Introduction
Use heat-resistant caulk and sheet metal to air seal flues and chimneys in the attic.
Why
There are often gaps in ceiling drywall around flue pipes and chimneys that can allow conditioned air from the house to escape into the attic in winter and hot dusty attic air to come into the home in the summer, wasting energy and making your home uncomfortable.
What You'll Need
- Heavy Gloves
- Eye Protection
- Tin Snips
- Aluminum flashing or sheet metal
- Screwdriver and screws or staple gun and heavy metal staples
- Fire-rated caulk
- Rigid mineral wool insulation board
- Box knife
1. Locate chimneys and flue pipes.
If you have a fireplace or woodstove, sketch or mentally note its location in reference to the closest exterior walls. You can also go outside and look for a chimney or large flue pipes exiting through the roof (Figure 1). Go into the attic and look in that location for a large round metal pipe or a brick square or rectangular column.
Source
If you have a gas furnace and/or a gas water heater located inside your home, look at the pipe coming out of the top of the furnace or water heater. Older gas furnaces and water heaters may have one large metal flue pipe coming from the top of the furnace or water heater that vents straight up through the attic and roof. Newer, sealed-combustion, direct-vent furnaces and water heaters may have a duct for intake air and a metal flue for exhaust air. These may exhaust through the attic or out through the wall. Newer high-efficiency condensing furnaces and water heaters will use PVC ducts instead of metal flues and typically vent out through a side wall rather than up through the attic.
If you have metal flues that exit the home through the roof, look at the flues where they enter the attic from the floor below or from the attic side to see if there are gaps between the sides of the flue and the surrounding drywall that need to be air sealed. If the attic is dark and the house lights are on, you may see light coming through the gaps around the flue. You may have to pull back attic insulation to look for gaps. Wear gloves and avoid touching the flue pipe as it will likely be warm if the furnace or water heater is running.
2. Install sheet metal around flue vent pipes.
Option A – Air Seal at the Bottom of the Framing
- Pull back any insulation (Figure 2).
- Cut two pieces of sheet metal to cover the chase opening. Allow 1 inch of overlap around the flue pipe.
- Fasten the sheet metal to the framing with screws or heavy staples.
- Seal all edges and seams in sheet metal with fire-rated caulk (Figures 3 and 4).
Source
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Option B – Air Seal at the Top of the Framing
- Pull back any insulation.
- If there are gaps larger than ¼ inch around the flue, install sheet metal to cover the openings. Cut two pieces of lumber equal in height to the ceiling joists (Figure 5). If the gaps are less than ¼ inch, seal them with fire-rated caulk, then proceed to Step 3.
- Fasten these wood cross pieces to the joists on either side of the flue pipe with screws, keeping at least 3 inches of clearance to the pipe. Caulk the joints between the wood blocking and the joists.
Source
- Cut two pieces of sheet metal or aluminum flashing to fit around the chimney pipe with 1 inch of overlap. Make the pieces large enough to extend onto the joists and blocking without extending past the joists or blocking.
- Apply a generous bead of fire-rated caulk to the top of the joists and blocking and to all joints between the blocking and joists.
- Fasten the sheet metal to the top of the blocking and joists with screws or heavy staples and seal around the flue and at sheet metal overlap seams with fire-rated caulk (Figure 6).
Source
Document with the Quality Control Checklist developed as both a guide to assist in the transition to high performance home building, and as a simplified tool to be used as part of any builder's on-site quality control procedures.
3. Make an insulation dam around the flue.
Create a dam around the flue pipe to keep insulation at least 3 inches away from the hot flue type. There are several options for making the dam. You can create a metal collar using sheet metal, or you can form a dam or wall of sheet metal, dry wall, or mineral wool board around the flue or chimney.
- Sheet metal collar –
- Use tin snips to cut sheet metal to fit around the flue pipe with at least 3 inches of clearance all the way around.
- Snip 3-inch tabs in the top edge and bottom edge.
- Fold the tabs in at the top and alternate tabs folded in and out at the bottom to maintain a 3-inch clearance. With tabs folded, the shield should be 4 inches taller than the finished insulation level. Staple the side seam of the collar together with heavy metal staples or overlap and adhere the seam with fire-rated caulk.
- Fasten the tabs to the flat sheet metal installed at the bottom of the ceiling joist bay (Option A, Figure 7) or at the top of the ceiling joist bay (Option B, Figure 8) with heavy metal staples or screws.
- Re-install insulation around the collar. Ideally the insulation in your attic should cover the rafters (Figure 9).
- Insulation dam or wall collar –
- Form a dam or wall of sheet metal (Figure 10) or a heat-resistant material like gypsum dry wall or rigid mineral wool (Figure 11).
- The dam should have at least 3 inches of clearance all the way around the flue pipe.
- The dam should be at least 4 inches higher than the finished insulation level.
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4. Air seal around the chimney.
- Pull back any insulation.
- If there are gaps larger than ¼ inch around a chimney in the attic, install sheet metal to cover the openings. Cut pieces of sheet metal to fit around the chimney and extend to surrounding framing. Install blocking if needed to attach the sheet metal to.
- Lay a thick bead of fire-rated caulk on the top of the framing.
- Place the sheet metal on the framing and fasten the sheet metal to the framing with screws or heavy staples.
- Seal all edges and seams in the sheet metal and around the chimney with fire-rated caulk or spray foam (Figure 12).
Source
5. Wrap the chimney with mineral wool.
Wrap the chimney with fire-resistant unfaced mineral wool batt insulation to a height at least 4 inches above the expected insulation level to keep flammable substances such as the paper facing on batt insulation from contacting the chimney brick (Figure 13). Replace the attic insulation and add more if desired.
