Best option for sealing chimney?
August 21, 2020 1:46 AM   Subscribe

We are experiencing some air leakage from our brick, wood-burning fireplace we never use and are wondering what the best option is for fixing this situation. We do have a damper in there but it's obviously not doing the job so we're looking into putting something additional, but what? Top damper? cap? balloon? fireplace door? What would work the best? Thanks!
posted by madonna of the unloved to Home & Garden (7 answers total) 1 user marked this as a favorite
My dad had this issue in a 1920 home in Colorado. He periodically used the fireplace, but it was mostly decorative. He created an insert of thick foam that fit the opening (like you would use for making couch cushions) attached (glued) to a piece of plywood. The plywood was covered with upholstery fabric that was stapled around the edges and it extended a few inches across the opening to seal it. The foam kept it in the fireplace box and it stopped the drafts. It was removable and relatively inexpensive.

I added doors to my fireplace and coupled with the damper, it seems to work pretty well. Have you had a chimney sweep check the damper to make sure it's closing properly? I had some water in my firebox and it turned out the damper was not closing fully, even though I could hear the "thunk" when I pulled it shut. It was adjusted and it solved that problem.
posted by XtineHutch at 6:37 AM on August 21, 2020

We had the same problem, and it was because the ancient cast-iron damper just doesn't seal up all that well, even thought it closes completely. Now we have custom fireplace doors which do the trick. I ordered them on-line and installed them myself, which was pretty easy. Just measure up the opening, and they custom made them to fit. Took a couple months from order to delivery though.

Before we got those I made a removable plug made of MDF board backed with a piece of hard blue styrofoam insulation sheet cut to fit the opening. I painted the MDF mat black. It was a cheap and easy fix that looked good. I put a couple nice brass handles on the plug so it could be easily inserted and removed.
posted by fimbulvetr at 7:14 AM on August 21, 2020

+1 to fireplace doors making a big difference. We ordered from Pleasant Hearth via Amazon.
posted by matildatakesovertheworld at 8:23 AM on August 21, 2020

Our house has a comically undersized fireplace in the living room. The box is too small to burn a wood fire and we're not plumbed for gas so we really don't know wtf the builder was thinking at the time.

In any case, we bought one of these to stop up the flue and it works great.
posted by jquinby at 9:55 AM on August 21, 2020

The really simple solution is to stuff a bunch of fiberglass or rock wool insulation up it. Cheap and it works. Just be sure to remove it if you want to use the fireplace.
posted by mareli at 12:18 PM on August 21, 2020

We also used thick, rigid foam cut to size to block off a fireplace flue. It's cheap and you can easily take it out if you need to. Oh, and it works well!
posted by thatone at 1:35 PM on August 21, 2020

I meant to answer this when first posted but got sidetracked: If you're sure you're never ever going to use the fireplace, I would spray the bottom of the damper with insulating foam, like "Great Stuff" which is available in most hardware stores. It air-seals, moisture seals and insulates, all in one. And it's reversible if you decide to start using the fireplace, just cut it out with a knife and scraper. (But obviously it's not a good solution for occasional use of the fireplace.) If you do this I would couple it with a good chimney cap so you don't get any pooling of water at the bottom of the chimney.
posted by beagle at 2:18 PM on August 23, 2020

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