Complete replacement of timber windows is seldom necessary,
as decay almost invariably starts at the bottom and works upwards,
and the lower components will need replacement and repair long
before the upper parts. Indeed, complete replacement may actually
be counterproductive, as some replacement windows inserted in
the last thirty years have now decayed, whereas many windows
from the 18th Century and earlier still survive.
Wrightson, David. The Conservation and Renewal of Timber
Windows, Public Information Leaflet, Cathedral
Communications, Wiltshire, England [PDF file]
|Two windows in a circa 1800 house. Left:
Ten-year-old replacement window with leaks along meeting
rail, condensation between double glazing indicating failed
seals. Right: Original sash, intact and performing well.
Preserving historic windows is always preferable to replacement.
As a rule of thumb, if 50% of the existing window can be repaired,
then the window should not be replaced.
3.0 Windows, Design
Guidelines for Residential Historic Districts in Salt Lake
City, Salt Lake City Historic Landmark Commission, Salt
Lake City Corporation, 1999.
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 existing windows.
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
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
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
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
windows; test standards for the contract work can then be specified.
Fisher, Chuck. Rehabilitating
Windows in Historic Buildings: An Overview (GSA 08500-03),
Preservation Technical Procedures, U.S.
General Services Administration