The SMART Filter Seal is a durable, yet flexible high-strength film designed to fit tightly over the air filter slot on the return air plenum and prevent air from bypassing the filter and thus save energy and improve indoor air quality. Magnets surrounding the perimeter hold it tight against the plenum to prevent air leakage. It can be easily removed to change out dirty air filters and replaced again.
The 2009 edition of the International Energy Conservation Code (IECC), section 403.2.2 states, “Sealing all ducts, air handlers, filter boxes, and building cavities used as ducts shall be sealed." The SMART Filter Seal fulfills that requirement.
Furthermore, both the U.S. Department of Energy and Energy Star recommend sealing the furnace slot.