U.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 ornamental 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 replacement can be a complex process involving both objective 

and subjective considerations. The Secretary of the Interior's Standards for Rehabilitation, and the accompanying 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, especiallyon simple facades such as this mill building. Replacement of the multipane windows here with larger panes could dramatically change the appearance of the building. The areas of missing windows convey the impression 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 information will be useful, however, for the architect, contractor, 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 treatments, and a general understanding of the function and 

history of windows is vital to making a proper evaluation. 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 interior, 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 eighteenth century single- and double-hung windows were introduced. 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. Sitespecific, 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 determination of significance should be made within the context 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 exceptional craftsmanship or design. Once this evaluation 

of significance has been completed, it is possible to proFor sale by the Superintendent 01 Documents, U.S. Government Printing Office, Washington, D.C. 20402 
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Muntin Profiles 

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of many possible profiles. Muntins can contribute substantially 

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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 window 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) hardware, and 7) the overall condition of the window (excellent, 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 contributing 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, especially 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 invisible treatment will insure proper water run-off, particu2 

larly if the bottom of the sill is flat. Any conditions, including 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 window. 

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 excessive 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 unsightly 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 between 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 readily absorbed into the end-grain of the wood. If severe 

deterioration exists in these areas, it will usually be apparent on visual inspection, but other less severely deteriorated 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 irregular 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 visible 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 

scope of the necessary repairs will be evident and a plan 

for the rehabilitation can be formulated. Generally the actions necessary to return a window to "like new" condition will fall into three broad categories: 1) routine maintenance 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 maintenance items and should be provided in a regular maintenance program for any building. The neglect of these 

routine items can contribute to many common window 

problems. 

Before undertaking any of the repairs mentioned in the 

following sections all sources of moisture penetration 

should be identified and eliminated, and all existing decay 

fungi destroyed in order to arrest the deterioration process. Many commercially available fungicides and wood 

preservatives are toxic, so it is extremely important to 

follow the manufacturer's recommendations for application, 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 otherwise 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. Regardless 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 identify 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 reglazing where necessary), 3) repairs to the frame , 4) weatherstripping 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 proper 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 Additional Reading section at end). 

Paint removal should begin on the interior frames, being 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 running 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 interior stop. The upper sash can be removed in the same 

manner as the lower one and both sash taken to a convenient 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 openings. 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 glazing points are pushed into the wood around the perimeter of the pane (see figure 4d). The final glazing compound 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 of 

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 for 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 for 

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 these 

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 and 

experience. To demonstrate the practicality of this approach, and photograph it, a Technical Preservation Services 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 of 

paint and the sash removed quite easily. 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 , replacement of the sash cords and reinstallation of the sash, parting bead, and stop required an hour and a half. These 

times refer only to individual operations; the entire process took several days due to the drying and curing times 

for putty, primer, and paint, however, work on other window 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 deterioration, especially in the vulnerable areas mentioned earlier, 

but even badly damaged windows can be repaired using 

simple processes. Partially decayed wood can be waterproofed, 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) waterproof 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. Caulking 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 homemade 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 consolidation, 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 section 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 Preservation 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 available 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 advanced 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 into existing members. The techniques require more skill 

and are more expensive than any of the previously discussed 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 factors 

controlling the practicality of this approach. Some woodworking 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 concentrate 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 techniques, and 3) if removal is necessary, thoroughly investigate the structural detailing and seek appropriate professional consultation. 

Another alternative may be considered if parts replacement is required, and that is sash replacement. If extensive 

replacement of parts is necessary and the job becomes 

prohibitively expensive it may be more practical to purchase new sash which can be installed into the existing 

frames. Such sash are available as exact custom reproductions, 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 building suppliers may provide a source of appropriate replacement 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 windows such as a commercial building or an industrial complex, there may be less of a problem arriving at a solution . 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 

volume 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 efficient as possible by the use of appropriate weatherstripping 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 contemporary weatherstripping should be considered an integral 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 accurate, they will usually be viewed as an acceptable contemporary 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 condensation problems must be addressed. Moisture which 

becomes trapped between the layers of glazing can condense on the colder, outer prime window, potentially 

leading to deterioration. The correct approach to using interior 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 window may clearly indicate replacement. The decision process 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 appearance 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 profiles; 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 understanding of how the window reflects the period, style, or 

regional characteristics of the building, or represents technological development. 

Armed with an awareness of the significance of the existing 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 windows. Continue looking until an acceptable replacement 

can be found. Check building supply firms, local woodworking mills, carpenters, preservation oriented magazines, 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 successfully 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 window added should thermally outperform a new doubleglazed metal window which does not have thermal 

breaks (insulation between the inner and outer frames intended 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. According 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 replaced 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 element 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 Services, U.S. Department of the Interior (Government Printing Office, 

Stock No. 024-016-00095-1), 1978. 

Rehab Right. Oakland, California: City of Oakland Planning Department, 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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