The best stuff for sealing ducts these days is duct mastic, a water-based adhesive that you simply slop on and spread around with your hands. Once the mastic dries, the seal is complete and shouldn’t have to be tampered with again. For spanning large gaps, it’s a good idea to cover the gap first with self-adhesive, fiberglass-mesh drywall joint tape. This comes in rolls and cuts easily with a sharp utility knife or scissors. You might also need some small sheet metal screws to replace any original screws that are missing or add some where the installer was neglectful.
Before you start, be sure to turn off your furnace. It must remain off until the mastic has cured (check the manufacturer’s directions for curing times). Cary Weiner stresses that “all joints should be sealed, including the connections between the air handler and the ducts, and the seams within the AHU (air handler unit) and the ducts themselves.”
Start at the air handler connections and work your way down the main supply duct and to the end of each branch duct, applying the mastic as directed. Pull away insulation as needed to access joints and other leak-prone areas, and reposition the insulation only after the mastic dries. When the supply ducts are done, repeat the process on all of the return ducts.
Bruce Harley, author of Cut Your Energy Bills Now, offers a great tip for applying mastic: wear vinyl gloves underneath a pair of cotton gloves. This allows you to slip off the cotton gloves and leave them in the mastic whenever you need to switch to a different task.
If you have any questions about what should be sealed and what should not, consult a pro (or at least a knowledgeable neighbor). For example, you should never seal any parts of the air handler itself or the furnace’s flue pipe, which looks like a small duct. You can also have your ducts sealed by a qualified HVAC (heating, ventilation and air conditioning) professional or a duct-sealing company. One professional service uses a special adhesive that seals holes from inside your ducts. This is a good option for sealing any inaccessible ductwork that runs through walls or between floors and finished ceilings.
How Much You Can Save by Sealing Your Ducts
Any energy or HVAC expert will tell you that sealing your ducts should be followed by insulating them, too, at least where they travel through unconditioned (heated or cooled) areas of the home. According to Weiner, “A homeowner can lose up to 30 percent of the energy used to heat or cool a space if ducts are not properly sealed and insulated, particularly if the ducts run through unconditioned space (such as an attic or basement).”
Weiner adds, “Assuming a homeowner can reduce energy use for space heating by 10 to 25 percent by sealing ductwork, the average homeowner could save $40 to $100 per year.” It’s important to note that sealing ductwork not only saves energy during the heating season; if you use central AC, it will save you money during the cooling season, too. And thanks to the improved performance of the system, sealed ducts help reduce the workload on your (expensive) heating and cooling plants -- that is, your furnace and AC unit. While this isn’t really a measurable factor, it’s worth noting that any mechanical equipment tends to last longer and require less maintenance when it’s working efficiently.
If you really want to know how much sealing your ducts will save, you can have your duct system tested by a home energy auditor or HVAC pro with special pressurizing equipment. Similar to a blower door test, duct testing equipment uses a fan to pressurize the duct network and detects leakage by how much air flow is present. By doing the test before and after you seal your ducts, an operator can calculate how much energy, and therefore money, you’ll save in an average year.