Q: I recently noticed a dirty patch of insulation in my attic. When I peeled the insulation back, I found a big gap that was letting air through (a large hole had been bored to allow a pipe to pass through the framing). I want to seal this hole with expanding foam. Can I also use the foam to seal around a sheet-metal chimney that passes through the attic?
A: It’s a good thing you spotted that dirt, which is proof that a draft is moving into the attic from the house below. The insulation acts as a filter, trapping the material moving by on the stream of air. Sealing the drafts will cut your utility bill, and a little sheet-metal work accompanied by some heat-resistant caulk will make the house more fire-resistant. First, deal with the draft. Spray foam is great for sealing drafts, but that doesn’t mean it’s also suitable for sealing against the chimney.
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“In most cases, you shouldn’t use standard foam insulation to seal and insulate against a sheet-metal chimney, or a masonry chimney for that matter,” says Michael Casey, an expert on building codes and co-author of “Code Check,” a manual that helps builders, remodelers and homeowners avoid running afoul of these regulations.
There are special foams for this, but unless you’re sure that they are permitted where you live, you’re better off sealing the air gap between the chimney and floor joists using sheet steel or aluminum flashing. This horizontal air seal is known as fire blocking or draft stopping, and it prevents the spread of fire between floors by sealing air spaces that permit its passage.
Once the metal is in place, seal the remaining gap between the metal’s edge and the chimney with a thin bead of high-temperature silicone caulk.
Finally, prepare for adding insulation (foam or additional fiberglass batts) by installing a second piece of vertical sheet metal that provides a 2-inch airspace around and parallel to the chimney. This barrier must be taller than any insulation is deep to prevent insulation from touching the chimney.
There are exceptions to this procedure. In some cases, a sheet-metal chimney that passes through an attic is already surrounded by a protective sleeve that provides the needed airspace. Also, the 2-inch air gap that I recommend is generous for some forms of insulated metal chimney, which require only a 1-inch airspace. Unless you’re sure of the type of chimney you have, use a 2-inch airspace.
“Remember, local building codes always have the last word,” Casey says. “Check with your building office before beginning work.”
