Mastic is the gooey sealant you should be paying your HVAC contractor a little extra for so that he puts it, instead of tape, on all joints in your ductwork. Its benefits include reduced callbacks thanks to better comfort; improved protection against backdrafting; and permanent instead of temporary sealing. It also can raise a home’s Energy Star rating one to three points.
Though more expensive than tape, mastic is just as easy to use. “It’s almost faster to get mastic out of a gallon bucket and smear it on with a 39-cent disposable brush than it is to install tape,” says Rob de Kieffer of Sun Power Inc., a weatherization association in Denver. At least a half-dozen products are on the market. “The main difference between products is thickness,” de Kieffer says. “We like thicker products versus the runnier ones because thickness helps you seal larger joints.”
His crews are trained to emphasize sealing all panning, end pans and plenums on the return-air side. Testing shows that returns are twice as leaky as supplies, and return-air leakage in the basement can backdraft combustion gases into the home. Sun Power also mastics all supplies; the objectives with supplies are improved comfort, better heating of remote rooms, and less overheating or overcooling of basements.