Skip to main content

Historic Preservation - Technical Procedures

Spectitle:

Preservation Briefs: 9 The Repair Of Historic Wooden Windows

Procedure code:

0861001S

Source:

Preservation Briefs 9, National Park Service, Pad

Division:

Doors And Windows

Section:

Wood Windows

Last Modified:

12/24/2013

Details:

Preservation Briefs: 9 The Repair Of Historic Wooden Windows



PRESERVATION BRIEFS:  9

THE REPAIR OF HISTORIC WOODEN WINDOWS
 

Link below connects to the National Park Service website and the latest version of Preservation Brief 9 as of 12/23/2013.  If NPS has changed the link address, do a general search for the Brief by its number and title.

 

http://www.nps.gov/tps/how-to-preserve/briefs/9-wooden-windows.htm

John H. Myers

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 Documents
              P.O. Box 371954
              Pittsburgh, PA  15250-7954

              GPO #024-005-01026-2
              Available ONLY in package 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.  


***INTRODUCTION***

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.  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.  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.  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
determination of significance should be made within the context of
the whole building, wherein the windows are one architectural
element.

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
proceed 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 ensure that it slopes downward away from the
building and allows 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 ensure proper water run-off,
particularly 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.  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 section 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***

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.
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 09910-01-S).  

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.
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, the glass should be removed or
protected from the sudden temperature change which can cause
breakage.  An 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.  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.  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.
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.  Epoxy patching compounds can be used to build up missing
sections of 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 by Morgan Phillips and Judith Selwyn (1978), 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 duplicated 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 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 an 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 ensure
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 01100-04-S "Preservation Briefs: 3
Conserving Energy in Historic Buildings").  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 suppliers 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, an 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



 


wood windows, window repair, wood window repair, Preservation Brief 9