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Historic Preservation - Technical Procedures
Rehabilitating Windows In Historic Buildings: An Overview
The Window Handbook - Nps/Pad And Cpb
Doors And Windows
Rehabilitating Windows In Historic Buildings: An Overview
REHABILITATING WINDOWS IN HISTORIC BUILDINGS: AN OVERVIEW
Charles E. Fisher
Preservation Assistance Division
National Park Service
This standard includes the bulk of information contained in the
original Preservation Tech Notes developed by the National Park
Service and the Center for Architectural Conservation at Georgia
Tech. The Preservation Tech Notes are case studies of exemplary
projects designed to provide specific examples of sound
preservation techniques. To obtain a complete copy of The Window
publications, including figures and illustrations, please contact:
Historic Preservation Education Foundation
P.O. Box 77160
Washington, DC 20013-7160
The Window Handbook, jointly prepared by the National Park Service,
Preservation Assistance Division and the Center for Architectural
Conservation at Georgia Tech, also contains all of the Tech Notes
on Windows and is available for purchase from the Historic
Preservation Education Foundation for $32.00. The Window Workbook
is available for $49.00. The two publications together can be
purchased for $72.00.
Today, greater attention is being placed on selecting appropriate
window rehabilitation treatments for historic buildings. As a
result, more contractors employ workers skilled in the repair of
wooden windows; a number of leading manufacturers of commercial
windows have modified existing window lines or introduced new ones
specifically for the historic market; and the process for selecting
appropriate windows has grown more complex as treatment options
have expanded and owners and historic preservation review boards
insist on more sensitive solutions.
With these changes, architects, building managers, and developers
are confronted with the need for sensitive rehabilitation of
windows in historic buildings; a subject about which little has
been written and few are sufficiently experienced. This is
particularly troublesome considering the high financial stakes
often involved. On a medium to large size building rehabilitation,
the window component typically accounts for ten percent of the
total project costs.
Windows are frequently replaced without careful and objective
examination of repair and upgrading alternatives, often leading to
unnecessary costs. Then, too, replacement windows are often
selected based on initial costs, without sufficient attention to
quality of construction, performance, and appearance.
Taking the time for proper architectural planning early in the
project is crucial. This is essential in order to explore
thoroughly the various repair and replacement options, test
possible solutions, and allow the time necessary for any custom
***ASSESSING HISTORIC WINDOWS***
The first step in the planning process is to assess the historic
significance and architectural qualities of the historic windows in
the building, since this assessment can limit the number of
suitable rehabilitation options. The most striking feature of a
1920's factory may very well be the large steel industrial windows
with their multiple lites, projecting operating units, and narrow
channel bars and mullions. And while the more common double-hung
window with multiple dividing lites may be only one of a number of
character-defining features of a late nineteenth-century hotel, it
may represent the most distinguishing architectural feature on a
New England textile mill. In each case, the relative importance of
the windows to the building must be understood and taken into
account. In assessing the significance of windows to a building,
there are at least four major areas that need to be examined.
The fact that windows are an integral part of the historic fabric
of a building must be acknowledged, particularly since windows may
comprise twenty to thirty percent of the surface area of an older
building. Historic windows should thus be preserved whenever
possible. To be considered historic, the windows need not be
original to a building; although each building needs to be assessed
individually, a fifty-year rule of thumb can often be applied in
establishing whether existing windows are historic and worthy of
DEGREE OF VISIBILITY:
The second factor is visibility. Where all elevations of the
building are highly visible and equally articulated, appropriate
window treatments are more limited than with a building that has
only one principal facade, side party walls, and a secondary rear
elevation not readily seen from the public right of way.
Similarly, there are numerous late nineteenth and twentieth-century
highrises with monumental windows of impressive design on the lower
two or three stories, simply detailed windows on the intermediate
floors, and more elaborate ones on the top one or two stories. The
degree of visibility in this case might lead to the use of several
window treatments on the same building.
A third and commonly overlooked factor in the assessment is that
windows are viewed from the inside as well as from the outside.
Windows frequently contribute to an interior design scheme, or are
located in significant interior spaces where appearance is an
HISTORIC WINDOW DESIGN:
The fourth factor in assessing windows directly concerns design and
detail. The design and operation of historic buildings may reflect
the technology of the time, as is the case with "Chicago style"
windows. Muntin pattern and muntin widths, mullion profiles,
decorative elements such as arched tops and brick molding,
construction detail such as O.G. lugs, the setback or reveal of the
window relative to the wall plane, the color of the sash and frame,
and even the reflective qualities of the glass all play important
roles in window design and appearance.
***THE CONDITION SURVEY***
The next planning step in almost every window study is to evaluate
the condition of the windows by undertaking a detailed survey.
Peeling paint, broken glass, loosely fitted windows, and apparent
sill rot are not necessarily solid evidence that windows are beyond
repair. For a majority of older buildings, failure to examine the
existing window conditions closely precludes an objective
evaluation of repairing and upgrading the existing windows versus
partial or total replacing.
The surveys are often conducted by a window consultant, although
some architectural firms have benefitted by having a staff member
develop the skills to perform the survey. Without an objective and
detailed condition survey (along with the window assessment) it is
difficult to weight accurately the rehabilitation alternatives to
ensure that the most appropriate window treatment is chosen.
Too often the contractor is expected to give repair and replacement
costs with little guidance from the architect. There are inherent
problems in such an approach. Most contractors will not examine
the windows carefully because it is a time-consuming process, and
instead will bid close to replacement cost for repair work.
Replacing windows admittedly is easier to plan and budget for, but
in the final analysis it is not necessarily most cost-effective or
If the window condition survey and the window significance
assessment clearly support retention of the historic windows, then
the next planning step is to examine the numerous options for
repairing and upgrading the existing units. Often the assessment
will prove inconclusive, thus requiring examination of other
alternatives, including replacing many or all of the windows with
others offering matching features and enhanced performance.
Based on the window assessment, there may be cases where
replacement windows could appropriately match only the overall
appearance of the historic window rather than the exact design.
This makes possible the use of modern commercial windows that have
been adapted to the historic rehabilitation market, yet that
consist of different materials. Once the options are identified
and preliminary cost estimates are derived, other factors may
influence final decisions about window work.
An energy study of the overall building -- not just the windows --
coupled with cost-payback analysis may lead to the conclusion that,
for a particular building, double glazing (retrofitted either to
existing windows or in replacement units) is simply not cost-
effective. Such findings are not necessarily confined to
geographic areas with mild climates. Taking steps to reduce air
infiltration, however, is usually always cost-effective where
older, poorly maintained windows are involved.
***REPAIRING AND UPGRADING EXISTING WINDOWS***
Generally, wood found in nineteenth and many early twentieth-
century windows is a dense or heart wood (often pine) and of higher
quality than most woods used today. A 100-year-old window, if
properly reconditioned and maintained, can reasonably be expected
to serve another century.
In many major cities, there is usually at least one firm
specializing in window maintenance work. With experienced teams,
such firms can be quite efficient at reconditioning hardware,
replacing sash ropes and broken pulleys, replacing or adding
weatherstripping, tightening loose sash joints, and replacing worn
or broken sash stops. They can undertake deferred maintenance work
at a reasonable cost, providing the building owner with a good
payback by reducing air infiltration and prolonging the life of the
In vacant or poorly maintained buildings, however, windows usually
require more extensive repairs. On wooden windows, extensive
deterioration is most prevalent at the sills, the lower ends of the
frames, and the bottom sash rails. For sills with surface
cracking, some of the newer paints on the market hold considerable
promise because of their durability; these are usually preferable
to metal panning, which can hide ongoing deterioration and tends to
promote decay over the long term, since tight permanently sealed
joints are difficult to achieve.
Epoxy consolidants and fillers may also be used where more
extensive sill deterioration occurs. This is a cost-effective
alternative to total sill replacement. Epoxy can be used to
recondition the bottom of sash frames at the sill junction,
although splicing-in new treated wood is another acceptable option.
Bottom sash rails sometimes require total replacement; this work
can be done easily and is less drastic than total sash replacement.
Establishing a complete workshop at the site to make repairs has
been a successful approach on a number of projects. Some millworks
will locate a field unit at a job site. Such work is labor
intensive, but material and transportation costs are low and the
onsite shops can undertake other project work, adjusting to work
schedules more easily.
Decisions must also be made about the amount of surface preparation
to undertake. Removing paint down to a sound surface; application
of water-repellant coatings on bare wood and at joints, and sanding
where ultra-violet degradation of exposed wood has occurred are
important steps that may be necessary to achieve a good substrate
for repainting and increase the length of the painting cycle.
Reducing air infiltration in existing windows is another principal
concern in upgrading existing windows. Air infiltration, rather
than single glazing, is the principal reason why older windows tend
to be poor energy performers. Reducing air infiltration is usually
the most cost-effective way of improving the energy performance of
older windows, even in cold weather climates. This can easily be
achieved by caulking around the frames, making sure the glazing
putty is sound, tightening loose-fitting sash, replacing cracked
panes, and most important, installing good weatherstripping.
Rather than running tests on existing windows, it is far more
practical to take a typical window, make necessary repairs, upgrade
its performance by adding high-quality weatherstripping, and then
run standard air infiltration tests. In most cases, it is possible
to surpass the minimum industry standards established for new
windows; test standards for the contract work can then be
EXTERIOR STORM WINDOWS:
The traditional method of double glazing is the use of exterior
storm windows, which achieves a U-value for the window comparable
to insulting glass. The typical exterior storm window greatly
reduces air infiltration, lowers the maintenance cost of the
historic window, and extends its useful life. Unfortunately,
exterior storms can affect the visual appearance of the historic
windows, although less so where single-lite historic sash are
involved. While it was common at the turn of the century to match
the divided lite pattern in the primary sash with that of the
exterior storm, today's single-lite storm panels tend to alter the
reflective qualities and shadow lines cast by the primary window,
and also obscure features such as muntins.
Several steps can be taken to lessen the visual impact of storm
windows. The simplest is to have the storm factory window painted
to match the color of the primary window and trim. The second step
is to specify a half-screen to be mounted on the inside, since it
is the screening material in the typical storm/screen combination
that most dramatically affects the appearance of a historic window.
By mounting a half-screen on the inside (a typical feature earlier
in the century), the sight lines of the storm unit are simplified
by the reduction from a typical three-track to a two-track frame.
Also commercially available are custom single-track, two-panel
units with a simple subframe set within the jamb. Some single-
track systems are designed so that the panels can be removed from
the inside for cleaning and, for summer use, screen panels inserted
for ventilation. This single-track design, can significantly
reduce the storm unit's impact on the window's historic appearance.
In either case, the storms will also result in considerable sound
reduction, which is important to buildings exposed to high street
INTERIOR STORM WINDOWS:
Generally, an interior-mounted storm unit preserves the visual
qualities of historic windows better than an exterior one. There
are unobtrusive, high-performance, commercial quality interior
storms intended to be jamb-mounted rather than affixed to the
casing. Some of the interior storms are side or top-hinged,
although the more common styles are double or triple-track units.
Where fixed windows are appropriate, single or double panels
attached to a subframe can be used, thus saving on initial costs
while still allowing occasional removal for cleaning and
maintenance. Condensation may be more of a problem with interior
rather than exterior storm applications, particularly with
residential buildings in extremely cold weather climates; however,
the problems encountered in buildings with interior storm
applications can be minimized with most windows if
weatherstripping, caulking, and weep holes are part of the
upgrading process and there is good quality construction and
EXISTING STEEL WINDOWS:
Double-glazing historic steel windows can be an easy operation.
With small residential steel casement units common in early
twentieth-century high-rise apartments, application of a
horizontally-sliding aluminum storm unit mounted on the inside
reduces noise from the outside while improving energy performance.
When finished in a dark color, this type of interior storm tends to
have little visual impact from the outside and can also be
unobtrusive from the inside. There is some sacrifice in the
optimum performance of the casement for natural cooling and
ventilation, since as much as 50 percent of the venting capacity of
the window is blocked by the slider window. In an age of
mechanical cooling, however, this may not be a concern.
On larger steel windows such as those found in medium-sized
factories, the steel sections sometimes were designed to
accommodate either single or dual glazing. Where these windows are
in repairable condition and sized to accept either dual or single
glazing, it may be possible to install sealed insulating glass
units in place of the single glazing. An evaluation should be made
beforehand, with particular attention to the operable portion of
the window, to determine whether the steel windows can accept the
additional weight of the insulating glass. While this retrofit
approach to double glazing has been effectively used, most steel
windows installed in the past were not designed to accept dual
glazing; the glazing bars (muntins) are too shallow or too narrow
EXISTING WOOD SASH FRAMES:
With single-lite wooden sash, it may be possible to retrofit
insulating glass within the existing sash frame. The sash frame
needs to be in relatively good condition and of sufficient size to
handle the additional glass weight. Mechanical routing of the
glazing rabbet is usually required to accommodate the additional
thickness of the insulating unit. In addition, the sash weights
probably will need to be augmented if operable windows are desired.
It is always important to establish beforehand whether there is
sufficient room in the weight pockets to allow for the additional
For the same single-lite sash, another alternative is to
"piggyback" an aluminum-frame storm panel onto the interior portion
of the sash. This procedure requires that the inside edge of the
rails and stiles on the room side be rabbeted to allow insertion of
the storm panel.
As with retrofitting insulating glass, the approach to double
glazing has an advantage over separate storms in ease of operation
and lower maintenance. In piggy-backing the storm panel onto the
sash, care must be taken to get as good a seal as possible where
the metal frame abuts the wood sash frame.
A good weatherstripping system should be used on the back side of
the metal storm frame, and the metal frame should fit snugly.
These measures are necessary to reduce the likelihood of
condensation between the two pieces of glass. Weep holes should be
provided, preferable on the stiles rather than the rails.
Removable clips or set screws to secure the storm panel to the wood
sash are recommended; these permit occasional cleaning of the glass
and allow access for maintenance work. Local glass shops can
provide the necessary materials.
The use of retrofitted insulating glass or piggyback storm panels
is generally limited to single-lite sash, although it may be
possible on some two-lite sash to achieve similar results, provided
the muntin is wide enough and strong enough.
One additional use of insulating glass with existing wood sash is
worth mentioning. For buildings in downtown urban areas or along
busy highways, there is an increased desire to reduce noise from
the outside. While sound reduction can be achieved through good
weather-stripping, the addition of a storm window with insulating
glass in combination with the existing window can yield superior
sound attenuation over a triple-glazed replacement sash.
***IN-KIND REPLACEMENT WINDOWS***
The window condition study may establish that a number of sash
needs to be replaced altogether. Windows exposed to harsh weather
and those prone to vandalism are often prime candidates for
selective replacement; this is a common practice, yet one that
requires careful attention. To compensate for the poorer quality
wood used in windows today, water repellents should be used. For
warm, moist climates, wood preservatives should be used as well.
Since the end grain is particularly susceptible to decay, water
repellents should be applied after pieces are cut, yet prior to
assembly where dip and spray preservative methods are used.
Knives may be custom-cut to replicate the historic shapes of
muntins and mullions. Various options for integral
weatherstripping, jamb liners, and sash balances exist -- all
variables that may be influenced by the number of replacement
windows involved and repair and maintenance work needed for the
remaining historic sash.
Special care must be taken in measuring the window opening
(especially where only the sash are being replaced) due to
irregularities in the historic sash and the racking of the window
over the years.
When total sash replacement is necessary, careful consideration
should be given to replacing the window with units that match in
color, detail, material (i.e., wood or steel), and manner of
Today, there are numerous retrofit features to consider. For
single-lite sash, insulating glass can easily be used. In some
instances, such as in Italianate style buildings with thin sash
rails, the bottom rail may need to be increased in width or
reinforcing rods inserted to provide sufficient strength when using
insulating glass. Where O.G. lugs are present on the historic sash
stiles, the decorative nature of this feature -- intended to give
additional joint strength -- can be retained, if only as an applied
Where circle or segmented top sash exist, this feature should be
duplicated in detail and applied aprons not used. Replacement
windows should be custom made to fit each opening. The opening
should not be blocked in to accommodate standard size units; the
width of the frame and sash and the amount of glass exposure should
be the same as the historic windows.
For the typical double-hung window, the upper and lower sash in the
replacement units should be in the same plane as the historic sash,
regardless of whether the new window is single or double hung or
fixed in place. The distinctive meeting rail and the strong shadow
lines it casts are important features to retain. With single-hung
windows, the common renovation practice of installing exterior
half-screens in the same plane as the upper sash should not be
used, since the historic double-hung appearance will be greatly
For the two-lite sash, common during the latter half of the
nineteenth century, it is often possible to duplicate the units
using insulating glass and a true wood muntin. Where the width of
the historic muntin is narrower than 1-1/16" on an average size
window, some problems may be encountered in keeping the muntin
width narrow enough when installing insulating glass. A typical
5/8" wide historic muntin on certain size windows can be increased
to perhaps 1" on the new sash without altering the appearance of
the window. It may be necessary to use one of the new thinner
metal spacers in the sealed insulating glass unit. If there is any
concern about the shiny aluminum being visible at an angle from the
outside, the commercially available brown color spacers could be
Increasing muntin width too much causes a marked change in a
window's historic appearance. Where excessive widening of the
muntin is necessary for adequate performance of the window and the
insulating glass, alternate solutions should be considered.
Obviously, with particularly thin historic muntins, such as a 5/8"
wide Gothic muntin, the sash will need to be single glazed, and
possibly combined with an interior storm unit.
For some windows, there is the possibility of using insulating
glass in four and six-lite wood replacement sash. Again, the
historic muntin width becomes a limiting factor. A mock-up may be
necessary to ascertain whether the metal spacer, which would be
close to the muntin edge, creates an serious visual problem;
generally it will not. In some instances, it may be advisable to
run wind-loading tests, particularly where the muntins are very
narrow. Where there are both vertical and horizontal muntins, some
millworks use oak to provide additional strength at the muntin
joint. In all cases, it is important to ensure that the sealed
insulating glass will properly function.
The use of insulating glass in multiple-lite replacement windows
can be quite expensive, especially when the lites are numerous.
One alternative is to copy a practice common in the first half of
the twentieth century, using a true divided lite sash, but
incorporating a single-lite storm panel piggybacked onto the room
side of the sash. Besides retaining the visual qualities of the
historic sash, this solution features ease of operation, resulting
from not having a completely separate storm frame. This window
design utilizes a metal frame, single-lite storm panel set into a
rabbet along the inside rail and stile edge on the room side,
secured with clips or set screws. There are instances where the
existing jamb tracks may be used with the replacement sash,
provided the original window was "overdesigned" so that the muntin
depth can be shaved to provide sufficient clearance to attach the
storm panel flush with the sash. In most cases, however, it would
be necessary to modify the jamb tracks, since the rails and stiles
would need to be of a thicker stock so as not to affect the
structural integrity and visual qualities of the muntins. As with
retrofitting a storm panel onto an existing single-lite sash,
careful attention to detail in the design and fabrication of the
piggyback unit is required.
Bow windows, popular in the Victorian era, require additional
planning efforts when duplicating in-kind. Generally, they contain
only single lites -- a distinct advantage in working with
replacement design. Glass in historic bow windows can be found in
both curved and straight pieces. In the latter case, standard
insulating glass can be used in the replacement wood window. On
the other hand, where curved glass is present, sufficient time
should be allotted in the planning stage to locate the sealed
insulating glass and have it custom-manufactured in a bow shape
that conforms to the original. There are a number of suppliers of
curved insulating glass; while this is expensive, most buildings
have only a few curved windows, and their distinctive appearance
should be retained in either a single or double-glazed replacement
REPLACEMENT STEEL WINDOWS:
When confronted with steel windows deteriorated beyond repair, the
owner or architect will often not consider replacement with
matching steel windows. Spalling concrete around the subframe,
racked windows, and rusted metal glazing channels suggest to many
that this is an antiquated window system with no place in the
rehabilitation market today.
Most steel windows being replaced in historic buildings are in old
factories or commercial buildings that have long been under-used or
vacant and have consequently suffered through several decades of
neglect. Steel windows that were originally hot-dip galvanized
have fared better under such circumstances, but even steel windows
still in relatively good condition must be maintained. This
includes painting to protect against rust, keeping the caulk and
putty glazing sound, and cleaning the weep holes. Steel windows in
50 to 70-year-old institutional buildings, offices and large
apartment buildings generally have been better maintained than
those in factories and often are in good or repairable condition.
Steel windows, with their multiple lites, typical narrow framing
members, glass set within the same building plane, and often large
glass exposure, have distinctive appearances that are extremely
difficult to match with either aluminum or wood. This reinforces
the need to consider carefully the use of steel replacement
While a steel unit might not outperform a good quality aluminum
window system, the differences are not as great as most would
believe. Steel windows are available with modern weatherstripping
around the venting unit, reducing air infiltration as well as
outside noise. Where flat T-bar glazing channels exist in the
historic windows, manufacturers can easily provide replacement
units that will accommodate sealed insulating glass (though thicker
or wider sections may be required). At least one manufacturer
markets a steel window with a partial thermal break feature. Also,
steel is a poorer conductor than aluminum and, with its greater
strength, less metal exposure to the weather is necessary. To help
reduce maintenance costs, a variety of modern priming systems and
paint finishes are available, with the best-performing ones
naturally costing a bit more.
Steel windows with more decorative metal sections, such as a cover-
bead profile, were designed with vary shallow glazing bars and are
available today for use only with single glazing. A few decorative
shapes are unavailable at this time altogether. While many
buildings with severely deteriorated steel windows can be replaced
with matching steel units that have upgraded performance features,
those with particularly thin glazing bars or decorative shapes may
require less than optimal solutions.
***REPLACEMENT WINDOWS WITH SUBSTITUTE MATERIALS***
For most small residences, offices, factories and institutional
buildings, replacement windows, should be of the same material with
features matching those on the historic windows, such as true
divided lites. Where significant interior spaces are involved with
windows as a contributing feature, in-kind replacement should also
be specified. On large buildings, however (especially late
nineteenth and early twentieth-century highrises), double-hung,
single-lite wooden sash that are beyond repair can often be
replaced successfully with an aluminum window. Differences in
construction, detail, and finish are not readily apparent from the
street below, if certain conditions are followed.
First, the appearance of a double-hung window should be retained
whether one or both sash are operable. Even for a fixed window,
two glass panels should be used, set in places corresponding to the
original glazing. A dividing bar matching the width and depth of
the historic meeting rails is also essential. For new double-hung
windows, this generally means higher quality units should be
specified because the thin sash frames found on cheaper windows
rarely maintain the shadow lines, planar qualities, and overall
appearance of the historic sash.
If the historic sash had a circle top, this feature should be
duplicated through matching framing and not through an applied
metal apron. As to finish, historically appropriate paint colors
for the building should be used, especially in light of other
changes being made to the window. For example, historic white sash
on a red brick building would be dramatically changed if a brown
color finish were used on the new sash. On a job with 150 or more
windows, specifying a custom color may well result in little or no
additional markup, depending on the painting facilities of the
manufacturer. Sometimes a custom color job requires additional
lead time in order to ensure a close color match, but this is
easily achieved through careful planning.
It is common practice to retain the existing wood frame when
replacing the window, since retention saves on demolition costs and
reduces the likelihood of damage to both interior and exterior
finishes. Whenever this is done, however, efforts should be taken
not to reduce the glazed surface area. This is difficult to
which is inexpensive.