Some contractors will offer to insulate the empty weight pockets at the sides of double hung windows.
Keep in mind that one of the reasons we have any old windows to restore is that the wood has been keep dry and decay free. Here is something we learned from sealing up hundreds of houses and thousands of windows during the 1970s energy crunches here in New England.
One of the functions of the weight pocket is that it helps keep the wood dry and decay free. It provides a large volume of air on the back side of the window frame jamb board and at the joint of the jamb and still. When the wood at the joint gets wet the air absorbs the moisture from the joint, helping to keep the wood at the joint decay free. When the weight pocket is filled with insulation of any kind, there is less air, and more water buildup in the joint, because it is trapped there by the insulation.
We learned this lesson the hard way. by the late 1980s and 1990s we started getting some call backs for "dropping sills". The joint of the jambs and sills were saturated with water and decaying to the point of joint failure, which let the sills drop down. As we did repairs and investigated the cause we discovered it was the insulation, because we found a few cases where one was pocket insulated and the other wasn't insulated and was not decayed.
Right after we figured this out, we were on a couple of projects where the HVAC engineers were doing studies and calculations on energy savings and the payback time for insulating in different parts of the building and in different ways. They figured that the payback time on the cost of insulating the small exterior wall area of the weight pockets was 190 to 315 years. In every house where they did this calculation the owner decided it wasn't worth doing, especially with the risk of rotting joints.
Limit air infiltration with narrow strips of various materials.
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