Air leakage caused by cold air outside and warm air inside takes place when the building contains cracks or openings at different levels. This results in the cold and heavy air entering at low levels and pushing the warm and light air out at high levels; the same draft takes place in a chimney. When storm sashes are applied to well-fitted windows, very little redirection in infiltration is secured, but the application of the sash does give an air space that reduces heat transmission and helps prevent window frosting. By applying storm sashes to poorly fitted windows, a reduction in leakage of up to 50 percent may be obtained. The effect, insofar as air leakage is concerned, will be roughly equivalent to that obtained by the installation of weather stripping.
