resources in areas
to be excavated.
Finally, mitigating the
effects of catastrophic moisture, such as floods, requires a
different approach and will not be addressed in this
Brief.
How and
Where to Look for Damaging Moisture
Finding, treating, and
managing the sources of damaging moisture requires a
systematic approach that takes time, patience, and a thorough
examination of all aspects of the problem-including a series
of variable conditions (See this page). Moisture problems may
be a direct result of one of these factors or may be
attributable to a combination of interdependent
variables.
Factors Contributing to
Moisture Problems
A variety of
simultaneously existing conditions contribute to moisture
problems in old buildings. For recurring moisture problems,
it may be necessary for the owner or preservation
professional to address many, if not all, of the following
variables:
-
Types of building
materials and construction systems
-
Type and condition of
roof and site drainage systems and their rates of
discharge
-
Type of soil, moisture
content, and surface /subsurface water flow adjacent to
building
-
Building usage and
moisture generated by occupancy
-
Condition and
absorption rates of materials
-
Type, operation, and
condition of heating, ventilating, cooling,
humidification/ dehumidification, and plumbing
systems
-
Daily and seasonal
changes in sun, prevailing winds, rain, temperature, and
relative humidity (inside and outside), as well as
seasonal or tidal variations in groundwater levels
-
Unusual site
conditions or irregularities of construction
-
Conditions in affected
wall cavities, temperature and relative humidity, and
dewpoints
-
Amount of air
infiltration present in a building
-
Adjacent landscape and
planting materials
Diagnosing and treating
the cause of moisture problems requires looking at both the
localized decay, as well as understanding the performance of
the entire building and site. Moisture is notorious for
traveling far from the source, and moisture movement within
concealed areas of the building construction make accurate
diagnosis of the source and path difficult. Obvious
deficiencies, such as broken pipes, clogged gutters, or
cracked walls that contribute to moisture damage, should
always be corrected promptly. For more complicated problems,
it may take several months or up to four seasons of
monitoring and evaluation to complete a full diagnosis.
Rushing to a solution without adequate documentation can
often result in the unnecessary removal of historic
materials-and worse-the creation of long-term problems
associated with an increase, rather than a decrease, in the
unwanted moisture.
Looking for Signs
Identifying the type of
moisture damage and discovering its source or sources usually
involves the human senses of sight, smell, hearing, touch,
and taste combined with intuition. Some of the more common
signs of visible as well as hidden moisture damage (see fig.
2) include:
-
Presence of standing
water, mold, fungus, or mildew
-
Wet stains, eroding
surfaces, or efflorescence (salt deposits) on interior and
exterior surfaces
-
Flaking paint and
plaster, peeling wallpaper, or moisture blisters on
finished surfaces
-
Dank, musty smells in
areas of high humidity or poorly ventilated spaces
-
Rust and corrosion
stains on metal elements, such as anchorage systems and
protruding roof nails in the attic
-
Cupped, warped,
cracked, or rotted wood
-
Spalled, cracked
masonry or eroded mortar joints
-
Faulty roofs and
gutters including missing roofing slates, tiles, or
shingles and poor condition of flashing or gutters
-
Condensation on window
and wall surfaces
-
Ice dams in gutters,
on roofs, or moisture in attics
Uncovering
and Analyzing Moisture Problems
Moisture comes from a
variety of external sources. Most problems begin as a result
of the weather in the form of rain or snow, from high ambient
relative humidity, or from high water tables. But some of the
most troublesome moisture damage in older buildings may be
from internal sources, such as leaking plumbing pipes,
components of heating, cooling, and climate control systems,
as well as sources related to use or occupancy of the
building. In some cases, moisture damage may be the result of
poorly designed original details, such as projecting
outriggers in rustic structures that are vulnerable to
rotting, and may require special treatment. The five most
common sources of unwanted moisture include:
-
Above grade exterior
moisture entering the building
-
Below grade ground
moisture entering the building
-
Leaking plumbing pipes
and mechanical equipment
-
Interior moisture from
household use and climate control systems
-
Water used in
maintenance and construction materials.
Above grade exterior
moisture generally results from weather related moisture
entering through deteriorating materials as a result of
deferred maintenance, structural settlement cracks, or damage
from high winds or storms (see fig. 3). Such sources as
faulty roofs, cracks in walls, and open joints around window
and door openings can be corrected through either repair or
limited replacement. Due to their age, historic buildings are
notoriously "drafty," allowing rain, wind, and damp air to
enter through missing mortar joints; around cracks in
windows, doors, and wood siding; and into uninsulated attics.
In some cases, excessively absorbent materials, such as soft
sandstone, become saturated from rain or gutter overflows,
and can allow moisture to dampen interior surfaces. Vines or
other vegetative materials allowed to grow directly on
building materials without trellis or other framework can
cause damage from roots eroding mortar joints and foundations
as well as dampness being held against surfaces. In most
cases, keeping vegetation off buildings, repairing damaged
materials, replacing flashings, rehanging gutters, repairing
downspouts, repointing mortar, caulking perimeter joints
around windows and doors, and repainting surfaces can
alleviate most sources of unwanted exterior moisture from
entering a building above grade.
Below grade ground
moisture is a major source of unwanted moisture for
historic and older buildings. Proper handling of surface
rain run-off is one of the most important measures of
controlling unwanted ground moisture. Rain water is often
referred to as "bulk moisture" in areas that receive
significant annual rainfalls or infrequent, but heavy,
precipitation. For example, a heavy rain of 2" per hour can
produce 200 gallons of water from downspout discharge alone
for a house during a one hour period. When soil is saturated
at the base of the building, the moisture will wet footings
and crawl spaces or find its way through cracks in foundation
walls and enter into basements (see fig. 4). Moisture in
saturated basement or foundation walls-also exacerbated by
high water tables-will generally rise up within a wall and
eventually cause deterioration of the masonry and adjacent
wooden structural elements.
Builders traditionally
left a working area, known as a builder's trench, around the
exterior of a foundation wall. These trenches have been known
to increase moisture problems if the infill soil is less than
fully compacted or includes rubble backfill, which, in some
cases, may act as a reservoir holding damp materials against
masonry walls. Broken subsurface pipes or downspout drainage
can leak into the builder's trench and dampen walls some
distance from the source. Any subsurface penetration of the
foundation wall for sewer, water, or other piping also can
act as a direct conduit of ground moisture unless these holes
are well sealed. A frequently unsuspected, but serious,
modern source of ground moisture is a landscape irrigation
system set too close to the building. Incorrect placement of
sprinkler heads can add a tremendous amount of moisture at
the foundation level and on wall surfaces.
The ground, and
subsequently the building, will stay much drier by 1)
re-directing rain water away from the foundation through
sloping grades, 2) capturing and disposing downspout water
well away from the building, 3) developing a controlled
ground gutter or effective drainage for buildings
historically without gutters and downspouts, and 4) reducing
splash-back of moisture onto foundation walls. The excavation
of foundations and the use of dampproof coatings and footing
drains should only be used after the measures of reducing
ground moisture listed above have been implemented.
Leaking plumbing pipes
and mechanical equipment can cause immediate or long-term
damage to historic building interiors. Routine maintenance,
repair, or, if necessary, replacement of older plumbing and
mechanical equipment are common solutions. Older water and
sewer pipes are subject to corrosion over time. Slow leaks at
plumbing joints hidden within walls and ceilings can
ultimately rot floor boards, stain ceiling plaster, and lead
to decay of structural members. Frozen pipes that crack can
damage interior finishes (see fig. 5 ). In addition to
leaking plumbing pipes, old radiators in some historic
buildings have been replaced with water-supplied fan coil
units which tend to leak. These heating and cooling units, as
well as central air equipment, have overflow and condensation
pans that require cyclical maintenance to avoid mold and
mildew growth and corrosion blockage of drainage channels.
Uninsulated forced-air sheet metal ductwork and cold water
pipes in walls and ceilings often allow condensation to form
on the cold metal, which then drips and causes bubbling
plaster and peeling paint. Careful design and vigilant
maintenance, as well as repair and insulating pipes or
ductwork, will generally rid the building of these common
sources of moisture.
Interior moisture
from building use and modern humidified heating and cooling
systems can create serious problems. In northern U.S.
climates, heated buildings will have winter-time relative
humidity levels ranging from 10%-35% Relative Humidity (RH).
A house with four occupants generates between 10 and 16
pounds of water a day (approximately 1 ‡- 2 gallons)
from human residents. Moisture from food preparation,
showering, or laundry use will produce condensation on
windows in winter climates. When one area or floor of a
building is air-conditioned and another area is not, there is
the chance for condensation to occur between the two areas.
Most periodic condensation does not create a long-term
problem.
Humidified climate
control systems are generally a major problem in museums
housed within historic buildings. They produce between
35%-55% RH on average which, as a vapor, will seek to
dissipate and equalize with adjacent spaces (see fig. 6).
Moisture can form on single-glazed windows in winter with
exterior temperatures below 30ƒF and interior
temperatures at 70ƒF with as little as 35% RH. Frequent
condensation on interior window surfaces is an indication
that moisture is migrating into exterior walls, which can
cause long-term damage to historic materials. Materials and
wall systems around climate controlled areas may need to be
made of moisture resistant finishes in order to handle the
additional moisture in the air. Moist interior conditions in
hot and humid climates will generate mold and fungal growth.
Unvented mechanical equipment, such as gas stoves, driers,
and kerosene heaters, generate large quantities of moisture.
It is important to provide adequate ventilation and find a
balance between interior temperature, relative humidity, and
airflow to avoid interior moisture that can damage historic
buildings.
Moisture from
maintenance and construction materials can cause damage
to adjacent historic materials. Careless use of liquids to
wash floors can lead to water seepage through cracks and
dislodge adhesives or cup and curl materials. High-pressure
power washing of exterior walls and roofing materials can
force water into construction joints where it can dislodge
mortar, lift roofing tiles, and saturate frame walls and
masonry. Replastered or newly plastered interior walls or the
construction of new additions attached to historic buildings
may hold moisture for months; new plaster, mortar, or
concrete should be fully cured before they are painted or
finished. The use of materials in projects that have been
damaged by moisture prior to installation or have too
high a moisture content may cause concealed damage (see fig.
7).
Transport or Movement
of Moisture
Knowing the five most
common sources of moisture that cause damage to building
materials is the first step in diagnosing moisture problems.
But it is also important to understand the basic mechanisms
that affect moisture movement in buildings. Moisture
transport, or movement, occurs in two states: liquid and
vapor. It is directly related to pressure differentials. For
example, water in a gaseous or vapor state, as warm moist
air, will move from its high pressure area to a lower
pressure area where the air is cooler and drier. Liquid water
will move as a result of differences in hydrostatic pressure
or wind pressure. It is the pressure differentials that
drive the rate of moisture migration in either state.
Because the building materials themselves resist this
moisture movement, the rate of movement will depend on two
factors: the permeability of the materials when affected by
vapor and the absorption rates of materials in contact with
liquid.
The mechanics, or
physics, of moisture movement is complex, but if the driving
force is difference in pressure, then an approach to reducing
moisture movement and its damage is to reduce the difference
in pressure, not to increase it. That is why the treatments
discussed in this Brief will look at managing moisture by
draining bulk moisture and ventilating vapor moisture
before setting up new barriers with impermeable coatings or
over-pressurized new climate control systems that threaten
aging building materials and archaic construction
systems.
Three forms of moisture
transport are particularly important to understand in regards
to historic buildings-infiltration, capillary action, and
vapor diffusion-remembering, at the same time, that the
subject is infinitely complex and, thus, one of continuing
scientific study (see fig. 8). Buildings were traditionally
designed to deal with the movement of air. For example,
cupolas and roof lanterns allowed hot air to rise and
provided a natural draft to pull air through buildings.
Cavity walls in both frame and masonry buildings were
constructed to allow moisture to dissipate in the air space
between external and internal walls. Radiators were placed in
front of windows to keep cold surfaces warm, thereby reducing
condensation on these surfaces. Many of these features,
however, have been altered over time in an effort to
modernize appearances, improve energy efficiency, or
accommodate changes in use. The change in use will also
affect moisture movement, particularly in commercial and
industrial buildings with modern mechanical systems.
Therefore, the way a building handles air and moisture today
may be different from that intended by the original builder
or architect, and poorly conceived changes may be partially
responsible for chronic moisture conditions.
Moisture moves into and
through materials as both a visible liquid (capillary action)
and as a gaseous vapor (infiltration and vapor diffusion).
Moisture from leaks, saturation, rising damp, and
condensation can lead to the deterioration of materials and
cause an unhealthy environment. Moisture in its solid form,
ice, can also cause damage from frozen, cracked water pipes,
or split gutter seams or spalled masonry from freeze-thaw
action. Moisture from melting ice dams, leaks, and
condensation often can travel great distances down walls and
along construction surfaces, pipes, or conduits. The amount
of moisture and how it deteriorates materials is dependent
upon complex forces and variables that must be considered for
each situation.
Determining the way
moisture is handled by the building is further complicated
because each building and site is unique. Water damage from
blocked gutters and downspouts can saturate materials on the
outside, and high levels of interior moisture can saturate
interior materials. Difficult cases may call for technical
evaluation by consultants specializing in moisture monitoring
and diagnostic evaluation. In other words, it may take a team
to effectively evaluate a situation and determine a proper
approach to controlling moisture damage in old
buildings.
Infiltration is
created by wind, temperature gradients (hot air rising),
ventilation fan action, and the stack or chimney effect that
draws air up into tall vertical spaces. Infiltration as a
dynamic force does not actually move liquid water, but is the
vehicle by which dampness, as a component of air, finds its
way into building materials. Older buildings have a natural
air exchange, generally from 1 to 4 changes per hour, which,
in turn, may help control moisture by diluting moisture
within a building. The tighter the building construction,
however, the lower will be the infiltration rate and the
natural circulation of air. In the process of infiltration,
however, moisture that has entered the building and saturated
materials can be drawn in and out of materials, thereby
adding to the dampness in the air. Inadequate air circulation
where there is excessive moisture (i.e., in a damp basement),
accelerates the deterioration of historic materials. To
reduce the unwanted moisture that accompanies infiltration,
it is best to incorporate maintenance and repair treatments
to close joints and weatherstrip windows, while providing
controlled air exchanges elsewhere. The worst approach is to
seal the building so completely, while limiting fresh air
intake, that the building cannot breathe.
Capillary action
occurs when moisture in saturated porous building materials,
such as masonry, wicks up or travels vertically as it
evaporates to the surface. In capillary attraction, liquid in
the material is attracted to the solid surface of the pore
structure causing it to rise vertically; thus, it is often
called "rising damp," particularly when found in conjunction
with ground moisture. It should not, however, be confused
with moisture that laterally penetrates a foundation wall
through cracks and settles in the basement. Not easily
controlled, most rising damp comes from high water tables or
a constant source under the footing. In cases of damp masonry
walls with capillary action, there is usually a whitish stain
or horizontal tide mark of efflorescence that seasonally
fluctuates about 1- 3 feet above grade where the excess
moisture evaporates from the wall (see fig. 9). This tide
mark is full of salt crystals, that have been drawn from the
ground and building materials along with the water, making
the masonry even more sensitive to additional moisture
absorption from the surrounding air. Capillary migration of
moisture may occur in any material with a pore structure
where there is a constant or recurring source of moisture.
The best approach for dealing with capillary rise in building
materials is to reduce the amount of water in contact with
historic materials. If that is not possible due to
chronically high water tables, it may be necessary to
introduce a horizontal damp-proof barrier, such as slate
course or a lead or plastic sheet, to stop the vertical rise
of moisture. Moisture should not be sealed into the wall with
a waterproof coating, such as cement parging or vinyl wall
coverings, applied to the inside of damp walls. This will
only increase the pressure differential as a vertical barrier
and force the capillary action, and its destruction of
materials, higher up the wall.
Vapor diffusion is
the natural movement of pressurized moisture vapor through
porous materials. It is most readily apparent as humidified
interior air moves out through walls to a cooler exterior. In
a hot and humid climate, the reverse will happen as moist hot
air moves into cooler, dryer, air-conditioned, interiors. The
movement of the moisture vapor is not a serious problem until
the dewpoint temperature is reached and the vapor changes
into liquid moisture known as condensation. This can
occur within a wall or on interior surfaces. Vapor diffusion
will be more of a problem for a frame structure with several
layers of infill materials within the frame cavity than a
dense masonry structure. Condensation as a result of vapor
migration usually takes place on a surface or film, such as
paint, where there is a change in permeability.
The installation of
climate control systems in historic buildings (mostly
museums) that have not been properly designed or
regulated and that force pressurized damp air to diffuse into
perimeter walls is an ongoing concern. These newer systems
take constant monitoring and back-up warning systems to avoid
moisture damage.
Long-term and undetected
condensation or high moisture content can cause serious
structural damage as well as an unhealthy environment, heavy
with mold and mildew spores. Reducing the interior/exterior
pressure differential and the difference between interior and
exterior temperature and relative humidity helps control
unwanted vapor diffusion. This can sometimes be achieved by
reducing interior relative humidity. In some instances, using
vapor barriers, such as heavy plastic sheeting laid over damp
crawl spaces, can have remarkable success in stopping vapor
diffusion from damp ground into buildings. Yet, knowledgeable
experts in the field differ regarding the appropriateness of
vapor barriers and when and where to use them, as well as the
best way to handle natural diffusion in insulated
walls.
Adding insulation to
historic buildings, particularly in walls of wooden frame
structures, has been a standard modern weatherization
treatment, but it can have a disastrous effect on historic
buildings. The process of installing the insulation destroys
historic siding or plaster, and it is very difficult to
establish a tight vapor barrier. While insulation has the
benefit of increasing the efficiency of heating and cooling
by containing temperature controlled air, it does not
eliminate surfaces on which damaging moisture can condense.
For insulated residential frame structures, the most obvious
sign of a moisture diffusion problem is peeling paint on
wooden siding, even after careful surface preparation and
repainting. Vapor impermeable barriers such as plastic
sheeting, or more accurately, vapor retarders, in cold
and moderate climates generally help slow vapor diffusion
where it is not wanted.
In regions where
humidified climate control systems are installed into
insulated frame buildings, it is important to stop
interstitial, or in-wall, dewpoint condensation. This
is very difficult because humidified air can penetrate
breaches in the vapor barrier, particularly around electrical
outlets (see fig. 10). Improperly or incompletely installed
retrofit vapor barriers will cause extensive damage to the
building, just in the installation process, and will allow
trapped condensation to wet the insulation and sheathing
boards, corrode metal elements such as wiring cables and
metal anchors, and blister paint finishes. Providing a tight
wall vapor barrier, as well as a ventilated cavity behind
wooden clapboards or siding appears to help insulated frame
walls, if the interior relative humidity can be adjusted or
monitored to avoid condensation. Correct placement of vapor
retarders within building construction will vary by region,
building construction, and type of climate control
system.
Surveying and Diagnosing Moisture Damage: Key Questions to
Ask
It is important for the
building to be surveyed first and the evidence and location
of suspected moisture damage systematically recorded before
undertaking any major work to correct the problem. This will
give a baseline from which relative changes in condition can
be noted.
When materials become
wet, there are specific physical changes that can be detected
and noted in a record book or on survey sheets. Every time
there is a heavy rain, snow storm, water in the basement, or
mechanical systems failure, the owner or consultant should
note and record the way moisture is moving, its appearance,
and what variables might contribute to the cause. Standing
outside to observe a building in the rain may answer many
questions and help trace the movement of water into the
building. Evidence of deteriorating materials that cover
more serious moisture damage should also be noted, even if it
is not immediately clear what is causing the damage. ( For
example, water stains on the ceiling may be from leaking
pipes, blocked fan coil drainage pans above, or from moisture
which has penetrated around a poorly sloped window sill
above.) Don't jump to conclusions, but use a systematic
approach to help establish an educated theory-or
hypothesis-of what is causing the moisture problem or what
areas need further investigation.
Surveying moisture
damage must be systematic so that relative changes can be
noted. Tools for investigating can be as simple as a
notebook, sketch plans, binoculars, camera, aluminum foil,
smoke pencil, and flashlight. The systematic approach
involves looking at buildings from the top down and from the
outside to the inside. Photographs, floor plans, site plan,
and exterior elevations-even roughly sketched-should be used
to indicate all evidence of damp or damaged materials, with
notations for musty or poorly ventilated areas. Information
might be needed on the absorption and permeability
characteristics of the building materials and soils. Exterior
drainage patterns should be noted and these base plans
referred to on a regular basis in different seasons and in
differing types of weather (see fig. 11). It is best to start
with one method of periodic documentation and to use this
same method each time. Because moisture is affected by
gravity, many surveys start with the roof and guttering
systems and work down through the exterior walls. Any obvious
areas of water penetration, damaged surfaces, or staining
should be noted. Any recurring damp or stain patterns, both
exterior and interior, should also be noted with a commentary
on the temperature, weather, and any other facts that may be
relevant (driving rains, saturated soil, high interior
humidity, recent washing of the building, presence of a lawn
watering system, etc.).
The interior should be
recorded as well, beginning with the attic and working down
to the basement and crawl space. It may be necessary to
remove damaged materials selectively in order to trace the
path of moisture or to pinpoint a source, such as a leaking
pipe in the ceiling. The use of a basic resistance moisture
meter, available in many hardware stores, can identify
moisture contents of materials and show, over time, if wall
surfaces are drying or becoming damper (see fig. 12). A smoke
pencil can chart air infiltration around windows or draft
patterns in interior spaces. For a quick test to determine if
a damp basement is caused by saturated walls or is a result
of condensation, tape a piece of foil onto a masonry surface
and check it after a day or two; if moisture has developed
behind the foil, then it is coming from the masonry. If
condensation is on the surface of the foil, then moisture is
from the air.
Comparing current
conditions with previous conditions, historic drawings,
photographs, or known alterations may also assist in the
final diagnosis. A chronological record, showing improvement
or deterioration, should be backed up with photographs or
notations as to the changing size, condition, or features of
the deterioration and how these changes have been affected by
variables of temperature and rainfall. If a condition can be
related in time to a particular event, such as efflorescence
developing on a chimney after the building is no longer
heated, it may be possible to isolate a cause, develop a
hypothesis, and then test the hypothesis (by adding some
temporary heat), before applying a remedial treatment. If the
owner or consultant has access to moisture survey and
monitoring equipment such as resistance moisture meters,
dewpoint indicators, salt detectors, infrared thermography
systems, psychrometer, fiber-optic boroscopes, and
miniaturized video cameras, additional quantified data can be
incorporated into the survey (see fig. 13). If it is
necessary to track the wetting and drying of walls over a
period of time, deep probes set into walls and in the soil
with connector cables to computerized data loggers or the use
of long-term recording of hygrothermographs may require a
trained specialist. Miniaturized fiber-optic video cameras
can record the condition of subsurface drain lines without
excavation (see fig. 14). It should be noted, however, that
instrumentation, while extremely useful, cannot take the
place of careful personal observation and analysis.
Relying on instrumentation alone rarely will give the owner
the information needed to fully diagnose a moisture problem.
To avoid jumping to a quick-potentially erroneous-conclusion,
a series of questions should be asked first. This will help
establish a theory or hypothesis that can be tested to
increase the chances that a remedial treatment will control
or manage existing moisture.
How is water
draining around building and site? What is the
effectiveness of gutters and downspouts? Are the slopes or
grading around foundations adequate? What are the locations
of subsurface features such as wells, cisterns, or drainage
fields? Are there subsurface drainage pipes (or drainage
boots) attached to the downspouts and are they in good
working condition? Does the soil retain moisture or allow it
to drain freely? Where is the water table? Are there window
wells holding rain water? What is the flow rate of area
drains around the site (can be tested with a hose for several
minutes)? Is the storm piping out to the street sufficient
for heavy rains, or does water chronically back up on the
site? Has adjacent new construction affected site drainage or
water table levels?
How does
water/moisture appear to be entering the building?
Have all five primary sources of moisture been evaluated?
What is the condition of construction materials and are there
any obvious areas of deterioration? Did this building have a
builder's trench around the foundation that could be holding
water against the exterior walls? Are the interior bearing
walls as well as the exterior walls showing evidence of
rising damp? Is there evidence of hydrostatic pressure under
the basement floor such as water percolating up through
cracks? Has there been moisture damage from an ice dam in the
last several months? Is damage localized, on one side of the
building only, or over a large area?
What are the
principal moisture dynamics? Is the moisture
condition from liquid or vapor sources? Is the attic moisture
a result of vapor diffusion as damp air comes up through the
cavity walls from the crawl space or is it from a leaking
roof? Is the exterior wall moisture from rising damp with a
tide mark or are there uneven spots of dampness from
foundation splash back, or other ground moisture conditions?
Is there adequate air exchange in the building, particularly
in damp areas, such as the basement? Has the height of the
water table been established by inserting a long pipe into
the ground in order to record the water levels?
How is the interior
climate handling moisture? Are there areas in the
building that do not appear to be ventilating well and where
mold is growing? Are there historic features that once helped
the building control air and moisture that can be
reactivated, such as operable skylights or windows? Could
dewpoint condensation be occurring behind surfaces, since
there is often condensation on the windows? Does the building
feel unusually damp or smell in an unusual way that suggest
the need for further study? Is there evidence of termites,
carpenter ants, or other pests attracted to moist conditions?
Is a dehumidifier keeping the air dry or is it, in fact,
creating a cycle where it is actually drawing moisture
through the foundation wall?
Does the moisture
problem appear to be intermittent, chronic, or tied to
specific events? Are damp conditions occurring within
two hours of a heavy rain or is there a delayed reaction?
Does rust on most nail heads in the attic indicate a
condensation problem? What are the wet patterns that appear
on a building wall during and after a rain storm? Is it
localized or in large areas? Can these rain patterns be tied
to gutter over-flows, faulty flashing, or saturation of
absorbent materials? Is a repaired area holding up well over
time or is there evidence that moisture is returning? Do
moisture meter readings of wall cavities indicate they are
wet, suggesting leaks or condensation in the wall?
Once a hypothesis of the
source or sources of the moisture has been developed from
observation and recording of data, it is often useful to
prove or disprove this hypothesis with interim treatments,
and, if necessary, the additional use of instrumentation to
verify conditions. For damp basements, test solutions can
help determine the cause. For example, surface moisture in
low spots should be redirected away from the foundation wall
with regrading to determine if basement dampness improves. If
there is still a problem, determine if subsurface downspout
collection pipes or cast iron boots are not functioning
properly. The above grade downspouts can be disconnected and
attached to long, flexible extender pipes and redirected away
from the foundation (see fig. 15). If, after a heavy rain or
a simulation using a hose, there is no improvement, look for
additional ground moisture sources such as high water tables,
hidden cisterns, or leaking water service lines as a cause of
moisture in the basement. New data will lead to a new
hypothesis that should be tested and verified. The process
of elimination can be frustrating, but is required if a
systematic method of diagnosis is to be
successful.
Selecting
an Appropriate Level of Treatment
The treatments in chart
format at the back of this publication are divided into
levels based on the degree of moisture problems. Level I
covers preservation maintenance; Level II focuses on repair
using historically compatible materials and essentially
mitigating damaging moisture conditions; and Level III
discusses replacement and alteration of materials that permit
continued use in a chronically moist environment. It is
important to begin with Level I and work through to a
manageable treatment as part of the control of moisture
problems. Buildings in serious decay will require treatments
in Level II, and difficult or unusual site conditions may
require more aggressive treatments in Level III. Caution
should always be exercised when selecting a treatment. The
treatments listed are a guide and not intended to be
recommendations for specific projects as the key is always
proper diagnosis.
Start with the repair of
any obvious deficiencies using sound preservation
maintenance. If moisture cannot be managed by maintenance
alone, it is important to reduce it by mitigating problems
before deteriorated historic materials are replaced.
Treatments should not remove materials that can be preserved;
should not involve extensive excavation unless there is a
documented need; and should not include coating buildings
with waterproof sealers that can exacerbate an existing
problem (see fig. 16). Some alteration to historic materials,
structural systems, mechanical systems, windows, or finishes
may be needed when excessive site moisture cannot be
controlled by drainage systems, or in areas prone to floods.
These changes, however, should, be sensitive to preserving
those materials, features, and finishes that convey the
historic character of the building and site.
Ongoing Care
Once the building has
been repaired and the larger moisture issues addressed, it is
important to keep a record of additional evidence of moisture
problems and to protect the historic or old building
through proper cyclical maintenance (see fig. 17) In some
cases, particularly in museum environments, it is critical to
monitor areas vulnerable to moisture damage. In a number of
historic buildings, in-wall moisture monitors are used to
ensure that the moisture purposely generated to keep relative
humidity at ranges appropriate to a museum collection does
not migrate into walls and cause deterioration. The potential
problem with all systems is the failure of controls, valves,
and panels over time. Back-up systems, warning devices,
properly trained staff and an emergency plan will help
control damage if there is a system failure.
Ongoing maintenance and
vigilance to situations that could potentially cause moisture
damage must become a routine part of the everyday life of a
building. The owner or staff responsible for the upkeep of
the building should inspect the property weekly and note any
leaks, mustiness, or blocked drains. Again, observing the
building during a rain will test whether ground and gutter
drainage are working well.
For some buildings a
back-up power system may be necessary to keep sump pumps
working during storms when electrical power may be lost. For
mechanical equipment rooms, condensation pans, basement
floors, and laundry areas where early detection of water is
important, there are alarms that sound when their sensors
come into contact with moisture.
Conclusion
Moisture in old and
historic buildings, though difficult to evaluate, can be
systematically studied and the appropriate protective
measures taken. Much of the documentation and evaluation is
based on common sense combined with an understanding of
historic building materials, construction technology, and the
basics of moisture and air movement. Variables can be
evaluated step by step and situations creating direct or
secondary moisture damage can generally be corrected. The
majority of moisture problems can be mitigated with
maintenance, repair, control of ground and roof moisture, and
improved ventilation. For more complex situations, however, a
thorough diagnosis and an understanding of how the building
handles moisture at present, can lead to a treatment
that solves the problem without damaging the historic
resource.
It is usually
advantageous to eliminate one potential source of moisture at
a time. Simultaneous treatments may set up a new dynamic in
the building with its own set of moisture problems.
Implementing changes sequentially will allow the owner or
preservation professional to track the success of each
treatment.
Moisture problems can be
intimidating to a building owner who has diligently tried to
control them. Keeping a record of evidence of moisture
damage, results of diagnostic tests, and remedial treatments,
is beneficial to a building's long-term care. The more
complete a survey and evaluation, the greater the success in
controlling unwanted moisture now and in the future.
Holding the line on
unwanted moisture in buildings will be successful if 1) there
is constant concern for signs of problems and 2) there is
ongoing physical care provided by those who understand the
building, site, mechanical systems, and the previous efforts
to deal with moisture. For properties with major or
difficult-to-diagnose problems, a team approach is often most
effective. The owner working with properly trained
contractors and consultants can monitor, select, and
implement treatments within a preservation context in order
to manage moisture and to protect the historic
resource.
Reading List
-
Conrad, Ernest A.,
P.E. "The Dews and Don'ts of Insulating." Old-House
Journal, May/June, 1996.
-
Cumberland, Don, Jr.
"Museum Collection Storage in an Historic Building Using a
Prefabricated Structure."
-
Preservation Tech
Notes. Washington, DC: National Park Service, issue
PTN-14. September, 1985.
-
Jessup, Wendy Claire,
Ed. Conservation in Context: Finding a Balance for the
Historic House Museum. Washington, DC: National Trust
for Historic Preservation (Symposium Proceedings March
7-8, 1994).
-
Labine, Clem.
"Managing Moisture in Historic Buildings" Special Report
and Moisture Monitoring Source List. Traditional
Building, Vol 9, No.2, May-June 1996.
-
Leeke, John.
"Detecting Moisture; Methods and Tools for Evaluating
Water in Old Houses." Old House Journal, May/June,
1996.
-
Moisture Control in
Buildings. Heinz R. Trechsel, Editor. Philadelphia:
American Society for Testing and Materials (ASTM manual
series: MNL 18), 1993.
-
Museums in Historic
Buildings (Special Issue). APT Bulletin. The
Journal of Preservation Technology, Vol 26, No. 3 .
Williamsburg, VA: APT, 1996.
-
Oxley, T.A. and A. E.
Gobert. Dampness in Buildings: Diagnosis, Treatment,
Instruments. London, Boston: Butterworth-Heinemann,
1994.
-
Park, Sharon C. AIA.
Preservation Brief 24: Heating, Ventilating, and
Cooling Historic Buildings: Problems and Recommended
Approaches. Washington, DC: Department of the
Interior, Government Printing Office, 1991.
-
Park, Sharon C. AIA.
Preservation Brief 31: Mothballing Historic
Buildings. Washington, DC: Department of the Interior,
Government Printing Office, 1993.
-
Rose, William.
"Effects of Climate Control on the Museum Building
Envelope," Journal of the American Institute for
Conservation, Vol. 33, No. 2. Summer, 1994.
-
Smith, Baird M.
Moisture Problems in Historic Masonry Walls; Diagnosis
and Treatment. Washington, DC.: Department of the
Interior, Government Printing Office, 1984.
-
Tolpin, Jim.
"Builder's Guide to Moisture Meters," Tools of the
Trade Vol 2, No. 1 (Quarterly Supplement to
The Journal of Light Construction). Richmond,
Vermont: Builderburg Group Inc. Summer, 1994.
Acknowledgments
Sharon C. Park,
AIA is the Senior Historical Architect, Technical
Preservation Services, Heritage Preservation Services
Program, National Park Service, Washington, D.C. The author
wishes to thank the following individuals and organizations
for providing technical review and other assistance in
developing this publication: The attendees, speakers, and
sponsors of the Diagnosing Moisture in Historic Buildings
Symposium held in Washington, DC in 1996 and funded by a
grant from the National Center for Preservation Technology
and Training, National Park Service ; Hugh C. Miller, FAIA;
Michael Henry, AIA, PE, PP; Baird M. Smith, AIA; Ernest A.
Conrad, P.E.; William B. Rose; Rebecca Stevens. AIA; Wendy
Claire Jessup; Elizabeth Sasser, AIA; Bryan Blundell; George
Siekkinen, AIA; Larry D. Dermody; Kimberly A. Konrad; Barbara
J. Mangum and the Isabella Stewart Gardner Museum, Boston;
Gunston Hall Plantation; Friends of Meridian Hill; Friends of
Great Falls Tavern; The National Trust for Historic
Preservation; Thomas McGrath, Douglas C. Hicks and The
Williamsport Preservation Training Center, NPS; the staff at
Heritage Preservation Services, NPS, Charles E. Fisher,
Brooks Prueher, Anne E. Grimmer, Antoinette Lee, and
especially Kay D. Weeks.This publication has been prepared
pursuant to the National Historic Preservation Act, as
amended, which directs the Secretary of the Interior to
develop and make available information concerning historic
properties. Comments about this publication should be
directed to de Teel Patterson Tiller, Acting Manager,
Heritage Preservation Services Program, National Park
Service, P.O. Box 37127, Washington, DC 20013-7127. This
publication is not copyrighted and can be reproduced without
penalty. Copyright photographs included in this publication
may not be used to illustrate publications other than as a
reference to this Preservation Brief, without permission of
the owners. Normal procedures for credit to the authors and
the National Park Service are appreciated.
ISSN: 0885-7016 October,
1996
fig. 1. Moisture
problems, if not properly corrected, will increase damage to
historic buildings. This waterproof coating trapped moisture
from the leaking roof , causing portions of the masonry
parapet to fail. Photo: NPS Files. fig. 2. Historic buildings
plagued by dampness problems will benefit from systematic
documentation to set a baseline against which moisture
changes can be measured. Exterior areas with higher moisture
levels may have algae growth or discoloration stains.
Drawing: John H. Stubbs. fig. 3. Deferred maintenance often
leads to blocked gutters and downspouts. This cracked gutter
system allowed moisture to penetrate the upper exterior wall,
erode mortar joints, and rot fascia boards. Photo: NPS files.
fig. 4. Excavating this foundation revealed that the
downspout pipe had corroded at the " u-trap" and was leaking
moisture into the soil. Openings around the horizontal water
supply line and cracks in the wall allowed moisture to
penetrate the basement in multiple locations. Photo: author.
fig. 5. Uninsulated plumbing pipes close to the exterior wall
froze and cracked, wetting this ornamental plaster ceiling
before the water supply line could be shut off. As a result,
limited portions of the ceiling needed reattaching. Photo:
author. fig. 6. Condensation dripping from the large overhead
courtyard skylight was damaging the masonry in this museum. A
new skylight with thermal glazing was installed, replacing
the deteriorated single-glazed unit. A new climate control
system monitors interior temperature and humidity. Photo:
© Isabella Stewart Gardner Museum, Boston. Fig. 7.
Damaging moisture conditions can occur during construction.
Peeling paint on this newly rehabilitated frame wall was
attributed to wall insulation that had become wet during the
project and was not discovered . Photo: NPS Files. Fig. 8.
The dynamic forces that move air and moisture through a
building are important to understand particularly when
selecting a treatment to correct a moisture problem. Air
infiltration, capillary action, and vapor diffusion all
affect the wetting and drying of materials. Drawing; NPS
Files.Fig. 9. Capillary rise of moisture in masonry is often
accompanied with a horizontal tide-mark line several feet
above the grade, as seen here. Removing or redirecting as
much ground moisture as possible usually helps reduce
moisture within a wall. Photo: NPS Files.Fig. 10. Vapor
diffusion can result in damp air migrating into absorbent
materials and condensing on colder surfaces, thereby wetting
insulation, damaging electrical conduits, and causing
deterioration of the wooden framing. Drawing: NPS Files. Fig.
11. Using sketch plans and elevation drawings to record the
moisture damage along with the date, time, and weather
conditions will show how moisture is affecting buildings over
time. Drawing: Courtesy, Quinn Evans Architects. Fig. 12.
Using instruments in this damp-check kit can help determine
the relative change in wet conditions over time. This
involves readings of air temperature, computing dewpoint
temperatures, and tracking the moisture content of materials
to indicate if they are drying properly. Photo: Dell
Corporation. Fig. 13. Psychrometric charts quantify the
amount of relative humidity a building can tolerate before
dewpoint condensation occurs. This is important when the
range of temperature and humidity are critical to both
collections management and historic building preservation.
Chart: Landmark Facilities Group. Fig. 14. Contractors
specializing in building diagnostics often have video cameras
or fiber optic equipment that allow the viewing of
inaccessible areas. This is particularly helpful in chimney
flues or subsurface drains, as shown here. In the past, these
areas would need to be excavated for visual inspection.
Photo: author. Fig. 15. In
testing a theory for the cause of basement wetness, the owner
used long black extender pipes to direct roof run-off away
from the foundation. This test established that the owner did
not need expensive waterproofing of the foundation, but a
better drainage system. Photo: Baird M. Smith. Fig. 16. This
detail drawing shows a shallow sub-surface perimeter drain in
conjunction with a historic brick ground gutter system to
help control roof run-off moisture from entering the historic
foundation. Detail: Courtesy, Gunston Hall Plantation. Photo:
Elizabeth Sasser. Fig. 17. Maintaining gutters and downspouts
in good operable condition, repairing exteriors to keep water
out, redirecting damaging moisture away from foundations and
controlling interior moisture and condensation are all
important when holding the line on moisture deterioration.
Photo: Nebraska State Historical Society. Back Cover: The
Diagnosing Moisture in Historic Building Symposium held in
Washington, DC, May, 1996, brought together practitioners in
the field of historic preservation to discuss the issues
contained in this Preservation Brief. Attendees are standing
in front of the cascading fountains at Meridian Hill Park, a
National Historic Landmark. Photo: Eric Avner.
Level I Preservation
Maintenance
Exterior: Apply
cyclical maintenance procedures to eliminate rain and
moisture infiltration.
Roofing/
guttering: Make weather-tight and operational; inspect
and clean gutters as necessary depending on number of nearby
trees, but at least twice a year; inspect roofing at least
once a year, preferably spring; replace missing or damaged
roofing shingles, slates, or tiles; repair flashing; repair
or replace cracked downspouts.
Walls: Repair
damaged surface materials; repoint masonry with appropriately
formulated mortar; prime and repaint wooden, metal, or
masonry elements or surfaces; remove efflorescence from
masonry with non-metallic bristle brushes.
Window and door
openings: Eliminate cracks or open joints; caulk or
repoint around openings or steps; repair or reset
weatherstripping; check
flashing; repaint, as
necessary.
Ground: Apply regular
maintenance procedures to eliminate standing water and
vegetative threats to building/site.
Grade: Eliminate
low spots around building foundations; clean out existing
downspout boots twice a year or add extension to leaders to
carry moisture away from foundation; do a hose test to verify
that surface drains are functioning; reduce moisture used to
clean steps and walks; eliminate the use of chlorides to melt
ice which can increase freeze/thaw spalling of masonry; check
operation of irrigation systems, hose bib leaks, and
clearance of air conditioning condensate drain
outlets.
Crawl space: Check
crawl space for animal infestation, termites, ponding
moisture, or high moisture content; check foundation grilles
for adequate ventilation; seasonally close grilles when
appropriate-in winter, if not needed, or in summer if hot
humid air is diffusing into air conditioned space.
Foliage: Keep
foliage and vines off buildings; trim overhanging trees to
keep debris from gutters and limbs from rubbing against
building; remove moisture retaining elements, such as
firewood, from foundations.
Basements and
foundations: Increase ventilation and maintain surfaces to
avoid moisture.
Equipment: Check
dehumidifiers, sump pump, vent fans, and water detection or
alarm systems for proper maintenance as required; check
battery back-up twice a year.
Piping/ductwork:
Check for condensation on pipes and insulate/seal joints, if
necessary.
Interior: Maintain
equipment to reduce leaks and interior moisture.
Plumbing pipes:
Add insulation to plumbing or radiator pipes located in areas
subject to freezing, such as along outside walls, in attics,
or in unheated basements.
Mechanical
equipment: Check condensation pans and drain lines to
keep clear; insulate and seal joints in exposed metal
ductwork to avoid drawing in moist air.
Cleaning:
Routinely dust and clean surfaces to reduce the amount of
water or moist chemicals used to clean building; caulk around
tile floor and wall connections; and maintain floor grouts in
good condition.
Ventilation:
Reduce household-produced moisture, if a problem, by
increasing ventilation; vent clothes driers to the outside;
install and always use exhaust fans in restrooms,
bathrooms, showers, and kitchens, when in use.
Illustration captions:
Level I
-
A. Inspecting the
overall building on at least an annual basis will identify
areas needing maintenance. A bucket lift is helpful for
large buildings. Photo: author.
-
B. Repair exterior
surfaces, paint, and recaulk as needed. Photo:
Williamsport Preservation Training Center (WPTC),
NPS.
-
C. Maintain drains,
gutters, and site drainage systems. Photo: WPTC,
NPS.
-
D. Protect the
building from damage by maintaining equipment and using
alarms, like this floor water sensor. Photo: Dell
Corporation.
Level II Repair and
Corrective Action
Exterior: Repair
features that have been damaged. Replace an extensively
deteriorated feature with a new feature that matches in
design, color, texture, and where possible,
materials.
Roofing: Repair
roofing, parapets and overhangs that have allowed moisture to
enter; add ice and water shield membrane to lower 3-4 feet or
roofing in cold climates to limit damage from ice dams;
increase attic ventilation, if heat and humidity build-up is
a problem. Make gutters slope @ 1/8" to the foot. Use
professional handbooks to size gutters and reposition, if
necessary and appropriate to historic architecture. Add
ventilated chimney caps to unused chimneys that collect rain
water.
Walls: Repair
spalled masonry, terra cotta, etc. by selectively installing
new masonry units to match; replace rotted clapboards too
close to grade and adjust grade or clapboards to achieve
adequate clearance; protect or cover open window
wells.
Ground: Correct
serious ground water problems; capture and dispose of
downspout water away from foundation; and control vapor
diffusion of crawlspace moisture.
Grade:
Re-establish positive sloping of grade; try to obtain 6" of
fall in the first 10' surrounding building foundation; for
buildings without gutter systems, regrade and install a
positive subsurface collection system with gravel, or
waterproof sheeting and perimeter drains; adjust pitch or
slope of eave line grade drains or French drains to reduce
splash back onto foundation walls; add subsurface drainage
boots or extension pipes to take existing downspout water
away from building foundation to the greatest extent
feasible.
Crawl space: Add
polyethylene vapor barrier (heavy construction grade or Mylar
) to exposed dirt in crawlspace if monitoring indicates it is
needed and there is no rising damp; add ventilation grilles
for additional cross ventilation, if determined
advisable.
Foundations and
Basements: Correct existing high moisture levels, if other
means of controlling ground moisture are
inadequate.
Mechanical
devices: Add interior perimeter drains and sump pump; add
dehumidifiers for seasonal control of humidity in confined,
unventilated space ( but don't create a problem with pulling
dampness out of walls); add ventilator fans to improve air
flow, but don't use both the dehumidifier and ventilator fan
at the same time.
Walls: Remove
commentates coatings, if holding rising damp in walls; coat
walls with vapor permeable lime based rendering plaster, if
damp walls need a sacrificial coating to protect mortar from
erosion; add termite shields, if evidence of termites and
dampness cannot be controlled.
Framing:
Reinforce existing floor framing weakened by moisture by
adding lolly column support and reinforcing joist ends with
sistered or parallel supports. Add a vapor impermeable
shield, preferably non-ferrous metal, under wood joists
coming into contact with moist masonry.
Interior: Eliminate
areas where moisture is leaking or causing a
problem
Plumbing: Replace
older pipes and fixtures subject to leaking or overflowing;
insulate water pipes subject to condensation.
Ventilation: Add
exhaust fans and whole house fans to increase air flow
through buildings, if areas are damp or need more ventilation
to control mold and mildew.
Climate: Adjust
temperature and relative humidity to manage interior
humidity; Correct areas of improperly balanced pressure for
HVAC systems that may be causing a moisture problem.
Illustrations: Level
II
-
A. Mitigate poor
drainage with gravel, filter cloth, or the use of
subsurface drainage mats. Photo: WPTC, NPS.
-
B. Repair roofs and
add ice and water shields at eaves and under valleys in
cold climates. Photo: WPTC, NPS.
-
C. Develop new
drainage system for roof run-off. Photo: WPTC,
NPS.
-
D. Install new
interior under-slab drainage and sump pump equipment for
basements that continue to collect water even after
surface drainage has been addressed. Photo: NPS
files.
Level III
Replacement / Alterations For Chronically Damp
Conditions
Exterior: Undertake exterior rehabilitation work that
follows professional repair practices-i.e., replace a
deteriorated feature with a new feature to match the existing
in design, color, texture, and when possible, materials. In
some limited situations, non-historic materials may be
necessary in unusually wet areas
Roofs: Add ventilator fans to exhaust roofs but avoid
large projecting features whose designs might negatively
affect the appearance of the historic roof. When replacing
roofs, correct conditions that have caused moisture problems,
but keep the overall appearance of the roof; for example,
ventilate under wooden shingles, or detail standing seams to
avoid buckling and cracking. Be attentive to provide extra
protection for internal or built-in gutters by using the best
quality materials, flashing, and vapor impermeable connection
details.
Walls: If insulation and vapor barriers are added to
frame walls, consider maintaining a ventilation channel
behind the exterior cladding to avoid peeling and blistering
paint occurrences.
Windows: Consider removable exterior storm windows,
but allow operation of windows for periodic ventilation of
cavity between exterior storm and historic sash. For stained
glass windows using protective glazing, use only ventilated
storms to avoid condensation as well as heat build-up.
Ground: Control excessive ground moisture. This may
require extensive excavations, new drainage systems, and the
use of substitute materials. These may include concrete or
new sustainable recycled materials for wood in damp areas
when they do not impact the historic appearance of the
building.
Grade: Excavate and install water collection systems
to assist with positive run-off of low lying or difficult
areas of moisture drainage; use drainage mats and under
finished grade to improve run-off control; consider the use
of column plinth blocks or bases that are ventilated or
constructed of non-absorbent substitute materials in
chronically damp areas. Replace improperly sloped walks;
repair non-functioning catch basins and site drains; repair
settled areas around steps and other features at grade.
Foundations: Improve performance of foundation walls with
damp-proof treatments to stop infiltration or damp course
layers to stop rising damp. Some substitute materials may
need to be selectively integrated into new features.
Walls: excavate, repoint masonry walls, add footing
drains, and waterproof exterior subsurface walls; replace
wood sill plates and deteriorated structural foundations with
new materials, such as pressure treated wood, to withstand
chronic moisture conditions; materials may change, but
overall appearance should remain similar. Add dampcourse
layer to stop rising damp; avoid chemical injections as these
are rarely totally effective, are not reversible, and are
often visually intrusive.
Interior: Control the amount of moisture and condensation
on the interiors of historic buildings. Most designs for new
HVAC systems will be undertaken by mechanical engineers, but
systems should be selected that are appropriate to the
resource and intended use.
Windows, skylights: Add double and triple glazing,
where necessary to control condensation. Avoid new metal
sashes or use thermal breaks where prone to heavy
condensation.
Mechanical systems: Design new systems to reduce
stress on building exterior. This might require insulating
and tightening up the building exterior, but provisions must
be made for adequate air flow. A new zoned system, with
appropriate transition insulation, may be effective in areas
with differing climatic needs.
Control devices/Interior spaces: If new climate
control systems are added, design back-up controls and
monitoring systems to protect from interior moisture damage.
Walls: If partition walls sit on floors that
periodically flood, consider spacers or isolation membranes
behind baseboards to stop moisture from wicking up through
absorbent materials.
Illustrations Level III
-
A. Wood sills set on grade were replaced with concrete
pier foundation and new wooden sill plates. Changes were
not visible on the exterior. Photo: WPTC, NPS
-
B. In a flood plain, rotted joists were replaced with a
concrete slab and sleepers designed to drain water. Spaced
flooring allowed drainage and room for damp wood to swell
without buckling. Harper's Ferry Center, NPS
-
C. Mechanical systems on the lower level were placed on
platforms above the flood line. Harper's Ferry Center,
NPS
-
D. This lead sheet was installed at the base of the
replacement column to stop rising damp. Photo: Bryan
Blundell.
-
E. Triple glazed windows replicated the originals to
control condensation. Photo: © Isabella Gardner
Museum, Boston.
-
F. The roof at Montpelier was redesigned to correct an
original design flaw. Narrower standing seam panels now
avoid buckling and cracking without changing the basic
appearance of the roof. Gutters were redesigned to handle
ice dams and water overflow. Photo: Courtesy, Larry D.
Dermody and the National Trust for Historic
Preservation.
-
G. Ground moisture was reduced by redirecting downspout
drainage away from the building. Critically damp
foundation walls were protected with a layer of bentonite
clay to reduce moisture penetration. Photo: Courtesy,
Larry D. Dermody and the National Trust for Historic
Preservation.
-
H. New sensors which monitor temperature and relative
humidity are located throughout this museum and tied to a
computer that controls the climate control system. Photo:
© Isabella Stewart Gardner Museum, Boston.
-
I. New computers tie a variety of monitoring and
security features into a comprehensive system which
provides warning and backup alerts when any of the system
components are not functioning properly. Photo: ©
Isabella Stewart Gardner Museum, Boston.
Glossary:
Air flow/ infiltration: The movement that carries
moist air into and through materials. Air flow depends on the
difference between indoor and outdoor pressures, wind speed
and direction as well as the permeability of materials.
Bulk water: The large quantity of moisture from roof
and ground run-off that can enter into a building either
above grade or below grade.
Capillary action: The force that moves moisture
through the pore structure of materials. Generally referred
to as rising damp, moisture at or below the foundation level
will rise vertically in a wall to a height at which the rate
of evaporation balances the rate at which it can be drawn up
by capillary forces.
Condensation: The physical process by which water
vapor is transformed into a liquid when the relative humidity
of the air reaches 100% and the excess water vapor forms,
generally as droplets, on the colder adjacent surface.
Convection: Heat transfer through the atmosphere by a
difference in force or air pressure is one type of air
transport. Sometimes referred to as the "stack effect,"
hotter less dense air will rise, colder dense air will fall
creating movement of air within a building.
Dewpoint: The temperature at which water vapor
condenses when the air is cooled at a constant pressure and
constant moisture content.
Diffusion: The movement of water vapor through a
material. Diffusion depends on vapor pressure, temperature,
relative humidity, and the permeability of a material.
Evaporation: The transformation of liquid into a
vapor, generally as a result of rise of temperature, is the
opposite of condensation. Moisture in damp soil, such as in a
crawl space, can evaporate into the air, raise the relative
humidity in that space, and enter the building as a vapor.
Ground moisture: The saturated moisture in the ground
as a result of surface run-off and naturally occuring water
tables. Ground moisture can penetrate through cracks and
holes in foundation walls or can migrate up from moisture
under the foundation base.
Monitoring instrumentation: These devices are
generally used for long term diagnostic analysis of a
problem, or to measure the performance of a treatment, or to
measure changes of conditions or environment. In-wall probes
or sensors are often attached to data-loggers which can be
down-loaded into computers.
Permeability: A characteristic of porosity of a
material generally listed as the rate of diffusion of a
pressurized gas through a material. The pore structure of
some materials allows them to absorb or adsorb more moisture
than other materials. Limestones are generally more permeable
than granites.
Relative humidity (RH): Dampness in the air is
measured as the percent of water vapor in the air at a
specific temperature relative to the amount of water vapor
that can be held in a vapor form at that specific
temperature.
Survey instrumentation: technical instrumentation that
is used on-site to provide quick readings of specific
physical conditions. Generally these are hand-held survey
instruments, such as moisture, temperature and relative
humidity readers, dewpoint sensors, and fiber optic
boroscopes.
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