Q: The article “Sealing a Chimney Chase” (Oct/17) pertains to a brick chimney. How would I seal the chase for an existing double-wall metal chimney?
A: Matt Damon, owner of Penobscot Home Performance, a weatherization company based in Bucksport, Maine, responds: The procedure for sealing around an existing cylindrical double-wall metal chimney is similar to that for sealing around a brick chimney. Be aware that metal-chimney companies make materials to air-seal new chimneys, but using them would require dismantling an existing chimney.
Start with a rectangular piece of 26-gauge sheet metal large enough to cover the framed chase. On the sheet, locate and draw a circle that has the same diameter as the outside of the pipe and cut it out.
As with a masonry chimney, take care to seal any gaps in the chase that are more than 2 inches away from the pipe, using rigid foam and one-part foam sealant. Note that double-wall pipe manufacturers may rate their products for zero clearance to combustibles, but I would try to maintain a gap of at least 2 inches to be on the safe side.
Cut the sheet metal in half and slip it around the pipe, as shown below. Screw the metal to the framing, sealing it with high-temperature caulk at the framing and where the metal contacts the pipe. To seal the edge of the metal where it was cut in half, bridge the caulked seam with a small strip of the same sheet metal.
Even if the pipe is rated for zero clearance, I would recommend wrapping it with 2-inch Roxul batts. Blown-in cellulose treated with borate is fire-resistant, but the fireproof Roxul would provide some additional peace of mind.