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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 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 specified.

Fisher, Chuck. Rehabilitating Windows in Historic Buildings: An Overview (GSA 08500-03), Historic Preservation Technical Procedures, U.S. General Services Administration

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