A side-by-side comparison shows where to apply mesh tape and sealant to close gaps around metal ducts.
Photo by Miranda Boutelle, Pioneer Utility Resources
One of the biggest energy wasters in your home might be out of sight: your ducts.
Around 20% to 30% of the air you pay to heat or cool is lost through leaks, holes or poorly installed ductwork in the average home, according to the U.S. Environmental Protection Agency. That's energy—and money—slipping right through the cracks, regardless of how stingy you are with the thermostat setting.
Properly sealing ducts can reduce wasted energy and help with indoor air quality. Leaking return ducts can pull dirty, dusty air from attics and crawl spaces. And if you have gas appliances, such as a gas water heater, dryer or furnace, leaking return ducts can cause backdraft, introducing combustion gases back into the home instead of outdoors where they belong.
You can seal ducts yourself or hire a professional. For a do-it-yourself approach, start by determining your duct type. Most homes have flexible or metal ducts.
Metal ductwork can be round or rectangular and vary in size, with assorted options for turns and connections. Insulate ducts when they run through unconditioned spaces, such as crawl spaces, attics and garages. If your metal ducts are insulated, you must remove the insulation to seal them properly, then reinsulate.
Flexible ducts typically have an inner and outer layer with insulation in between. The inner layer is made of bendable plastic reinforced with wire, like a stretched-out Slinky covered in plastic. The outer layer holds the insulation in place. The inner liner in flexible ducts can become brittle over time, cracking and causing leaks. If your inner liner is in bad shape, replace the flexible duct.
Check for any obvious disconnections, gaps, extra bends or sagging ducts. Improve airflow by using short, straight, taut runs. Avoid sags and excess turns.
Fasten metal ductwork at each joint using sheet metal screws. Make sure your ducts are properly supported to keep the joints from separating over time. For flexible ducts, use Panduit straps. These heavy-duty nylon zip ties offer long-term durability and a tensioning tool specifically designed to tighten them.
When it comes to sealing ducts, use products designed for the job. Seal every joint and seam with duct-sealing mastic—a thick paste you can apply with a paintbrush or gloved hand. When it dries, mastic creates a flexible, airtight seal that eliminates duct leakage. Use mesh tape to bridge larger gaps before applying mastic. Remove register covers inside the home to seal the joints and seams there. You also want a good seal where the duct meets drywall or flooring.
In easier-to-reach areas, you can use UL-listed foil-backed tape designed for ductwork.
Do not use duct tape, as temperature fluctuations can cause the adhesive to fail.
Duct sealing is messy, so protect yourself and your home. Use drop cloths to protect flooring. Metal tape and the edges of metal ducts are sharp, so wear coated work or garden gloves under nitrile or latex disposable gloves. To protect your arms, wear a long-sleeve shirt tucked into your gloves.
After air sealing, have an HVAC professional test the airflow during your next annual maintenance visit.
If you don't feel comfortable getting your hands dirty, hire a professional. Most companies that install ductwork or heating and cooling systems can also seal ducts.
Sealing your ductwork gives you peace of mind that your home is not wasting energy or introducing unhealthy air into your living space.
Miranda Boutelle writes on energy efficiency topics for the National Rural Electric Cooperative Association, the national trade association representing nearly 900 electric co-ops.