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.


By | 2016-11-27T11:54:10+00:00 November 26th, 2007|Conservation|16 Comments


  1. JR November 3, 2008 at 8:37 am

    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. DiyGuy November 3, 2008 at 6:36 pm

    Thanks for the tip. Let me know if you’ve had a good experience with a certain brand.

  3. Adventures In DIY November 3, 2008 at 7:27 pm

    […] Inexpensively sealing a drafty fireplace […]

  4. school grants December 4, 2010 at 4:08 pm

    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!

  5. DiyGuy December 6, 2010 at 7:06 pm

    Thanks for the comment. I’ve been busy with projects. 🙂 I’ll get some more posts up.

  6. Christy January 4, 2011 at 6:01 am

    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.

  7. cett January 17, 2011 at 2:03 pm

    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…

  8. Sarah February 3, 2011 at 6:58 pm

    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?

  9. Jim October 12, 2012 at 6:26 pm

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

  10. Carrie December 1, 2012 at 8:52 am

    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!

  11. mnights April 6, 2013 at 11:48 am

    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.

  12. Dr. Odd January 18, 2014 at 12:07 pm

    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?


  13. DiyGuy January 18, 2014 at 4:53 pm

    HAHAHA! Thanks for catching that.

  14. Put an End to Fireplace Drafts October 27, 2014 at 4:02 pm

    […] the very least you can cut a piece of rigid foam insulation to fit under the flue. Just remember to remove it before you light a […]

  15. dean gould January 3, 2016 at 6:16 pm

    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.

  16. Don Olson November 13, 2017 at 11:49 pm

    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.

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