Part IV: Air Seal and Insulate your Chimney to Save Money

Series: Installing Blown-in InsulationPermalink

Save money by adding blown-in insulation to your attic.

Chimneys are a great conduit for removing smoke and gases from your house, but they’re also notorious for allowing conditioned air to escape to the outside via the chase that surrounds them.

If that weren’t enough of a concern, they’re also fire hazards when carelessly insulated with materials that aren’t properly fire-rated.

Fear not, you can resolve both issues with a little know-how and the right type of materials.

Initial State of the Chimney Permalink

My natural gas furnace exhausts into a block chimney that is within a chase (a framed enclosed space). There are several inches of space on all four sides of the chimney. This void spans the distance from the bottom floor to the attic and it’s through here that our conditioned air escapes.

Cut to the Chase Permalink

It’s often recommended to use spray foam insulation when sealing gaps in a building’s envelope. This is bad advice when sealing around chimneys and vents because they have the potential to get very hot. The ideal way to air seal in this situation is with a piece of sheet metal and high temperature silicone sealant.

You’ll see above that I cut the sheet metal to fit with snips, screwed it into nearby studs, and sealed the remaining gaps with the heat-resistant silicone sealant.

Wrap the Chimney in Mineral Wool, Ideally Permalink

Fiberglass has a lower melting point than mineral wool, so it’s recommended that you wrap any chimney or flue with the latter material. I initially couldn’t find mineral wool batts, so I just used fiberglass. I used a length of metal wire to secure the fiberglass to the chimney with the aid of the needle-nose pliers.

In regards to fire risk, this particular chimney is for my natural gas furnace and is lined with clay tiles. The outer layer of cinderblock doesn’t get too hot due to an air gap between the brick and the tile. Know the risks before you wrap your brick chimney with fiberglass!

Once my chimney was properly air sealed and insulated I no longer worried about surrounding it with the blown-in fiberglass insulation. One task down, many more to go!

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