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The Repair and Thermal Upgrading
of Historic Steel Windows
Sharon C. Park, AIA
The Secretary of the
Interior's "Standards for Rehabilitation" require that where
historic windows are individually significant features, or
where they contribute to the character of significant
facades, their distinguishing visual qualities must not be
destroyed. Further, the rehabilitation guidelines recommend
against changing the historic appearance of windows through
the use of inappropriate designs, materials, finishes, or
colors which radically change the sash, depth of reveal, and
muntin configuration; the reflectivity and color of the
glazing; or the appearance of the frame.
Windows are among the
most vulnerable features of historic buildings undergoing
rehabilitation. This is especially the case with rolled steel
windows, which are often mistakenly not deemed worthy of
preservation in the conversion of old buildings to new uses.
The ease with which they can be replaced and the mistaken
assumption that they cannot be made energy efficient except
at great expense are factors that typically lead to the
decision to remove them. In many cases, however, repair and
retrofit of the historic windows are more economical than
wholesale replacement, and all too often, replacement units
are unlike the originals in design and appearance. If the
windows are important in establishing the historic character
of the building (see fig. 1), insensitively designed
replacement windows may diminish--or destroy--the building's
historic character.
This Brief identifies
various types of historic steel windows that dominated the
metal window market from 18901950. It then gives criteria for
evaluating deterioration and for determining appropriate
treatment, ranging from routine maintenance and
weatherization to extensive repairs, so that replacement may
be avoided where possible.(1) This information applies to
do-it-yourself jobs and to large rehabilitations where the
volume of work warrants the removal of all window units for
complete overhaul by professional contractors.
This Brief is not
intended to promote the repair of ferrous metal windows in
every case, but rather to insure that preservation is always
the first consideration in a rehabilitation project. Some
windows are not important elements in defining a building's
historic character; others are highly significant, but so
deteriorated that repair is infeasible. In such cases, the
Brief offers guidance in evaluating appropriate replacement
windows.
Historical Development
Although metal windows
were available as early as 1860 from catalogues published by
architectural supply firms, they did not become popular until
after 1890. Two factors combined to account for the shift
from wooden to metal windows about that time. Technology
borrowed from the rolling industry permitted the mass
production of rolled steel windows. This technology made
metal windows cost competitive with conventional wooden
windows. In addition, a series of devastating urban fires in
Boston, Baltimore, Philadelphia, and San Francisco led to the
enactment of strict fire codes for industrial and multistory
commercial and office buildings.
As in the process of
making rails for railroads, rolled steel windows were made by
passing hot bars of steel through progressively smaller,
shaped rollers until the appropriate angled configuration was
achieved (see fig. 2). The rolled steel sections, generally
1/8" thick and 1" - 1-1/2" wide, were used for all the
components of the windows: sash, frame, and subframe (see
fig. 3). With the addition of wire glass, a fire-resistant
window resulted. These rolled steel windows are almost
exclusively found in masonry or concrete buildings.
A byproduct of the
fire-resistant window was the strong metal frame that
permitted the installation of larger windows and windows in
series. The ability to have expansive amounts of glass and
increased ventilation dramatically changed the designs of
late 19th and early 20th century industrial and commercial
buildings.
The newly available,
reasonably priced steel windows soon became popular for more
than just their fire-resistant qualities. They were
standardized, extremely durable, and easily transported.
These qualities led to the use of steel windows in every type
of construction, from simple industrial and institutional
buildings to luxury commercial and apartment buildings.
Casement, double-hung, pivot, projecting, austral, and
continuous windows differed in operating and ventilating
capacities. Figure 4 outlines the kinds and properties of
metal windows available then and now. In addition, the thin
profiles of metal windows contributed to the streamlined
appearance of the Art Deco, Art Moderne, and International
Styles, among others.
The extensive use of
rolled steel metal windows continued until after World War II
when cheaper, noncorroding aluminum windows became
increasingly popular. While aluminum windows dominate the
market today, steel windows are still fabricated. Should
replacement of original windows become necessary, replacement
windows may be available from the manufacturers of some of
the earliest steel windows. Before an informed decision can
be made whether to repair or replace metal windows, however,
the significance of the windows must be determined and their
physical condition assessed.
Evaluation
Historic and
Architectural Considerations
An assessment of the
significance of the windows should begin with a consideration
of their function in relation to the building's historic use
and its historic character. Windows that help define the
building's historic character should be preserved even if the
building is being converted to a new use. For example,
projecting steel windows used to introduce light and an
effect of spaciousness to a warehouse or industrial plant can
be retained in the conversion of such a building to offices
or residences.
Other elements in
assessing the relative importance of the historic windows
include the design of the windows and their relationship to
the scale, proportion, detailing and architectural style of
the building. While it may be easy to determine the aesthetic
value of highly ornamented windows, or to recognize the
importance of streamlined windows as an element of a style,
less elaborate windows can also provide strong visual
interest by their small panes or projecting planes when open,
particularly in simple, unadorned industrial buildings (see
fig. 5).
One test of the
importance of windows to a building is to ask if the overall
appearance of the building would be changed noticeably if the
windows were to be removed
or radically altered. If so, the windows are important in
defining the building's historic character, and should be
repaired if their physical condition permits.
Physical
Evaluation
Steel window repair
should begin with a careful evaluation of the physical
condition of each unit. Either drawings or photographs,
liberally annotated, may be used to record the location of
each window, the type of operability, the condition of all
three parts--sash, frame and subframe--and the repairs
essential to its continued use.
Specifically, the
evaluation should include: presence and degree of corrosion;
condition of paint; deterioration of the metal sections,
including bowing, misalignment of the sash, or bent sections;
condition of the glass and glazing compound; presence and
condition of all hardware, screws, bolts, and hinges; and
condition of the masonry or concrete surrounds, including
need for caulking or resetting of improperly sloped
sills.
Corrosion, principally
rusting in the case of steel windows, is the controlling
factor in window repair; therefore, the evaluator should
first test for its presence. Corrosion can be light, medium,
or heavy, depending on how much the rust has penetrated the
metal sections. If the rusting is merely a surface
accumulation or flaking, then the corrosion is light. If the
rusting has penetrated the metal (indicated by a bubbling
texture), but has not caused any structural damage, then the
corrosion is medium. If the rust has penetrated deep into the
metal, the corrosion is heavy. Heavy corrosion generally
results in some form of structural damage, through
delamination, to the metal section, which must then be
patched or spliced.
A sharp probe or tool,
such as an ice pick, can be used to determine the extent of
corrosion in the metal. If the probe can penetrate the
surface of the metal and brittle strands can be dug out, then
a high degree of corrosive deterioration is present.
In addition to corrosion,
the condition of the paint, the presence of bowing or
misalignment of metal sections, the amount of glass needing
replacement, and the condition of the masonry or concrete
surrounds must be assessed in the evaluation process. These
are key factors in determining whether or not the windows can
be repaired in place. The more complete the inventory of
existing conditions, the easier it will be to determine
whether repair is feasible or whether replacement is
warranted.
Rehabilitation Work
Plan
Following inspection and
analysis, a plan for the rehabilitation can be formulated.
The actions necessary to return windows to an efficient and
effective working condition will fall into one or more of the
following categories: routine maintenance, repair, and
weatherization. The routine maintenance and weatherization
measures described here are generally within the range of
do-it-yourselfers. Other repairs, both moderate and major,
require a professional contractor. Major repairs normally
require the removal of the window units to a workshop, but
even in the case of moderate repairs, the number of windows
involved might warrant the removal of all the deteriorated
units to a workshop in order to realize a more economical
repair price. Replacement of windows should be considered
only as a last resort.
Since moisture is the
primary cause of corrosion in steel windows, it is essential
that excess moisture be eliminated and that the building be
made as weathertight as possible before any other work is
undertaken. Moisture can accumulate from cracks in the
masonry, from spalling mortar, from leaking gutters, from air
conditioning condensation runoff, and from poorly ventilated
interior spaces.
Finally, before beginning
any work, it is important to be aware of health and safety
risks involved. Steel windows have historically been coated
with lead paint. The removal of such paint by abrasive
methods will produce toxic dust. Therefore, safety goggles, a
toxic dust respirator, and protective clothing should be
worn. Similar protective measures should be taken when acid
compounds are used. Local codes may govern the methods of
removing lead paints and proper disposal of toxic
residue.
Typical Rolled Steel Windows Available from 1890 to the
Present
DOUBLE-HUNG industrial
windows duplicated the look of traditional wooden windows.
Metal double-hung windows were early examples of a building
product adapted to meet stringent new fire code requirements
for manufacturing and high-rise buildings in urban areas.
Soon supplanted in industrial buildings by less expensive
pivot windows, double-hung metal windows regained popularity
in the 1940s for use in speculative suburban housing.
PIVOT windows were an
early type of industrial window that combined inexpensive
first cost and low maintenance. Pivot windows became standard
for warehouses and power plants where the lack of screens was
not a problem. The window shown here is a horizontal pivot.
Windows that turned about a vertical axis were also
manufactured (often of iron). Such vertical pivots are rare
today.
PROJECTING windows,
sometimes called awning or hopper windows, were perfected in
the 1920s for industrial and institutional buildings. They
were often used in "combination" windows, in which upper
panels opened out and lower panels opened in. Since each
movable panel projected to one side of the frame only, unlike
pivot windows, for example, screens could be
introduced.
AUSTRAL windows were also
a product of the 1920s. They combined the appearance of the
double-hung window with the increased ventilation and ease of
operation of the projected window. (When fully opened, they
provided 70% ventilation as compared to 50% ventilation for
double-hung windows.) Austral windows were often used in
schools, libraries and other public buildings.
CASEMENT windows adapted
the English tradition of using wrought iron casements with
leaded cames for residential use. Rolled steel casements
(either single, as shown, or paired) were popular in the
1920s for cottage style residences and Gothic style campus
architecture. More streamlined casements were popular in the
1930s for institutional and small industrial
buildings.
CONTINUOUS windows were
almost exclusively used for industrial buildings requiring
high overhead lighting. Long runs of clerestory windows
operated by mechanical tension rod gears were typical. Long
banks of continuous windows were possible because the frames
for such windows were often structural elements of the
building.
Routine Maintenance
A preliminary step in the
routine maintenance of steel windows is to remove surface
dirt and grease in order to ascertain the degree of
deterioration, if any. Such minor cleaning can be
accomplished using a brush or vacuum followed by wiping with
a cloth dampened with mineral spirits or denatured
alcohol.
If it is determined that
the windows are in basically sound condition, the following
steps can be taken: 1) removal of light rust, flaking and
excessive paint; 2) priming of exposed metal with a
rust-inhibiting primer; 3) replacement of cracked or broken
glass and glazing compound; 4) replacement of missing screws
or fasteners; 5) cleaning and lubrication of hinges; 6)
repainting of all steel sections with two coats of finish
paint compatible with the primer; and 7) caulking the masonry
surrounds with a high quality elastomeric caulk.
Recommended methods for
removing light rust include manual and mechanical abrasion or
the application of chemicals. Burning off rust with an
oxyacetylene or propane torch, or an inert gas welding gun,
should never be attempted because the heat can distort the
metal. In addition, such intense heat (often as high as 3800
deg. F) vaporizes the lead in old paint, resulting in highly
toxic fumes. Furthermore, such heat will likely result in
broken glass. Rust can best be removed using a wire brush, an
aluminum oxide sandpaper, or a variety of power tools adapted
for abrasive cleaning such as an electric drill with a wire
brush or a rotary whip attachment. Adjacent sills and window
jambs may need protective shielding.
Rust can also be removed
from ferrous metals by using a number of commercially
prepared anticorrosive acid compounds. Effective on light and
medium corrosion, these compounds can be purchased either as
liquids or gels. Several bases are available, including
phosphoric acid, ammonium citrate, oxalic acid and
hydrochloric acid. Hydrochloric acid is generally not
recommended; it can leave chloride deposits, which cause
future corrosion. Phosphoric acid-based compounds do not
leave such deposits, and are therefore safer for steel
windows. However, any chemical residue should be wiped off
with damp cloths, then dried immediately. Industrial
blow-dryers work well for thorough drying. The use of running
water to remove chemical residue is never recommended because
the water may spread the chemicals to adjacent surfaces, and
drying of these surfaces may be more difficult. Acid cleaning
compounds will stain masonry; therefore plastic sheets should
be taped to the edge of the metal sections to protect the
masonry surrounds. The same measure should be followed to
protect the glazing from etching because of acid
contact.
Measures that remove rust
will ordinarily remove flaking paint as well. Remaining loose
or flaking paint can be removed with a chemical paint remover
or with a pneumatic needle scaler or gun, which comes with a
series of chisel blades and has proven effective in removing
flaking paint from metal windows. Well-bonded paint may serve
to protect the metal further from corrosion, and need not be
removed unless paint buildup prevents the window from closing
tightly. The edges should be feathered by sanding to give a
good surface for repainting.
Next, any bare metal
should be wiped with a cleaning solvent such as denatured
alcohol, and dried immediately in preparation for the
application of an anticorrosive primer. Since corrosion can
recur very soon after metal has been exposed to the air, the
metal should be primed immediately after cleaning. Spot
priming may be required periodically as other repairs are
undertaken. Anticorrosive primers generally consist of
oil-alkyd based paints rich in zinc or zinc chromate.(2) Red
lead is no longer available because of its toxicity. All
metal primers, however, are toxic to some degree and should
be handled carefully. Two coats of primer are recommended.
Manufacturer's recommendations should be followed concerning
application of primers.
Repair
Repair in Place
The maintenance
procedures described above will be insufficient when
corrosion is extensive, or when metal window sections are
misaligned. Medium to heavy corrosion that has not done any
structural damage to the metal sections can be removed either
by using the chemical cleaning process described under
"Routine Maintenance" or by sandblasting. Since sandblasting
can damage the masonry surrounds and crack or cloud the
glass, metal or plywood shields should be used to protect
these materials. The sandblasting pressure should be low,
80100 pounds per square inch, and the grit size should be in
the range of #10#45. Glass peening beads (glass pellets) have
also been successfully used in cleaning steel sections. While
sandblasting equipment comes with various nozzle sizes,
pencil-point blasters are most useful because they give the
operator more effective control over the direction of the
spray. The small aperture of the pencil-point blaster is also
useful in removing dried putty from the metal sections that
hold the glass. As with any cleaning technique, once the bare
metal is exposed to air, it should be primed as soon as
possible. This includes the inside rabbeted section of sash
where glazing putty has been removed. To reduce the dust,
some local codes allow only wet blasting. In this case, the
metal must be dried immediately, generally with a blowdrier
(a step that the owner should consider when calculating the
time and expense involved). Either form of sandblasting metal
covered with lead paints produces toxic dust. Proper
precautionary measures should be taken against toxic dust and
silica particles.
Bent or bowed metal
sections may be the result of damage to the window through an
impact or corrosive expansion. If the distortion is not too
great, it is possible to realign the metal sections without
removing the window to a metal fabricator's shop. The glazing
is generally removed and pressure is applied to the bent or
bowed section. In the case of a muntin, a protective 2 x 4
wooden bracing can be placed behind the bent portion and a
wire cable with a winch can apply progressively more pressure
over several days until the section is realigned. The 2 x 4
bracing is necessary to distribute the pressure evenly over
the damaged section. Sometimes a section, such as the bottom
of the frame, will bow out as a result of pressure exerted by
corrosion and it is often necessary to cut the metal section
to relieve this pressure prior to pressing the section back
into shape and making a welded repair.
Once the metal sections
have been cleaned of all corrosion and straightened, small
holes and uneven areas resulting from rusting should be
filled with a patching material and sanded smooth to
eliminate pockets where water can accumulate. A patching
material of steel fibers and an epoxy binder may be the
easiest to apply. This steel-based epoxy is available for
industrial steel repair; it can also be found in auto body
patching compounds or in plumber's epoxy. As with any
product, it is important to follow the manufacturer's
instructions for proper use and best results. The traditional
patching technique--melting steel welding rods to fill holes
in the metal sections--may be difficult to apply in some
situations; moreover, the window glass must be removed during
the repair process, or it will crack from the expansion of
the heated metal sections. After these repairs, glass
replacement, hinge lubrication, painting, and other cosmetic
repairs can be undertaken as necessary.
To complete the checklist
for routine maintenance, cracked glass, deteriorated glazing
compound, missing screws, and broken fasteners will have to
be replaced; hinges cleaned and lubricated; the metal windows
painted, and the masonry surrounds caulked. If the glazing
must be replaced, all clips, glazing beads, and other
fasteners that hold the glass to the sash should be retained,
if possible, although replacements for these parts are still
being fabricated. When bedding glass, use only glazing
compound formulated for metal windows. To clean the hinges
(generally brass or bronze), a cleaning solvent and fine
bronze wool should be used. The hinges should then be
lubricated with a non-greasy lubricant specially formulated
for metals and with an anticorrosive agent. These lubricants
are available in a spray form and should be used periodically
on frequently opened windows.
Final painting of the
windows with a paint compatible with the anticorrosive primer
should proceed on a dry day. (Paint and primer from the same
manufacturer should be used.) Two coats of finish paint are
recommended if the sections have been cleaned to bare metal.
The paint should overlap the glass slightly to insure
weathertightness at that connection. Once the paint dries
thoroughly, a flexible exterior caulk can be applied to
eliminate air and moisture infiltration where the window and
the surrounding masonry meet.
Caulking is generally
undertaken after the windows have received at least one coat
of finish paint. The perimeter of the masonry surround should
be caulked with a flexible elastomeric compound that will
adhere well to both metal and masonry. The caulking used
should be a type intended for exterior application, have a
high tolerance for material movement, be resistant to
ultraviolet light, and have a minimum durability of 10 years.
Three effective compounds (taking price and other factors
into consideration) are polyurethane, vinyl acrylic, and
butyl rubber. In selecting a caulking material for a window
retrofit, it is important to remember that the caulking
compound may be covering other materials in a substrate. In
this case, some compounds, such as silicone, may not adhere
well. Almost all modern
caulking compounds can be painted after curing completely.
Many come in a range of colors, which eliminates the need to
paint. If colored caulking is used, the windows should have
been given two coats of finish paint prior to
caulking.
Repair in Workshop
Damage to windows may be
so severe that the window sash and sometimes the frame must
be removed for cleaning and extensive rust removal,
straightening of bent sections, welding or splicing in of new
sections, and reglazing. These major and expensive repairs
are reserved for highly significant windows that cannot be
replaced; the procedures involved should be carried out only
by skilled workmen. (see fig. 6a-6f.)
As part of the orderly
removal of windows, each window should be numbered and the
parts labeled. The operable metal sash should be dismantled
by removing the hinges; the fixed sash and, if necessary, the
frame can then be unbolted or unscrewed. (The subframe is
usually left in place. Built into the masonry surrounds, it
can only be cut out with a torch.) Hardware and hinges should
be labeled and stored together.
The two major choices for
removing flaking paint and corrosion from severely
deteriorated windows are dipping in a chemical bath or
sandblasting. Both treatments require removal of the glass.
If the windows are to be dipped, a phosphoric acid solution
is preferred, as mentioned earlier. While the dip tank method
is good for fairly evenly distributed rust, deep set rust may
remain after dipping. For that reason, sandblasting is more
effective for heavy and uneven corrosion. Both methods leave
the metal sections clean of residual paint. As already noted,
after cleaning has exposed the metal to the air, it should be
primed immediately after drying with an anticorrosive primer
to prevent rust from recurring.
Sections that are
seriously bent or bowed must be straightened with heat and
applied pressure in a workshop. Structurally weakened
sections must be cut out, generally with an oxyacetylene
torch, and replaced with sections welded in place and the
welds ground smooth. Finding replacement metal sections,
however, may be difficult. While most rolling mills are
producing modern sections suitable for total replacement, it
may be difficult to find an exact profile match for a
splicing repair. The best source of rolled metal sections is
from salvaged windows, preferably from the same building. If
no salvaged windows are available, two options remain. Either
an ornamental metal fabricator can weld flat plates into a
built-up section, or a steel plant can mill bar steel into
the desired profile.
While the sash and frame
are removed for repair, the subframe and masonry surrounds
should be inspected. This is also the time to reset sills or
to remove corrosion from the subframe, taking care to protect
the masonry surrounds from damage.
Missing or broken
hardware and hinges should be replaced on all windows that
will be operable. Salvaged windows, again, are the best
source of replacement parts. If matching parts cannot be
found, it may be possible to adapt ready-made items. Such a
substitution may require filling existing holes with steel
epoxy or with plug welds and tapping in new screw holes.
However, if the hardware is a highly significant element of
the historic window, it may be worth having reproductions
made.
Following are
illustrations of the repair and thermal upgrading of the
rolled steel windows in a National Historic Landmark (fig.
6). Many of the techniques described above were used during
this extensive rehabilitation. The complete range of repair
techniques is then summarized in the chart titled Steps for
Cleaning and Repairing Historic Steel Windows (see fig.
7).
Weatherization
Historic metal windows
are generally not energy efficient; this has often led to
their wholesale replacement. Metal windows can, however, be
made more energy efficient in several ways, varying in
complexity and cost. Caulking around the masonry openings and
adding weatherstripping, for example, can be do-it-yourself
projects and are important first steps in reducing air
infiltration around the windows. They usually have a rapid
payback period. Other treatments include applying fixed
layers of glazing over the historic windows, adding operable
storm windows, or installing thermal glass in place of the
existing glass. In combination with caulking and
weatherstripping, these treatments can produce energy ratings
rivaling those achieved by new units.(3)
Weatherstripping
The first step in any
weatherization program, caulking, has been discussed above
under "Routine Maintenance." The second step is the
installation of weatherstripping where the operable portion
of the sash, often called the ventilator, and the fixed frame
come together to reduce perimeter air infiltration (see fig.
8). Four types of weatherstripping appropriate for metal
windows are spring-metal, vinyl strips, compressible foam
tapes, and sealant beads. The spring-metal, with an integral
friction fit mounting clip, is recommended for steel windows
in good condition. The clip eliminates the need for an
applied glue; the thinness of the material insures a tight
closure. The weatherstripping is clipped to the inside
channel of the rolled metal section of the fixed frame. To
insure against galvanic corrosion between the
weatherstripping (often bronze or brass), and the steel
window, the window must be painted prior to the installation
of the weatherstripping. This weatherstripping is usually
applied to the entire perimeter of the window opening, but in
some cases, such as casement windows, it may be best to avoid
weatherstripping the hinge side. The natural wedging action
of the weatherstripping on the three sides of the window
often creates an adequate seal.
Vinyl weatherstripping
can also be applied to metal windows. Folded into a "V"
configuration, the material forms a barrier against the wind.
Vinyl weatherstripping is usually glued to the frame,
although some brands have an adhesive backing. As the vinyl
material and the applied glue are relatively thick, this form
of weatherstripping may not be appropriate for all
situations.
Compressible foam tape
weatherstripping is often best for large windows where there
is a slight bending or distortion of the sash. In some very
tall windows having closure hardware at the sash midpoint,
the thin sections of the metal window will bow away from the
frame near the top. If the gap is not more than 1/4", foam
weatherstripping can normally fill the space. If the gap
exceeds this, the window may need to be realigned to close
more tightly. The foam weatherstripping comes either with an
adhesive or plain back; the latter variety requires
application with glue. Compressible foam requires more
frequent replacement than either spring-metal or vinyl
weatherstripping.
A fourth type of
successful weatherstripping involves the use of a caulking or
sealant bead and a polyethylene bond breaker tape. After the
window frame has been thoroughly cleaned with solvent,
permitted to dry, and primed, a neat bead of low modulus
(firm setting) caulk, such as silicone, is applied. A bond
breaker tape is then applied to the operable sash covering
the metal section where contact will occur. The window is
then closed until the sealant has set (27 days, depending on
temperature and humidity). When the window is opened, the
bead will have taken the shape of the air infiltration gap
and the bond breaker tape can be removed. This
weatherstripping method appears to be successful for all
types of metal windows with varying degrees of air
infiltration.
Since the several types
of weatherstripping are appropriate for different
circumstances, it may be necessary to use more than one type
on any given building. Successful weatherstripping depends
upon using the thinnest material adequate to fill the space
through which air enters. Weatherstripping that is too thick
can spring the hinges, thereby resulting in more gaps.
FIGURE 8. Appropriate
Types of Weatherstripping
for Metal Windows
* SPRING-METAL comes in
bronze, brass or stainless steel with an integral friction
fit clip. The weatherstripping is applied after the repaired
windows are painted to avoid galvanic corrosion. This type of
thin weatherstripping is intended for windows in good
condition.
* VINYL STRIPS are scored
and fold into a "V" configuration. Applied adhesive is
necessary which will increase the thickness of the
weatherstripping, making it inappropriate for some
situations. The weatherstripping is generally applied to the
window after painting.
* Closed cell FOAM TAPE
comes either with or without an adhesive backing. It is
effective for windows with a gap of approximately 1/4" and is
easy to install. However, this type of weatherstripping will
need frequent replacement on windows in regular use. The
metal section should be cleaned of all dirt and grease prior
to its application.
* SEALANT BEAD. This very
effective type of weatherstripping involves the application
of a clean bead of firm setting caulk on the primed frame
with a polyethelene bond breaker tape on the operable sash.
The window is then closed until the bead has set and takes
the form of the gap. The sash is then opened and the tape is
removed leaving the set caulk as the weatherstripping.
# # #
Thermal Glazing
The third weatherization
treatment is to install an additional layer of glazing to
improve the thermal efficiency of the existing window. The
decision to pursue this treatment should proceed from careful
analysis. Each of the most common techniques for adding a
layer of glazing will effect approximately the same energy
savings (approximately double the original insulating value
of the windows); therefore, cost and aesthetic considerations
usually determine the choice of method. Methods of adding a
layer of glazing to improve thermal efficiency include adding
a new layer of transparent material to the window; adding a
separate storm window; and replacing the single layer of
glass in the window with thermal glass.
The least expensive of
these options is to install a clear material (usually rigid
sheets of acrylic or glass) over the original window. The
choice between acrylic and glass is generally based on cost,
ability of the window to support the material, and long-term
maintenance outlook. If the material is placed over the
entire window and secured to the frame, the sash will be
inoperable. If the continued use of the window is important
(for ventilation or for fire exits), separate panels should
be affixed to the sash without obstructing operability (see
fig. 9). Glass or acrylic panels set in frames can be
attached using magnetized gaskets, interlocking material
strips, screws or adhesives. Acrylic panels can be screwed
directly to the metal windows, but the holes in the acrylic
panels should allow for the expansion and contraction of this
material. A compressible gasket between the prime sash and
the storm panel can be very effective in establishing a
thermal cavity between glazing layers. To avoid condensation,
1/8" cuts in a top corner and diagonally opposite bottom
corner of the gasket will provide a vapor bleed, through
which moisture can evaporate. (Such cuts, however, reduce
thermal performance slightly.) If condensation does occur,
however, the panels should be easily removable in order to
wipe away moisture before it causes corrosion.
The second method of
adding a layer of glazing is to have independent storm
windows fabricated. (Pivot and austral windows, however,
which project on either side of the window frame when open,
cannot easily be fitted with storm windows and remain
operational.) The storm window should be compatible with the
original sash configuration. For example, in paired casement
windows, either specially fabricated storm casement windows
or sliding units in which the vertical meeting rail of the
slider reflects the configuration of the original window
should be installed. The decision to place storm windows on
the inside or outside of the window depends on whether the
historic window opens in or out, and on the visual impact the
addition of storm windows will have on the building. Exterior
storm windows, however, can serve another purpose besides
saving energy: they add a layer of protection against air
pollutants and vandals, although they will partially obscure
the prime window. For highly ornamental windows this
protection can determine the choice of exterior rather then
interior storm windows.
The third method of
installing an added layer of glazing is to replace the
original single glazing with thermal glass. Except in rare
instances in which the original glass is of special interest
(as with stained or figured glass), the glass can be replaced
if the hinges can tolerate the weight of the additional
glass. The rolled metal sections for steel windows are
generally from 1" 1-1/2" thick. Sash of this thickness can
normally tolerate thermal glass, which ranges from 3/8" 5/8".
(Metal glazing beads, readily available, are used to
reinforce the muntins, which hold the glass.) This treatment
leaves the window fully operational while preserving the
historic appearance. It is, however, the most expensive of
the treatments discussed here. (See fig. 6f).
Window Replacement
Repair of historic
windows is always preferred within a rehabilitation project.
Replacement should be considered only as a last resort.
However, when the extent of deterioration or the
unavailability of replacement sections renders repair
impossible, replacement of the entire window may be
justified. In the case of significant windows, replacement in
kind is essential in order to maintain the historic character
of the building. However, for less significant windows,
replacement with compatible new windows may be acceptable. In
selecting compatible replacement windows, the material,
configuration, color, operability, number and size of panes,
profile and proportion of metal sections, and reflective
quality of the original glass should be duplicated as closely
as possible.
A number of metal window
manufacturing companies produce rolled steel windows. While
stock modern window designs do not share the multi-pane
configuration of historic windows, most of these
manufacturers can reproduce the historic configuration if
requested, and the cost is not excessive for large orders
(see figs. 10a and 10b). Some manufacturers still carry the
standard pre-World War II multi-light windows using the
traditional 12" x 18" or 14" x 20" glass sizes in industrial,
commercial, security, and residential configurations. In
addition, many of the modern steel windows have integral
weatherstripping, thermal break construction, durable vinyl
coatings, insulating glass, and other desirable
features.
Windows manufactured from
other materials generally cannot match the thin profiles of
the rolled steel sections. Aluminum, for example, is three
times weaker than steel and must be extruded into a boxlike
configuration that does not reflect the thin historic
profiles of most steel windows. Wooden and vinyl replacement
windows generally are not fabricated in the industrial style,
nor can they reproduce the thin profiles of the rolled steel
sections, and consequently are generally not acceptable
replacements.
For product information
on replacement windows, the owner, architect, or contractor
should consult manufacturers' catalogues, building trade
journals, or the Steel Window Institute, 1230 Keith Building,
Cleveland, Ohio 44115.
Summary
The National Park Service
recommends the retention of significant historic metal
windows whenever possible. Such windows, which can be a
character-defining feature of a historic building, are too
often replaced with inappropriate units that impair rather
than complement the overall historic appearance. The repair
and thermal upgrading of historic steel windows is more
practicable than most people realize. Repaired and properly
maintained metal windows have greatly extended service lives.
They can be made energy efficient while maintaining their
contribution to the historic character of the
building.
NOTES
(1) The technical
information given in this brief is intended for most ferrous
(or magnetic) metals, particularly rolled steel. While
stainless steel is a ferrous metal, the cleaning and repair
techniques outlined here must not be used on it as the finish
will be damaged. For information on cleaning stainless steel
and nonferrous metals, such as bronze, Monel, or aluminum,
refer to Metals in America's Historic Buildings (see
bibliography).
(2) Refer to Table IV.
Types of Paint Used for Painting Metal in Metals in America's
Historic Buildings, p. 139. (See bibliography).
(3) One measure of energy
efficiency is the U-value (the number of BTUs per hour
transferred through a square foot of material). The lower the
U-value, the better the performance. According to ASHRAE
HANDBOOK 1977 Fundamentals, the U-value of historic rolled
steel sash with single glazing is 1.3. Adding storm windows
to the existing units or reglazing with 5/8" insulating glass
produces a Uvalue of .69. These methods of weatherizing
historic steel windows compare favorably with rolled steel
replacement alternatives: with factory installed 1"
insulating glass (.67 Uvalue); with added thermal break
construction and factory finish coatings (.62 Uvalue).
Bibliography
ASHRAE Handbook 1977
Fundamentals. New York: American Society of Heating,
Refrigerating and Air-conditioning Engineers, 1978.
Crittal, W. F. A Metal
Window Dictionary. London: Curwen Press, 1926. Reprinted by
B.T. Batsford. Ltd., 1953.
Gayle, Margot; David W.
Look, AIA; John G. Waite. Metals in America's Historic
Buildings: Uses and Preservation Treatments. Technical
Preservation Services, U.S. Department of the Interior.
Washington,D.C.: U.S. Government Printing Office,
1980.
Gillet, William. "Steel
Windows." Windows and Glass in the Exterior of Buildings.
National Academy of Sciences Publication 478. Washington,
D.C.: 1957,7578.
Sarton, R. H. "Selecting
and Specifying an Appropriate Type of Steel Window."
Metalcraft. Vol. 6, No. 1 (January, 1931): 4348, 6465.
Sweet's Architectural
Catalogue. 13th Edition, New York, Sweets Catalogue Service,
Inc., 1918.
The author gratefully
acknowledges the invaluable assistance of co-worker Michael
Auer in preparing this brief for publication. This
publication is an extension of research initiated by Frederec
E. Kleyle. Special thanks are given to Hope's Architectural
Products, Inc., Jamestown, NY, for their generous
contribution of historic metal window catalogues which were
an invaluable source of information. The following
individuals are also to be thanked for reviewing the
manuscript and making suggestions: Hugh Miller, Chief, Park
Historic Architecture Division, National Park Service;
Barclay L. Rogers, Museum Services, National Park Service;
Susan M. Young, Steel Window Institute, and Danny
Schlichenmaier, State Building Division, Lincoln, Nebraska.
Finally, thanks go to Technical Preservation Services Branch
staff and to cultural resources staff of the National Park
Service Regional Offices, whose valuable comments were
incorporated into the final text and who contributed to the
publication of this brief.
Washington, D. C.
September, 1984
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