Duct air leakage wastes energy and reduces home energy efficiency during both the heating and cooling seasons. If your ducts are located in a crawl space, attic, or attached garage, air leaking out of supply ducts and into return ducts may increase your heating or cooling costs by 10 to 30 percent. In hot humid weather, return-duct leaks pull in a steady stream of hot humid outdoor air that your air conditioner must cool and dehumidify, driving air conditioning costs up.
Inspect your duct system for gaps and loose joints. Clean the areas you’d like to seal with soap and water. Common gray “duct tape” is not a good duct-sealing material because its adhesive often fails. Duct mastic, available in buckets and caulking tubes from heating suppliers, is the preferred sealant. Silicone caulking works well, too, but should be used only during moderate weather when you can ventilate with outdoor air for a day to reduce its odor.
The most important duct leaks to seal are the ones near the furnace because the air pressure there is greatest. The furnace itself isn’t usually airtight, although it should be. Sealing holes and cracks in the furnace and its connections to the main ducts is a good way to begin air sealing.
You or your contractor should be particularly careful to seal all return duct leaks near the furnace. These can be dangerous because they create a vacuum that can pull combustion gases down your chimney and cause backdrafting. Seal these leaks first, before sealing any supply leaks near the heating equipment. This potential backdrafting problem is one good reason to have your ducts sealed by a professional, who should have measuring equipment to test for possible safety problems.
Once your ducts are sealed, you can rest assured that the air you have paid to heat, cool, or de-humidify will stay inside your home where it belongs.
For more information on air conditioning see The Homeowners Handbook to Energy Efficiency , Your Home Cooling Energy Guide, Chapter 8 of Residential Energy: Cost Savings and Comfort for Existing Buildings or for specific information about servicing air conditioners for energy efficiency, see Saturn Hydronic Systems Field Guide.