Sealing around the chimney

Posted Wednesday, December 14, 2011 in Sustainable Maine

Sealing around the chimney

by Paul Kando

In many houses the biggest air leak is found around the chimney.

A chimney 24 inches square with an average 2-inch gap around it, for example, represents a 1.5-square-foot hole in the heated envelope, open to the outdoors (via the cold attic) 24 hours a day, seven days a week. If the same chimney traverses unheated space between floors as well, the hole can be as large as 3 square feet. Invariably this is an outbound leak, with warm air flowing out of the heated space, creating a compensating cold, inbound air stream somewhere else, like in the damp basement.

Because the warm air flows away from you, you don’t necessarily feel a leak around a chimney. You only feel its effect: It is awfully hard to keep the place warm. Other, hidden, often more insidious impacts of this gap may be damage caused by air-transported moisture in the attic and perhaps even an ice dam or two.

For all its huge energy waste and other problems, a leaky chimney-chase is not hard to fix. Fire codes require a minimum air space of 2 inches around a chimney (1 inch around the round, insulated “metalbestos” flues). Therefore you cannot stuff the gap full of non-fire-rated material. This includes insulation (fibrous insulation does not air-seal in any case). On the other hand, it is perfectly OK for fire-resistant materials to be installed right against a chimney.

Here is a simple fix: In the attic, first move all insulation away from the chimney and locate the nearest wooden joists or other framing around it. The gap between the framing and the chimney may vary from 2 to 6 inches and even more in some houses. If the gap is more than 6 inches, you may want to nail in pieces of 2-inch board, preferably similar in dimensions to the existing framing, to reduce the gap. (There are many possibilities here, as long as the fire-gap is maintained; this extra piece of framing does not have to be load-bearing. Feel free to improvise.)

Next wipe the chimney wall clean and dust-free all around at the level of the top of the surrounding wood framing. Cut metal flashing of appropriate width to size, and screw sections of it to the wood framing all around the chimney, making sure that the sections overlap ½ to 3/4 inch. One edge of the flashing should touch the chimney wall. Seal the flashing sections together and against the chimney with high-temperature caulk, filling all gaps, as shown on the drawings. (Caution: Caulk will not adhere to a dusty surface). This will complete the air and moisture seal.

Pieces of roof drip-edge, installed with the bent edge up, will do a better job on a square chimney than plain flashing. The bend will make for a firm, stiff joint and the bent-up part will hold a generous amount of caulk and help seal the metal against the chimney well. On round flues use two sections of plain sheet metal, cut out to fit the outer diameter of the flue in such a way that, when joined, the sections overlap enough to be sealed together.

Since you can’t insulate against the chimney, the next step is to install an insulation dam about 2 inches away from and all around the chimney. Use a single piece of metal flashing of sufficient width and length to fashion such a dam, then add the desired amount of attic insulation. A series of narrow, 2-inch-long “ears” cut along both edges of the flashing strip forming the dam and bent at a 90-degree angle toward the chimney will act as handy spacers. Two-and-a-half-inch-long drywall screws driven through the flashing the right amount will do likewise, permitting adjustments to compensate for irregularities in the chimney surface. (It does not matter if the screw heads protrude outward since the dam will be covered with attic insulation.)

It is also possible to seal a chimney chase from below, if the framing is accessible. The procedure is the same as above, except you will be working overhead rather than at ground level. You will still need to install an insulation dam in the attic if you plan to add insulation.

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