Windows should be Energy Star labeled and bear the American Architectural Manufacturers Association label or National Wood Window and Door Association “Hallmark” label. They should also have National Fenestration Rating Council labels showing that they meet the specified performance ratings for U-factor (0.40 or less) and solar heat gain coefficient (0.55 or less). To avoid basement insulation a lower U-factor window is required when rehabbing certain building types.

To improve the energy efficiency of existing windows, use rope caulk to seal air leaks between sashes and any other part of the window that moves. Caulk should be used to seal between the window frame and the wall. Plastic storm windows, or permanent storm windows, either interior or exterior, are extremely useful in creating a comfortable room.

Insulating shades are also a useful treatment, especially over large, single pane windows.