Introduction: Basics - Gas Log Fireplace, Heat Thief

I've always wondered if the gas log fireplace was a useful feature of my home. When I bought an infrared thermometer, I discovered that it is not. It is a heat thief. The temperature at the fireplace on a cold day is around 59 degrees Fahrenheit. The rest of the room is about 72. I decided that something is going on here that is not in my favor. So, I decided to close the fireplace. There are actually two places that you can close this type of fireplace--inside at the fireplace opening and outside at the flue. I did both.

Step 1: Tools and Material


  • Thermometer - any room thermometer is adequate for this project.
  • Tape measure
  • Ceramic knife - this is ideal and makes the best edge. A box cutter works, though.


  • One foam plastic insulation board 1/2" x 4' x 8' - if possible have a building supply associate do the cutting. They have a giant T-square that they can use for cutting a nice, accurate edge. Also, two small, cut boards are easy to transport in a small car.

Step 2: Check If It Is a Problem

The best first step is to determine whether or not your fireplace is actually losing heat. I happened to buy an infrared thermometer, but a normal alcohol thermometer is just fine. Place it at the bottom of the fireplace, and check it a few minutes later, if it is significantly (10-15 degrees) lower than ambient room temperature, then you are losing heat. Time to do something.

Step 3: Measure the Openings

Carefully measure the openings that need to be closed. Measure the width at the top and bottom to make sure that it is rectangular. If not take the measure of the trapezoid. Likewise, measure the height at both sides.

Step 4: Buy the Insulation Board

I went to my local building supply, purchased one 1/2" x 4' x 8' foam plastic sheathing and had the associate cut the insulation board to the dimensions that I acquired previously. You should have two sets of measurements. One set is for the smaller opening and the other is for the larger opening. The insulation board will be 4' x 8'. I strongly suggest that you bring a ceramic knife for the associate to use when he/she cuts the board. The box cutter will make a mess of the edges. A ceramic knife can produce a nice, smooth edge. A word of caution: DO NOT try to bend the ceramic knife. Ceramic knives do not bend, they break. Only cut very straight lines with the ceramic knife.

Step 5: Before You Install...

Before you install anything in the fireplace, make sure that the gas is turned off inside the fireplace and at the valve in the floor--see the picture. The pilot light should not be burning at all. This is critical. You do NOT want a house fire because of this project.

Step 6: Install the Smaller Insulation Board

This insulation board will fit in the smaller opening. It will be the main barrier to prevent loss of heat. If you measured it correctly, it should fit in the smaller opening, and be held there by friction. It should not need anything to fasten it.

Step 7: Install the Larger Insulation Board

This is a secondary barrier to heat loss. Again, if you measured it properly and it was cut to size, it should stay in place by friction and not need any kind of fastener to hold it.

Step 8: You Are Done Inside

Your finished project should look like the picture. You can check the temperature. It should be the same at the top and bottom of the closed opening. That temperature should be about the same as ambient room temperature. The heat thief has been foiled.

Step 9: You Could Decorate Your Project

I had thought of putting this picture to replace the dull pink of the insulation board, but my wife put a stop to that idea. Your family might be OK with it though.