One of the most underappreciated and most easily fixed problems in homes is that of poorly sealed duct work. Homes which control their interior climate with ducted air systems can only do so as effectively as the distribution system moves the conditioned air. If you seal the return and supply air ducts, it can help improve the efficiency of the heating and cooling.
How to Seal Air Ducts – Supply and Return
Any duct work system has supply and return ducts. The supply ducts run from the furnace/A/C and supply conditioned air to the building to control temperature in the living space. There will be a slight positive pressure in the duct work, meaning any seams or cracks will allow conditioned air to leak out.
This conditioned air leaking out will cause the HVAC unit to work harder. If for example your house were in Minnesota and your duct work lost 10% of the heat through leaks, you’d have a slightly warmer basement but the main living space would take longer to heat and have a harder time maintaining that temperature.
Return ducts are unsurprisingly the opposite. They pull in conditioned air from the living space, heat/cool it and send it down the supply duct. The duct has slightly negative pressure meaning that leaks will pull unconditioned air into the duct lines. Imagine that same house in Minnesota. If the duct work was in the cold basement, it would be sucking in 45 – 50 F air. This cold air would force the furnace to work that much harder to heat the air.
How To Seal Air Ducts – Securing and Sealing the Ducts
The first thing to understand is that, much like with attic insulating and air sealing, insulating a duct is not the same thing as sealing it. Wrapping the duct in bubble wrap, boxing it in with foam board or especially wrapping it with fiberglass does not count as ‘sealing.’ OK, then on to the (actual) sealing.
Secure The Ducts – First make sure the joints between different duct pieces are secured and tight. If any of the connections are loose or won’t tighten, secure it with self-tapping sheet metal screws.
Duct Supports – Make sure the ducts are adequately supported. Overly long unsupported duct runs will sag and could force the connection seams apart. Ducting should be fastened to overhead joists every three to four feet using duct supports or with metal wire. For plastic or flex tube ducts, there are mesh strapping materials that can be fastened to the joists.
Apply Mastic – Now we apply mastic (or your one chance as an adult to play with mud). The surface of the duct needs to be entirely clean of dirt and dust before applying the mastic. Soap and water usually do the trick but heavier cleaning chemicals may be needed. Apply the mastic to the connection gap.
If the gap is larger than a hairline, mesh duct tape can be used to bridge the gap. Cover the gap with mastic, giving an inch on either side of the gap to approximating 1/4″ thick layer. (Aside: there are mastics designed specifically for sheet metal. It has consistancy like peanut butter and can be spread by hand). Once dried, the ducts can be insulated however you’d like.
Leaky air ducts can be a significant impediment to maintaining appropriate temperature in your house. Properly sealing (and knowing how to seal air ducts) the distribution system can address this issue.