Air leakage can occur around the window frame and through the cracks within the window assembly, particularly along operable sashes. Caulking and weatherstripping can reduce air leakage through these pathways. According to the U.S. Department of Energy , the costs of properly applied caulking and weather stripping can usually be recovered in energy savings within one year.
Caulk provides an effective, flexible seal for cracks, gaps and joints that are no more than a quarter inch wide. There is a range of caulking compounds varying in strength and price. These compounds and half-barrel caulking gun applicators can be purchase at any local hardware store.
Old caulk or paint residue should be removed before new caulk is applied to all joints in the window frame and the joint between the frame and the wall. Relatively warm temperatures (above 45°F) are necessary for the caulk to set properly and adhere to the surface. Low humidity is also important during application to prevent cracks from swelling with moisture.
Weatherstripping helps prevent air leaks around windows sashes. It should be applied to clean, dry surfaces at temperatures above 20°F. Ideally, weatherstripping compresses when the sash is shut. To seal an irregularly shaped space, more than one type of weatherstripping may have to be combined.
After some years, any kind of weatherstripping should be inspected in case it warrants to be replaced. Organic felts age fairly quickly, and all felts absorb moisture, reducing their effectiveness. Brush or wiper type weatherstripping eventually gets matted down like a carpet that has had too much traffic. Metal strips are easily dented or bent. Plastics and rubber can become brittle or sticky. Nevertheless, some recent synthetic weatherstripping can be expected to last longer.
Caulking and weatherstripping reduces air leakage but does not insulate or address solar heat gain. For better insulation of existing windows, consider storm windows; for solar heat gain control, see window film or exterior shading.