Replacing windows in older & historic houses
Although the retention of original or existing windows is always desirable and this article 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:
- the pattern of the openings and their size;
- proportions of the frame and sash;
- configuration of window panes;
- muntin profiles;
- type of wood;
- paint color;
- characteristics of the glass; and
- 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. (Click here for a list of window suppliers from The Old House Web's Guide to Suppliers.) 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, a historic wooden window with a high quality storm window added should thermally outperform a new double-glazed 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 ASHRAE 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.
The Old House Web