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Historic Preservation - Technical Procedures

Spectitle:

Preservation Briefs: 13 The Repair And Thermal Upgrading Of Historic Steel Windows

Procedure code:

0851001S

Source:

Preservation Briefs 13, National Park Service, Pad

Division:

Doors And Windows

Section:

Steel Windows

Last Modified:

09/10/2012

Details:

Preservation Briefs: 13 The Repair And Thermal Upgrading Of Historic Steel Windows



PRESERVATION BRIEFS:  13

THE REPAIR AND THERMAL UPGRADING OF HISTORIC STEEL WINDOWS

 

The link immediately below connects to the latest version of National Park Service Preservation Brief 13:

 

http://www.nps.gov/history/hps/tps/briefs/brief13.htm


Sharon C. Park, AIA

This standard includes the bulk of information contained in the
original Preservation Brief developed by the National Park Service.
To obtain a complete copy of this brief, including figures and
illustrations, please contact:  

              Superintendent of Documen= ts
              P.O. Box 371954
              Pittsburgh, PA  1525= 0-7954

              GPO #024-005-01026-2
              Available ONLY in packaged sets
              Briefs 1-14 - $13.00

Please call the Publication Order Information Desk at 202/783-3238
or FAX 202/512-2250 to verify price and availability.  


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.

***INTRODUCTION***

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, insensitively
designed replacement windows may diminish - or destroy - the
building's historic character.

This Brief identifies various types of historical windows that
dominated the metal window market from 1890-1950.  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.  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 multi-story 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.  The rolled steel sections, generally
1/8" thick and 1" to 1- 1/2" wide, were used for all the components
of the windows: sash, frame, and subframe.  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 l9th
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.

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.


***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 oxy-acetylene 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=F8F) 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 anti-corrosive 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
blowdryers 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 build-up 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 anti-corrosive 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.  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, 80-100 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 blow-drier (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 anti-corrosive 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
anti-corrosive 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.

As part of the orderly removal of windows, each window should be
numbered and the parts labelled.  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 labelled 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<= br> 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 anti-corrosive 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 oxy-acetylene 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.


***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.

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.  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
mid-point, the thin sections of the metal window will bow away from
the frame near the top.  If the gap is not more than l/4", foamweatherstripping 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 (2-7 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 each type of weatherstripping is 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.

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.  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




 


Preservation Brief 13,repair and thermal upgrading of historic steel windows, steel windows, repair of steel windows