Looking for the leaky ducts
During the winter I lose heat through my air-conditioning ceiling vents, even though I close the vents at the end of the cooling season. I know I'm losing heat because last year I could see snow melting off the roof above the ductwork that runs through the attic. Is there anything I can do about this?
— David, Temple, PA
Richard Trethewey replies: You're doing the right thing by closing the vents' registers (or diffusers, as they're sometimes called). Registers aren't exactly airtight, though. If they're metal, seal them with magnetic register covers — flexible sheets that can be cut to fit and painted to match the ceiling. Don't disguise them too well, though; you'll need to remove them in the summertime, when the air-conditioning is needed. If the registers are plastic, there's no ready-made fix, unless you replace them with metal.
But the fact that you're losing so much heat into your attic makes me suspect something else is going on as well. If all the other obvious sources for air leakage are plugged — such as the chimney chase or the attic hatch — check to see that all your duct joints are sealed properly. If not, they're leaking air and costing you money in winter and summer. Mastic is the best sealant because it stays flexible over time; look for it in heating supply stores or on the Web. Shiny foil-faced metal tape will also do the job, but stay away from duct tape. Despite it's name, duct tape uses an adhesive that dries out, making it almost worthless as a sealant, even if the tape looks okay.
After your ducts are sealed, make sure you insulate them well with foil-faced fiberglass. Usually R-4 to R-6 is adequate, depending on your climate. Your municipal building official can tell you how much insulation is recommended for your locale.