Weatherization Measures for Existing Windows

Depending on the condition of existing windows, local climate and energy prices, window replacement may or may not qualify as a weatherization measure. If window replacement does not qualify, there are more inexpensive measures for improving the energy efficiency of existing windows. Examples are:

• Caulking
• Weatherstripping
• Storm windows
• Solar control window film

Caulking and weatherstripping

For leaky windows, caulking and weatherstripping is usually the most cost-effective measure. Caulking and weatherstripping reduce air leakage around the window frame and through the cracks within the window assembly and along operable sashes. The costs of properly applied caulking and weatherstripping 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. 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. More information on caulking.

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 replacement. More information on weatherstripping.

Storm windows

Storm windows can reduce the air leakage and improve the insulating value of existing windows. They can be installed on the interior or exterior side of windows, and can be mounted permanently or for seasonal use. Interior storm windows can be more easily installed and removed for ventilation or cleaning.

Storm windows can reduce air leakage. However, it is important to allow humid air caught between a storm window and a window to escape to the outdoors as otherwise, trapped moisture can condense on cold surfaces and cause damage. This means that in a window/storm window pair, the outside unit of the pair must not be too airtight. Exterior storm windows are therefore typically designed with weep holes. In the case of interior storm windows, the primary window must permit some air leakage to prevent harmful condensation. The interior unit of the window/storm window pair, on the other hand, should be airtight.

Aside from air leakage, storm windows reduce heat loss from conduction and radiation: by roughly one third if a regular storm window is installed inside or outside of a typical single-pane wood window; by at least half if the storm window has a low-E coating which reduces heat transfer from warm to cold glass. Since storm windows do not hermetically seal the air space they create, only certain low-E options that are moisture resistant can be used in storm windows. These low-E options reduce heat loss but do not typically have a significant effect on solar heat gain.

Solar control window film

Solar control window film reduces solar heat gain by reflection and absorption. As they block solar heat gain year-round, these films are ideal for cooling-dominated climates. Window films can be tinted for solar heat and glare control, but some recent window film options reflect solar heat while maintaining a relatively clear appearance. Window films can have Solar Heat Gain Coefficient (SHGC) and Visible Transmittance (VT) ratings by the National Fenestration Ratings Council (NFRC). The lower a film's SHGC, the less solar heat it transmits. The higher the VT rating, the more visible light is transmitted.

Window film is applied to the room-side glass surface of windows. Since window film absorbs the portion of solar heat that it does not reflect or transmit, it increases the glass temperature and may cause thermal stress on the glass or insulated glazing seals, particularly on sunny but cold days. Weatherization professionals should check your windows' location, type and condition before having window film installed. For more information on the factors to take into account when installing window film, check the Window Film Information Center by the International Window Film Association.