Windows: The Importance of Weatherstripping

By: Elizabeth Wanga

Windows: The Importance of WeatherstrippingWhile weatherstripping around windows keeps the cold air out in the winter months, it is also important for preventing the cold air in air conditioned spaces from escaping. The energy efficiency of a window can be greatly increased by providing adequate weatherstripping between the sash and the frame. Weatherstripping also reduces unwanted draftiness. Weatherstripping should be evaluated and addressed as part of  routine maintenance. For the relative reduction in energy costs, the investment in new weatherstripping is inexpensive.

There are a myriad of products that can be used to weatherstrip a window. The appropriate product choice depends on the window type and where the weatherstripping will fit within the window. Many wood windows were originally stripped with felt. Felt strips compress easily into voids, greatly decreasing air infiltration. Because itas easily cut and thinned to fit the variable openings between window components, felt is still used in window restoration. However, felt often degrades over time and absorbs water, which causes it to tear and hold water against the window.

Weatherstripping is generally suitable for locations where it can be compressed or where it can slide on the sides of the sash. Weatherstripping can be mechanically fastened or installed in a kerf cut into the sash, or in a channel in manufactured windows.

Interlocking spring weatherstripping, which bears against the closed sash, is suitable for both sides of the window: where the window slides, and at the base and head of the window where the spring would be in compression. These springs are manufactured in metal and plastic and can be formed in bronze, brass, aluminum, galvanized steel, stainless steel and plastic.  To avoid galvanic corrosion in metal window assemblies, choose a compatible metal.

Bulb type weatherstripping is recommended for compression. The bulb can vary in size and can be useful when there is an uneven gap between window components. Pile and brush weatherstripping is mainly used for sliding doors and at the base of swinging doors.

To be effective, weatherstripping must fully engage all surfaces between two components. If weatherstripping is in good condition, slight adjustments to the sash locks can help to compress the bulb or spring weatherstripping, and ensure the gap between the components is closed.

Air infiltration around a window can be tested using a vacuum chamber. This information aids in designing the appropriate weatherstripping. Once the weatherstripping is installed,adjustments can be made in the field to determine what combination best suits the window configuration. NYSERDA, the US Department of Energy’s website and local power companies are excellent resources for information on weatherizing windows.

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