HomeMy WebLinkAboutPreserve-Brief-Wooden-WindowsU.S. Department of the Interior
National Park Service
Cultural Resources
Heritage Preservation Services
Preservation
Briefs: 9
The Repair of
Historic Wooden Windows
JohnH. ~yers------------------------------------------------------------------
The windows on many historic buildings are an important
aspect of the architectural character of those buildings.
Their design, craftsmanship, or other qualities may make
them worthy of preservation . This is self-evident for or-
namental windows, but it can be equally true for
warehouses or factories where the windows may be the
most dominant visual element of an otherwise plain
building (see figure 1). Evaluating the significance of
these windows and planning for their repair or replace-
ment can be a complex process involving both objective
and subjective considerations . The Secretary of the In-
terior's Standards for Rehabilitation, and the accompany-
ing guidelines, call for respecting the significance of
original materials and features , repairing and retaining
them wherever possible, and when necessary, replacing
them in kind. This Brief is based on the issues of
significance and repair which are implicit in the standards,
but the primary emphasis is on the technical issues of
planning for the repair of windows including evaluation
of their physical condition , techniques of repair, and
design considerations when replacement is necessary.
Figure 1. Windows are frequently important visual focal points, especial-
lyon simple facades such as this mill building. Replacement of the multi-
pane windows here with larger panes could dramatically change the ap-
pearance of the building. The areas of missing windows convey the im-
pression of such a change . Photo: John T. Lowe
Much of the technical section presents repair techniques as
an instructional guide for the do-it-yourselfer. The infor-
mation will be useful, however, for the architect, contrac-
tor, or developer on large-scale projects. It presents a
methodology for approaching the evaluation and repair of
existing windows, and considerations for replacement,
from which the professional can develop alternatives and
specify appropriate materials and procedures.
Architectural or Historical Significance
Evaluating the architectural or historical significance of
windows is the first step in planning for window treat-
ments, and a general understanding of the function and
history of windows is vital to making a proper evalua-
tion. As a part of this evaluation, one must consider four
basic window functions: admitting light to the interior
spaces, providing fresh air and ventilation to the in-
terior, providing a visual link to the outside world, and
enhancing the appearance of a building . No single factor
can be disregarded when planning window treatments; for
example, attempting to conserve energy by closing up or
reducing the size of window openings may result in the
use of more energy by increasing electric lighting loads
and decreasing passive solar heat gains.
Historically, the first windows in early American houses
were casement windows; that is, they were hinged at the
side and opened outward. In the beginning of the eigh-
teenth century single-and double-hung windows were in-
troduced. Subsequently many styles of these vertical
sliding sash windows have come to be associated with
specific building periods or architectural styles, and this is
an important consideration in determining the significance
of windows, especially on a local or regional basis. Site-
specific, regionally oriented architectural comparisons
should be made to determine the significance of windows
in question. Although such comparisons may focus on
specific window types and their details, the ultimate deter-
mination of significance should be made within the con-
text of the whole building, wherein the windows are one
architectural element (see figure 2).
After all of the factors have been evaluated, windows
should be considered significant to a building if they; 1)
are original, 2) reflect the original design intent for the
building, 3) reflect period or regional styles or building
practices, 4) reflect changes to the building resulting
from major periods or events, or 5) are examples of ex-
ceptional craftsmanship or design . Once this evaluation
of significance has been completed, it is possible to pro-
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Window Sections
Muntin Profiles
These are only three examples
of many possible profiles. Mun-
tins can contribute substantially
to window significance.
Figure 2. These drawings of window details identify major components, terminology , and installation details for a wooden double-hung window.
ceed with planning appropriate treatments, beginning
with an investigation of the physical condition of the
windows.
Physical Evaluation
The key to successful planning for window treatments is
a careful evaluation of existing physical conditions on a
unit-by-unit basis. A graphic or photographic system may
be devised to record existing conditions and illustrate the
scope of any necessary repairs. Another effective tool is a
window schedule which lists all of the parts of each win-
dow unit. Spaces by each part allow notes on existing
conditions and repair instructions. When such a schedule
is completed, it indicates the precise tasks to be performed
in the repair of each unit and becomes a part of the
specifications. In any evaluation, one should note at a
minimum, 1) window location, 2) condition of the paint,
3) condition of the frame and sill, 4) condition of the sash
(rails, stiles and muntins), 5) glazing problems, 6) hard-
ware, and 7) the overall condition of the window (ex-
cellent, fair, poor, and so forth).
Many factors such as poor design, moisture, vandalism,
insect attack, and lack of maintenance can contribute to
window deterioration, but moisture is the primary con-
tributing factor in wooden window decay. All window
units should be inspected to see if water is entering around
the edges of the frame and, if so, the joints or seams
should be caulked to eliminate this danger. The glazing
putty should be checked for cracked, loose, or missing
sections which allow water to saturate the wood, especial-
ly at the joints. The back putty on the interior side of the
pane should also be inspected, because it creates a seal
which prevents condensation from running down into the
joinery. The sill should be examined to insure that it
slopes downward away from the building and Cl-llows
water to drain off. In addition, it may be advisable to cut
a dripline along the underside of the sill. This almost in-
visible treatment will insure proper water run-off, particu-
2
larly if the bottom of the sill is flat. Any conditions, in-
cluding poor original design, which permit water to come
in contact with the wood or to puddle on the sill must be
corrected as they contribute to deterioration of the win-
dow.
One clue to the location of areas of excessive moisture
is the condition of the paint; therefore, each window
should be examined for areas of paint failure. Since ex-
cessive moisture is detrimental to the paint bond, areas of
paint blistering, cracking, flaking, and peeling usually
identify points of water penetration, moisture saturation,
and potential deterioration. Failure of the paint should
not, however, be mistakenly interpreted as a sign that the
wood is in poor condition and hence, irreparable. Wood
is frequently in sound physical condition beneath unsight-
ly paint. After noting areas of paint failure, the next step
is to inspect the condition of the wood, particularly at the
points identified during the paint examination.
Each window should be examined for operational
soundness beginning with the lower portions of the frame
and sash. Exterior rainwater and interior condensation can
flow downward along the window, entering and collecting
at points where the flow is blocked. The sill, joints be-
tween the sill and jamb, corners of the bottom rails and
muntin joints are typical points where water collects and
deterioration begins (see figure 3). The operation of the
window (continuous opening and closing over the years
and seasonal temperature changes) weakens the joints,
causing movement and slight separation. This process
makes the joints more vulnerable to water which is readi-
ly absorbed into the end-grain of the wood. If severe
deterioration exists in these areas, it will usually be ap-
parent on visual inspection, but other less severely deteri-
orated areas of the wood may be tested by two traditional
methods using a small ice pick.
An ice pick or an awl may be used to test wood for
soundness. The technique is simply to jab the pick into a
wetted wood surface at an angle and pry up a small sec-
Figure 3. Deterioration of poorly maintained windows usually begins on
horizontal surfaces and at joints where water can collect and saturate the
wood. The problem areas are clearly indicated by paint failure due to
moisture. Photo: Baird M. Smith, AlA
tion of the wood . Sound wood will separate in long
fibrous splinters , but decayed wood will lift up in short ir-
regular pieces due to the breakdown of fiber strength.
Another method of testing for soundness consists of
pushing a sharp object into the wood, perpendicular to
the surface. If deterioration has begun from the hidden
side of a member and the core is badly decayed, the visi-
ble surface may appear to be sound wood. Pressure on
the probe can force it through an apparently sound skin
to penetrate deeply into decayed wood. This technique is
especially useful for checking sills where visual access to
the underside is restricted.
Following the inspection and analysis of the results, the
sco pe of the necessary repairs will be evident and a plan
for the rehabilitation can be formulated. Generally the ac-
tions necessary to return a window to "like new" condi-
tion will fall into three broad categories: 1) routine main-
tenance procedures, 2) structural stabilization, and 3)
parts replacement. These categories will be discussed in
the following sections and will be referred to respectively
as Repair Class I, Repair Class II, and Repair Class III.
Each successive repair class represents an increasing level
of difficulty, expense , and work time. Note that most of
the points mentioned in Repair Class I are routine main-
tenance items and should be provided in a regular main-
tenance program for any building . The neglect of the se
routine items can contribute to many common window
problems.
Before undertaking any of the repairs mentioned in the
followi ng sections all sources of moisture penetration
should be identified and eliminated, and all existing decay
fungi destroyed in order to arrest the deterioration pro-
cess. Many commercially available fungicides and wood
preservatives are toxic, so it is extremely important to
follow the manufacturer's recommendations for applica-
tion, and store all chemical materials away from children
and animals. After fungicidal and preservative treatment
the windows may be stabilized , retained, and restored
with every expectation for a long service life.
Repair Class I: Routine Maintenance
Repairs to wooden windows are usually labor intensive
and relatively uncomplicated . On small scale projects this
allows the do-it-yourselfer to save money by repairing
all or part of the windows. On larger projects it presents
the opportunity for time and money which might other-
wise be spent on the removal and replacement of existing
windows, to be spent on repairs, subsequently saving all
or part of the material cost of new window units. Regard-
less of the actual costs , or who performs the work, the
evaluation process described earlier will provide the
knowledge from which to specify an appropriate work
program, establish the work element priorities, and iden-
tify the level of skill needed by the labor force.
The routine maintenance required to upgrade a window
to "like new" condition normally includes the following
steps: 1) some degree of interior and exterior paint
removal, 2) removal and repair of sash (inCluding reglaz-
ing where necessary), 3) repairs to the frame , 4) weather-
stripping and reinstallation of the sash, and 5) repainting.
These operations are illustrated for a typical double-hung
wooden window (see figures 4a-f) , but they may be
adapted to other window types and styles as applicable.
Historic windows have usually acquired many layers of
paint over time. Removal of excess layers or peeling and
flaking paint will facilitate operation of the window and
restore the clarity of the original detailing. Some degree of
paint removal is also necessary as a first step in the prop-
er surface preparation for subsequent refinishing (if paint
color analysis is desired , it should be conducted prior to
the onset of the paint removal). There are several safe and
effective techniques for removing paint from wood,
depending on the amount of paint to be removed . Several
techniques such as scraping, chemical stripping, and the
use of a hot air gun are discussed in "Preservation Briefs:
10 Paint Removal from Historic Woodwork" (see Addi-
tional Reading section at end).
Paint removal should begin on the interior frames , be-
ing careful to remove the paint from the interior stop and
the parting bead, particularly along the seam where these
stops meet the jamb . This can be accomplished by run-
ning a utility knife along the length of the seam , breaking
the paint bond . It will then be much easier to remove the
stop, the parting bead and the sash. The interior stop may
be initially loosened from the sash side to avoid visible
scarring of the wood and then gradually pried loose using
a pair of putty knives, working up and down the stop in
small increments (see figure 4b). With the stop removed,
the lower or interior sash may be withdrawn . The sash
cords should be detached from the sides of the sash and
their ends may be pinned with a nail or tied in a knot to
prevent them from falling into the weight pocket.
Removal of the upper sash on double-hung units is
similar but the parting bead which holds it in place is set
into a groove in the center of the stile and is thinner and
more delicate than the interior stop. After removing any
paint along the seam , the parting bead should be carefully
pried out and worked free in the same manner as the in-
terior stop. The upper sash can be removed in the same
manner as the lower one and both sash taken to a conve-
nient work area (in order to remove the sash the interior
stop and parting bead need only be removed from one
side of the window). Window openings can be covered
with polyethylene sheets or plywood sheathing while the
sash are out for repair.
The sash can be stripped of paint using appropriate
techniques, but if any heat treatment is used (see figure
4c), the glass should be removed or protected from the
sudden temperature change which can cause breakage . An
3
Figure 4a. The following series of photographs of
the repair of a historic double-hung window use a
unit which is structurally sound but has many
layers of paint, some cracked and missing putty,
slight separation at the joints, broken sash cords,
and one cracked pane. Photo: John H . Myers
Figure 4d. Reglazing or replacement of the putty
requires that the existing putty be removed
manually, the glazing points be extracted, the
glass removed, and the back putty scraped out. To
reglaze, a bed of putty is laid around the perimeter
of the rabbet, the pane is pressed into place,
glazing points are inserted to hold the pane
(shown), and a final seal of putty is beveled
around the edge of the glass. Photo: John H.
Myers
4
Figure 4b. After removing paint from the seam
between the interior stop and the jamb, the stop
can be pried out and gradually worked loose using
a pair of putty knives as shown. To avoid visible
scarring of the wood, the sash can be raised and
the stop pried loose initially from the outer side.
Photo: John H. Myers
Figure 4e. A common repair is the replacement of
broken sash cords with new cords (shown) or with
chains. The weight pocket is often accessible
through a removable plate in the jamb, or by
removing the interior trim . Photo: John H. Myers
Figure 4c. Sash can be removed and repaired in a
convenient work area. Paint is being removed from
this sash with a hot air gun while an asbestos
sheet protects the glass from sudden temperature
change. Photo: John H. Myers
( ( 1
Figure 4£. Following the relatively simple repairs,
the window is weathertight, like new in
appearance, and serviceable for many years to
come. Both the historic material and the detailing
and craftsmanship of this original window have
been preserved. Photo: John H. Myers
overlay of aluminum foil on gypsum board or asbestos
can protect the glass from such rapid temperature
change. It is important to protect the glass because it
may be historic and often adds character to the window.
Deteriorated putty should be removed manually, taking
care not to damage the wood along the rabbet . If the
glass is to be removed, the glazing points which hold the
glass in place can be extracted and the panes numbered
and removed for cleaning and reuse in the same open-
ings. With the glass panes out, the remaining putty can be
removed and the sash can be sanded, patched, and
primed with a preservative primer. Hardened putty in
the rabbets may be softened by heating with a soldering
iron at the point of removal. Putty remaining on the
glass may be softened by soaking the panes in linseed
oil, and then removed with less risk of breaking the
glass. Before reinstalling the glass, a bead of glazing
compound or linseed oil putty should be laid around the
rabbet to cushion and seal the glass . Glazing compound
should only be used on wood which has been brushed
with linseed oil and primed with an oil based primer or
paint. The pane is then pressed into place and the glaz-
ing points are pushed into the wood around the perim-
eter of the pane (see figure 4d). The final glazing com-
pound or putty is applied and beveled to complete the
seal. The sash can be refinished as desired on the inside
and painted on the outside as soon as a "skin " has formed
on the putty, usually in 2 or 3 days . Exterior paint should
cover the beveled glazing compound or putty and lap
over onto the glass slightly to complete a weathertight
seal. After the proper curing times have elapsed for paint
and putty, the sash will be ready for reinstallation.
While the sash are out of the frame, the condition o f
the wood in the jamb and sill can be evaluated. Repair
and refinishing of the frame may proceed concurrently
with repairs to the sash, taking advantage of the curing
times f or the paints and putty used on the sash. One of
the most common work items is the replacement of the
sash cords with new rope cords or with chains (see figure
4e). The weight pocket is frequently accessible through a
door on the face of the frame near the sill , but if no door
exists , the trim on the interior face may be removed fo r
access . Sash weights may be increased for easier window
operation by elderly or handicapped persons . Additional
repairs to the frame and sash may include consolidation
or replacement of deteriorated wood. Techniques for th ese
repairs are discussed in the following sections .
The operations just discussed summarize the efforts
necessary to restore a window with minor deterioration to
"like new " condition (see figure 4f). The techniques can be
applied by an unskilled person with minimal training a nd
experience. To demonstrate the practicality of this ap-
proach, and photograph it , a Technical Preservation Ser-
vices staff member repaired a wooden double-hung, two
over two window which had been in service over ninety
years. The wood was structurally sound but the window
had one broken pane, many layers of paint , broken sash
cords and inadequate, worn-out weatherstripping. The
staff member found that the frame could be stripped o f
paint and the sash removed quite ea sily . Paint , putty and
glass removal required about one hour for each sash, and
the reglazing of both sash was accomplished in about one
hour. Weatherstripping of the sash and frame , replace-
ment of the sash cords and reinstallation of the sash, part-
ing bead, and stop required an hour and a half. These
times refer only to individual operations; the entire proc-
ess took several days due to the drying and curing times
for putty, primer, and paint, however, work on other win-
dow units could have been in progress during these lag
times .
Repair Class II: Stabilization
The preceding description of a window repair job focused
on a unit which was operationally sound. Many windows
will show some additional degree of physical deteriora-
tion, especially in the vulnerable areas mentioned earlier,
but even badly damaged windows can be repaired using
simple processes. Partially decayed wood can be water-
proofed, patched, built-up, or consolidated and then
painted to achieve a sound condition, good appearance,
and greatly extended life . Three techniques for repairing
partially decayed or weathered wood are discussed in this
section, and all three can be accomplished using products
available at most hardware stores.
One established technique for repairing wood which is
split, checked or shows signs of rot, is to: 1) dry the
wood, 2) treat decayed areas with a fungicide, 3) water-
proof with two or three applications of boiled linseed oil
(applications every 24 hours), 4) fill cracks and holes with
putty, and 5) after a "skin" forms on the putty, paint the
surface. Care should be taken with the use of fungicide
which is toxic. Follow the manufacturers' directions and
use only on areas which will be painted. When using any
technique of building up or patching a flat surface, the
finished surface should be sloped slightly to carry water
away from the window and not allow it to puddle . Caulk-
ing of the joints between the sill and the jamb will help
reduce further water penetration.
When sills or other members exhibit surface weathering
they may also be built-up using wood putties or home-
made mixtures such as sawdust and resorcinol glue, or
whiting and varnish. These mixtures can be built up in
successive layers, then sanded, primed, and painted. The
same caution about proper slope for flat surfaces applies
to this technique.
Wood may also be strengthened and stabilized by con-
solidation, using semi-rigid epoxies which saturate the
porous decayed wood and then harden. The surface of the
consolidated wood can then be filled with a semi-rigid
epoxy patching compound, sanded and painted (see figure
5). Epoxy patching compounds can be used to build up
Figure 5 . This illustrates a two-part epoxy patching compound used to fill
the surface of a weathered sill and rebuild the missing edge. When the epoxy
cures , it can be sanded smooth and painted to achieve a durable and
waterproof repair. Photo: John H . Myers
5
missing sections or decayed ends of members. Profiles can
be duplicated using hand molds, which are created by
pressing a ball of patching compound over a sound sec-
tion of the profile which has been rubbed with butcher's
wax . This can be a very efficient technique where there
are many typical repairs to be done. Technical Preserva-
tion Services has published Epoxies for Wood Repairs
in Historic Buildings (see Additional Reading section at
end), which discusses the theory and techniques of epoxy
repairs. The process has been widely used and proven in
marine applications; and proprietary products are avail-
able at hardware and marine supply stores. Although
epoxy materials may be comparatively expensive, they
hold the promise of being among the most durable and
long lasting materials available for wood repair.
Any of the three techniques discussed can stabilize and
restore the appearance of the window unit. There are
times, however, when the degree of deterioration is so ad-
vanced that stabilization is impractical, and the only way
to retain some of the original fabric is to replace damaged
parts.
Repair Class III: Splices and Parts Replacement
When parts of the frame or sash are so badly deteriorated
that they cannot be stabilized there are methods which
permit the retention of some of the existing or original
fabric. These methods involve replacing the deteriorated
parts with new matching pieces, or splicing new wood in-
to existing members. The techniques require more skill
and are more expensive than any of the previously dis-
cussed alternatives. It is necessary to remove the sash
and / or the affected parts of the frame and have a
carpenter or woodworking mill reproduce the damaged or
missing parts. Most millwork firms can duplicate parts,
such as muntins, bottom rails, or sills , which can then be
incorporated into the existing window, but it may be
necessary to shop around because there are several factor s
controlling the practicality of this approach. Some wood-
working mills do not like to repair old sash because nails
or other foreign objects in the sash can damage expensive
knives (which cost far more than their profits on small
repair jobs); others do not have cutting knives to
duplicate muntin profiles. Some firms prefer to concen-
trate on larger jobs with more profit potential, and some
may not have a craftsman who can duplicate the parts. A
little searching should locate a firm which will do
the job, and at a reasonable price. If such a firm does not
exist locally , there are firms which undertake this kind of
repair and ship nationwide. It is possible , however, for
the advanced do-it-yourselfer or craftsman with a table
saw to duplicate moulding profiles using techniques
discussed by Gordie Whittington in "Simplified Methods
for Reproducing Wood Mouldings," Bulletin of the
Association for Preservation Technology, Vol. III, No .4,
1971, or illustrated more recently in The Old House ,
Time-Life Books, Alexandria, Virginia, 1979 .
The repairs discussed in this section involve window
frames which may be in very deteriorated condition ,
possibly requiring removal; therefore, caution is in
order. The actual construction of wooden window frames
and sash is not complicated. Pegged mortise and tenon
units can be disassembled easily , if the units are out of the
building. The installation or connection of some frames to
the surrounding structure, especially masonry walls, can
complicate the work immeasurably, and may even require
6
dismantling of the wall. It may be useful , therefore , to
take the following approach to frame repair: 1) conduct
regular maintenance of sound frames to achieve the
longest life possible , 2) make necessary repairs in place
wherever possible, using stabilization and splicing tech-
niques , and 3) if removal is necessary, thoroughly in-
vestigate the structural detailing and seek appropriate pro-
fessional consultation.
Another alternative may be considered if parts replace-
ment is required, and that is sash replacement. If extensive
r eplacement of parts is necessary and the job becomes
prohibitively expensive it may be more practical to pur-
chase new sash which can be installed into the existing
frames . Such sash are available as exact custom reproduc-
tions, reasonable facsimiles (custom windows with similar
profiles ), and contemporary wooden sash which are
similar in appearance . There are companies which still
manufacture high quality wooden sash which would
duplicate most historic sash. A few calls to local build-
ing suppliers may provide a source of appropriate replace-
ment sash, but if not, check with local historical
associations, the state historic preservation office ,
or preservation related magazines and supply catalogs for
information.
If a rehabilitation project has a large number of win-
dows such as a commercial building or an industrial com-
plex , there may be less of a problem arriving at a solu-
tion . Once the evaluation of the windows is completed
and the scope of the work is known, there may be a
potential economy of scale. Woodworking mills may be
interested in the work from a large project; new sash in
v olume may be considerably less expensive per unit ;
crews can be assembled and trained on site to perform all
of the window repairs ; and a few extensive repairs can be
absorbed (without undue burden) into the total budget
for a large number of sound windows. While it may be
expensive for the average historic home owner to pay
seventy dollars or more for a mill to grind a custom knife
to duplicate four or five bad muntins, that cost becomes
negligible on large commercial projects which may have
several hundred windows.
Most windows should not require the extensive repairs
discussed in this section . The ones which do are usually in
buildings which have been abandoned for long periods or
have totally lacked maintenance for years. It is necessary
to thoroughly investigate the alternatives for windows
which do require extensive repairs to arrive at a solution
which retains historic significance and is also economically
feasible . Even for projects requiring repairs identified in
this section, if the percentage of parts replacement per
window is low, or the number of windows requiring
repair is small, repair can still be a cost effective solution.
Weatherization
A window which is repaired should be made as energy ef-
ficient as possible by the use of appropriate weather-
stripping to reduce air infiltration. A wide variety of
products are available to assist in this task . Felt may be
fastened to the top, bottom, and meeting rails, but may
have the disadvantage of absorbing and holding moisture,
particularly at the bottom rail. Rolled vinyl strips may
also be tacked into place in appropriate locations to
reduce infiltration. Metal strips or new plastic spring
strips may be used on the rails and, if space permits, in
the channels between the sash and jamb. Weatherstripping
is a historic treatment, but old weatherstripping (felt) is
not likely to perform very satisfactorily. Appropriate con-
temporary weatherstripping should be considered an in-
tegral part of the repair process for windows. The use of
sash locks installed on the meeting rail will insure that the
sash are kept tightly closed so that the weatherstripping
will function more effectively to reduce infiltration.
Although such locks will not always be historically accu-
rate, they will usually be viewed as an acceptable contem-
porary modification in the interest of improved thermal
performance.
Many styles of storm windows are available to improve
the thermal performance of existing windows. The use of
exterior storm windows should be investigated whenever
feasible because they are thermally efficient, cost-effective,
reversible, and allow the retention of original windows
(see "Preservation Briefs: 3"). Storm window frames may
be made of wood, aluminum, vinyl, or plastic; however,
the use of unfinished aluminum storms should be
avoided. The visual impact of storms may be minimized
by selecting colors which match existing trim color.
Arched top storms are available for windows with special
shapes. Although interior storm windows appear to offer
an attractive option for achieving double glazing with
minimal visual impact, the potential for damaging con-
densation problems must be addressed. Moisture which
becomes trapped between the layers of glazing can con-
dense on the colder, outer prime window, potentially
leading to deterioration. The correct approach to using in-
terior storms is to create a seal on the interior storm while
allowing some ventilation around the prime window. In
actual practice, the creation of such a durable, airtight
seal is difficult.
Window Replacement
Although the retention of original or existing windows is
always desirable and this Brief is intended to encourage
that goal, there is a point when the condition of a win-
dow may clearly indicate replacement. The decision proc-
ess for selecting replacement windows should not begin
with a survey of contemporary window products which
are available as replacements, but should begin with a
look at the windows which are being replaced. Attempt to
understand the contribution of the window(s) to the ap-
pearance of the facade including: 1) the pattern of the
openings and their size; 2) proportions of the frame and
sash; 3) configuration of window panes; 4) muntin pro-
files; 5) type of wood; 6) paint color; 7) characteristics of
the glass; and 8) associated details such as arched tops,
hoods, or other decorative elements. Develop an under-
standing of how the window reflects the period, style, or
regional characteristics of the building, or represents tech-
nological development.
Armed with an awareness of the significance of the ex-
isting window, begin to search for a replacement which
retains as much of the character of the historic window as
possible. There are many sources of suitable new win-
dows. Continue looking until an acceptable replacement
can be found. Check building supply firms, local wood-
working mills, carpenters, preservation oriented maga-
zines, or catalogs or sUl'pliers of old building materials,
for product information. Local historical associations and
state historic preservation offices may be good sources of
information on products which have been used success-
fully in preservation projects.
Consider energy efficiency as one of the factors for
replacements, but do not let it dominate the issue. Energy
conservation is no excuse for the wholesale destruction of
historic windows which can be made thermally efficient
by historically and aesthetically acceptable means. In fact ,
a historic wooden window with a high quality storm win-
dow added should thermally outperform a new double-
glazed metal window which does not have thermal
breaks (insulation between the inner and outer frames in-
tended to break the path of heat flow). This occurs
because the wood has far better insulating value than the
metal, and in addition many historic windows have high
ratios of wood to glass, thus reducing the area of highest
heat transfer. One measure of heat transfer is the U-value,
the number of Btu's per hour transferred through a square
foot of material. When comparing thermal performance ,
the lower the U-value the better the performance. Accord-
ing to ASHRAf 1977 Fundamentals, the U-values for
single glazed wooden windows range from 0.88 to 0.99.
The addition of a storm window should reduce these
figures to a range of 0.44 to 0.49. A non-thermal break,
double-glazed metal window has a U-value of about 0.6.
Conclusion
Technical Preservation Services recommends the retention
and repair of original windows whenever possible . We
believe that the repair and weatherization of existing
wooden windows is more practical than most people
realize, and that many windows are unfortunately re-
placed because of a lack of awareness of techniques for
evaluation, repair, and weatherization. Wooden windows
which are repaired and properly maintained will have
greatly extended service lives while contributing to the
historic character of the building. Thus, an important ele-
ment of a building's significance will have been preserved
for the future.
Additional Reading
ASHRAE Handbook-1977 Fundamentals. New York: American Society
of Heating, Refrigerating and Air-conditioning Engineers , 1978
(chapter 26).
Ferro, Maximillian. Preservation: Present Pathway to Fall River's Future.
Fall River, Massachusetts: City of Fall River , 1979 (chapter 7).
"Fixing Double-Hung Windows." Old House Journal (no. 12, 1979): 135 .
Look, David W. "Preservation Briefs: 10 Paint Removal from Historic
Woodwork." Washington, DC: Technical Preservation Services,
U.S. Department of the Interior, forthcoming.
Morrison, Hugh. Early American Architecture. New York: Oxford
University Press, 1952.
Phillips, Morgan, and Selwyn, Judith, Epoxies for Wood Repairs in
Historic Buildings. Washington, DC : Technical Preservation Ser-
vices, U.S. Department of the Interior (Government Printing Office,
Stock No. 024-016-00095-1), 1978.
Rehab Right. Oakland, California: City of Oakland Planning Depart-
ment, 1978 (pp. 78-83).
"Sealing Leaky Windows." Old House Journal (no. 1, 1973): 5.
Smith, Baird M, "Preservation Briefs: 3 Conserving Energy in Historic
Buildings," Washington, DC: Technical Preservation Services, U.S.
Department of the Interior, 1978.
1981
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