I recently conducted an energy audit for a homeowner who purchased a home about a year ago that had a newly installed energy-star furnace. The homeowner was happy to point out this energy-saving feature. However, the home had a duct system that was located mostly in the unheated basement, had many leaks, and was not insulated. As a result, a portion of the heated air was pouring out into a cold basement and then through gaps around basement windows to the outside.
Additionally, when the furnace was on, cold air was being drawn into the return ducts, thereby reducing air supply temperature and furnace efficiency. When the furnace was not running, cold air was seeping up through the duct system and into the house, just as it would anywhere else at weak points in the home’s thermal envelope.
Leaky air ducts have the potential to lower heating and cooling efficiency up to 40%. Research has shown that they can also increase the air exchange in a house from 30 to 300% when the air handler (the unit that blows conditioned air through a house) is running. Fortunately, air ducts that are located in basements, attics, and crawl spaces can be sealed and insulated. Before undertaking this, an air duct leakage test can be performed by an energy auditor to determine both overall leakiness as well as pinpoint exactly where the leaks are.
The first piece of advice regarding sealing and insulating air ducts is to not use duct tape-it leaks. Duct mastic, which is a latex-based material, should be spread by hand over holes, cracks, and seams. Be sure to wear some vinyl gloves for this task, not only because mastic is messy, but because ducts can have sharp edges. To be safe, wear more than one layer, or put heavy duty work gloves on over the latex ones. Fiberglass-mesh tape can be used for reinforcement over larger gaps. Sealing all the way from the air handler to where the registers meet the floor or ceiling is necessary.
If the duct is insulated, do not seal the outside face of the foil or vinyl insulation, just the duct itself. You will have to remove the insulation first, seal the ducts, and then reattach insulation with a clamp stapler.
After the ducts are sealed, if they weren’t previously insulated, it’s a good time to insulate using only insulation made specifically for air ducts. It is typically vinyl-faced and filled with fiberglass providing a minimum R-value of 6. If ducts are close enough to the attic floor, cover them with loose-fill insulation-making sure they are sealed first.
Sealing your air ducts can be messy, but it is well worth the energy and cost savings.
© 2008 Tim Gould. All rights reserved.