You do not need to replace the original windows in your historic house or building to make your structure more energy efficient. You can improve the energy efficiency of your historic windows by adding storm windows, weatherizing your original windows and preventing condensation.

Do Three Simple Weatherizing Tasks

You can stop over 98% of outside air infiltration into your windows by doing these three simple weatherizing tasks:

1. Caulk and paint. Make sure the exterior casing trim around your windows is caulked and painted well. The exterior casing trim is an area of massive air infiltration. Testing by the Window Preservation Standards Collaborative in 2011 demonstrated that when the casings on the exterior of the window are caulked and painted well, air infiltration around the window opening and from the sash weight pocket is greatly reduced.
2. Add a storm window. Reduce heat loss from your historic windows by adding a well-fitted, exterior storm window to each of your windows. A storm will also stop air infiltration and protect your primary window sashes from the weather.
3. Repair the glazing compound. Repair glazing compound that has failed or is missing. Glazing compound seals the glass to the window sash. If it is loose or missing, air can infiltrate your window and reduce its energy efficiency.

Prevent Window Condensation

If you see condensation on your windows, it is a sign that warm interior air is entering the cavity between your window sashes and storm windows. When the warm air hits the cold storm window, it causes condensation. The effect is the same in the summer when the cooled interior air hits the warm storm window. Condensation can damage your window sills and sashes. To prevent condensation and the resulting damage to your windows, make sure you replace cracked or broken glass and apply weatherstripping at the perimeter of your windows.