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.