My house has a brick chimney that used to exhaust the furnace and water-heater gasses. I’ve since decommissioned those mechanicals and capped the chimney’s metal liner, basically retiring the chimney completely. The openings in the basement have been sealed with metal caps, and there is a chimney cap on the exterior. Is the chimney still an energy penalty?
— J. Pritzen, via GreenBuildingAdvisor.com

M.H.: There are two ways that this type of chimney hurts energy performance. First, the caps on the thimbles and the cleanout door at the base of the chimney are probably leaky, so the stack effect is pulling conditioned indoor air through these gaps and up the flue. Second, the brick chimney probably represents an uninsulated thermal bridge that allows heat to escape via conduction and convection.

The first step in addressing the air-leakage issue is to build a better cap. The top of the flue can be sealed by stuffing fiberglass insulation down the flue and sealing above the fiberglass with canned spray foam. You can build a plywood cap roofed with copper or EPDM, or a concrete cap. Whatever type of cap you build, aim for airtightness.

Once you’ve installed the cap, go indoors and seal the cleanout door at the base of the chimney and every single thimble you have access to by stuffing some fiberglass batts in there and then going to town with spray foam to provide the air seal. An alternative is to build a custom plug out of plywood, sealing the edges of the plywood with caulk.

After the exterior and interior holes are taken care of, you can add insulation on the interior portion of the chimney to address the thermal-bridging issue. I recommend 4 in. of rigid foam applied to the face of the masonry, followed by a layer of gypsum drywall over the foam to provide fire resistance.