How to Close an Unused Chimney

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The fireplace flue can be a major source of energy loss. Cooled or heated air from inside your home can escape through the chimney, causing your HVAC system to work harder. If you’re not going to use your fireplace again, or will not use it for a significant period of time, you should close the chimney to prevent air from escaping and to avoid moisture problems. This means sealing the chimney at the top and the bottom.

Climb onto your roof to inspect the chimney. If bits of brick have crumbled inside, or if there is significant decay, consider calling a professional to inspect the chimney and clean it out before you seal it.

Remove the cap on the chimney. You may need to use a screwdriver to loosen screws holding it in place.

Measure all sides of the opening at the top of the chimney. Put on heavy work gloves, and use tin snips to cut a piece of sheet metal to the same measurements as the chimney opening.

Apply a bead of butyl rubber caulk around three sides of the chimney at the top, and set the cut piece of metal on the chimney so the edges line up with the caulk bead.

Press the metal into the caulk. The goal of this metal cap is to prevent rain from coming in, but it shouldn’t be an airtight seal. Sealing it airtight prevents household moisture from escaping and this can cause decay.

Replace the chimney cap and go inside the house. Close the fireplace damper completely.

Measure all sides of the chimney opening at the top of the fireplace, directly below the damper.

Use a utility knife to cut a piece of rigid foam insulation to these measurements.

Place the foam up inside the chimney so that it fits tightly into place.

Apply a bead of butyl rubber caulking around all sides of the insulation where they rest against the sides of the chimney.

Things You Will Need

  • Ladder
  • Screwdriver
  • Measuring tape
  • Tin snips
  • Butyl rubber caulk
  • Caulk gun
  • Foam insulation
  • Utility knife

Warning

  • Do not attempt to use the chimney after it has been closed. If you decide to reopen it, have a professional inspect the chimney before you use it again.
  • Warn guests in your home that the chimney is not in use and has been closed.