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Community and Q&A

Insulating a concrete workshop floor

GBA Editor | Posted in General Questions on

My cellar workshop has a concrete floor and fairly low headroom. If I used inch blue board under 1/2 or 3/4 inch plywood, how much warmer would my floor be. Assume average winter floor temp of, say, 40 degree F. House is old contstruction. Furnace is new, forced air with some vents in cellar, so I do have some heat. Cold floor is murder, however.


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  1. GBA Editor
    Martin Holladay | | #1

    I don't know where you are located, but it's hard to imagine that your basement floor is actually 40 degrees, especially if you have a furnace and some furnace registers down there.

    I'll take your word for it, though, and assume that your floor is colder than you want it to be. Maybe some reader will do some calculations to tell you what your floor temperature will be once you insulate it with 1 inch of XPS and 1/2 inch of plywood — but I imagine there are several variables we would need to know to do the calculation. (What is the actual concrete temperature? How far away are your hot-air registers? What is your air temperature?)

    Here's what I would do, however. Buy a couple of sheets of foam and a sheet of plywood and put them down on your floor. Compare the temperature of the concrete to the temperature of the plywood with an infrared thermometer. (One of these thermometers costs $100 or less. It's a great tool to have.)

    The bottom line: 1 inch of foam is R-5, and it will make a significant difference. Of course, 2 inches would be even better.

  2. Riversong | | #2

    With 1" XPS and plywood (I would suggest 3/4" for a shop floor on 16" oc sleepers), not only will the floor be close to the air temperature at floor level, but it will feel much warmer on your feet as the surface will be far less conductive (concrete is 15-20 times as conductive as softwood).

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