Sealing A Drafty Fireplace

During the inspection for air leaks in our home I came across a drafty fireplace. The previous owner put a glass door in front, but that didn’t seem to stop the draft completely. Because we won’t use the fireplaces, we have two, I decided to seal both up with rigid foam insulation.

Drafty Fireplace
Drafty Fireplace

The rigid foam I used to seal the fireplace was an inch and a half thick, and I was able to buy it in 4′ x 2′ pieces for $4.15 each.

Rigid Foam Insulation
Rigid Foam Insulation

When surveying the interior of the fireplace, I noticed the installation bracket for the glass doors on the front of the fireplace stuck out inside the fireplace. This was too convenient as a ledge for one side of the insulation to rest on.

Glass Front Installation Bracket
Glass Front Installation Bracket

After measuring the fireplace interior, it was time to layout the cuts on the foam. A regular pen worked just fine on the foam. The fireplace interior is a trapazoid. Here are the steps I found that helped me quickly layout the trapazoid on the foam.

Trapazoid Layout Guide
Trapazoid Layout Guide

  1. Measure out the bottom width first on one of the edges
  2. Make a perpendicular line at half the base width
  3. Measure and mark the depth
  4. Draw a rough top line
  5. Use the half way line to mark the top points
  6. Draw the sides using the bottom and top points.

When drawing the lines, I flipped the measuring tape over and used it as a straight edge.

Drawing Lines On Foam
Drawing Lines On Foam

An old serrated steak knife was used to cut the foam.

Using Old Steak Knife To Cut Foam
Using Old Steak Knife To Cut Foam

Cut Fireplace Insert
Cut Fireplace Insert

Inserting the final cut piece required a little patiences and some brute force. Because the foam was pretty thick, I was able to really wedge it in place without worrying about it breaking.

Installed Foam Piece
Installed Foam Piece

I no longer feel the draft from that fireplace, and the foam piece is not visible unless you look up into the fireplace.


21 thoughts on “Sealing A Drafty Fireplace”

  1. Chris,
    There are chimney balloons that are made to plug the flue down by the fireplace damper as well. Kind of like the plug you fashioned here from rigid foam board, but the balloons inflate into place in the flue. You may find them to be easier to put in and take out.

  2. Pingback: Adventures In DIY
  3. Pretty nice post. I just stumbled upon your blog and wanted to say that I have really enjoyed browsing your blog posts. In any case I’ll be subscribing to your feed and I hope you write again soon!

  4. You can cover the vents with a magnetic product called Draft Decor. It seals out all of the air coming in over and under the doors. You can buy it online.

  5. hi, thanks for the idea!! will try this weekend! wish i had seen this before i bought the 70$ chimney balloon – didn’t fit in my fireplace and lost 40$ between shipping it back and restocking charges…

  6. Hey, I just also read another idea – – to buy some rolled insulation, roll it tight, then put it in the chimney, let it go to expand to press against the chimney walls. Has anyone tried this?

  7. I had a house with six fireplaces and sealed them all with fiberglass rolls inside trash bags. Worked great.

  8. tried the fiberglass rolls last winter and it still leaked a ton of air, hoping the flat insulation board works better, thanks for the idea!

  9. I spent $50+(shipping) on a chimney balloon. It did not work out well in my option. It was very difficult to install, but it did hold for about a year. I still felt some draft after it was installed. The tube dropped down frequently and the kids liked to pull on it, so it didn’t stay in once they knew it was there. I will try the rigid foam next.

  10. From the (incomplete) instructions:
    “Inserting the final cut piece required a little patients and some brute force. ”

    How small should each patient be (size of a jockey, circus midget, mini-me…?) and what kind of disorder (physical, mental…both?) should he have? How many patients will I need (semi-private room’s worth, a hospital wing…)?

    Finally, just how brutal with them should I be?


  11. I like this idea. I’d like to run a Duraflame Electric Log Heater in my fireplace. I thought the balloon plug is a great idea but they don’t recommend heat as I’m sure it will expand the balloon and pop it. So this rigid foam idea looks good. the question whether the foam becomes a fire hazard or will it throw off any toxic gases.. The Duraflame unit works on 4600 BTU.

  12. i think I’ll insert the insulation vertically, with maybe a sheet of plywood over that, painted with multiple coats of gloss black. That would make it clear to the next owner to NOT start up the gas log. I’ll leave the gas valve handle INSIDE the fireplace, as well.

  13. Hey there, I tried this as a cheap option to stop the draft from our chimney and it worked well. We unfortunately had a flue handle somewhat in the way. But cutting the foam a little larger did the trick. The foam just molded around it. Saved me minimum $40.

  14. I tried this, but I still draft so removed the screen cut a piece of plywood the size if the opening. Covered the front with black bristol board and inserted in the opening and placed weather stripping around the edge. I popped the scene back on you can’t see the board because the screen covered the edges. Works like a charm.

  15. Hello from 2019 (just). After a successful run of a few years with a chimney balloon in the fireplace in our lounge, it started leaking, but I managed to buy a replacement. Unfortunately, after a couple of days, it was clear that the new balloon had developed a slow leak, so it would initially stay in position, but deflate after about 12 hours 🙁

    Wasn’t sure if PVC could be patched with a bicycle puncture repair kit. But I came across this page and remembered I had some 25mm insulating foam. Even more fortunately I released that there was a portion of the flue I could reach that was square rather than trapezoid (the lower portion of the flue was a bit too irregular) so I tried this.

    I did need to wear goggles though as gritty dirt was falling on my face, but otherwise pretty straight forward.

    Seems to have worked really well 🙂

    Thank you and Happy New Year!

  16. That’s great! Glad you found the post helpful and you didn’t have to spend any more money 🙂 Happy New Year!!

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