About 12% of residential energy is lost or gained (depending on the season) through air leaks around window frames, edges etc. Here is a pie-chart which shows you the sources of heat loss/gain in the residential environment. A window's energy efficiency is measured by its R-value (resistance to heat) and U-value (conductance of heat). A higher R-value means that the window will lose less heat than one with a lower R-value. The converse will be true for the U-value. So the U-value is a converse of the R-value.
Several methods are available to reduce energy loss in windows. Some of these are discussed below.
Caulking and Weather Stripping
Through using these methods, the cracks and holes around existing windows can be plugged. Caulking is made of chemical compounds of latex or silicone that can be used to fill up cracks and holes. Old caulking must be removed before new caulking can be applied. When applying caulking, the weather should ideally be warm and dry.
Weather-stripping is a thin piece of metal, vinyl, rubber, felt or foam that can seal a joint. In the case of windows, the joint is between the fixed frame and the movable window sash.
Here is a good resource for the DIY Homeowner giving detailed instructions about the installation and the benefits of caulking and weather-stripping.
Window Frames
The type and quality of the window frame can also affect the infiltration and loss of energy through windows.
- Aluminum frames are used for their strength and durability. But aluminum conducts heat, hence is prone to condensation. But by placing insulation plastic strips between the exterior and interior of the frame, heat loss can be minimized.
- Wood frames tend to swell and warp when exposed to wet conditions.
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