It has been estimated that 20% of home heating is lost through
windows, much of it through the gaps around window frames; all
the rest is lost through ventilated and suspended timber floors
and through open chimneys. It can be seen that under these circumstances
double glazing alone cannot address the question of heat loss,
and that measures to prevent air infiltration will be more effective.
Even heavy curtains can be a better investment.
Wrightson, David. The Conservation and Renewal of Timber
Windows, Public Information Leaflet, Cathedral
Communications, Wiltshire, England [PDF file]
In some cases, owners may be concerned that an older window
is less efficient in terms of energy conservation. In winter,
for example, heat loss associated with an older window may make
a room uncomfortable and increase heating costs. In fact, most
heat loss is associated with air leakage hough gaps in an older
window that are the result of a lack of maintenance, rather
than loss of energy through the single pane of glass found in
historic windows. Glazing compound may be cracked or missing,
allowing air to move around the glass. Sash members also may
have shifted, leaving a gap for heat loss.
The most cost-effective energy conservation measures for most
historic windows are to replace glazing compound, repair wood
members and install weather stripping. These steps will dramatically
reduce heat loss while preserving historic features.
If additional energy savings are a concern, consider installing
a storm window. This may be applied to the interior or the exterior
of the window. It should be designed to match the historic window
divisions such that the exterior appearance of the original
window is not obscured.
3.0 Windows, Design
Guidelines for Residential Historic Districts in Salt Lake
City, Salt Lake City Historic Landmark Commission, Salt
Lake City Corporation, 1999.
Increased Light & View, Efficient Windows Collaborative,
College of Architecture and Landscape Architecture, Twin Cities
Campus, University of Minnesota