I have a barn-garage type outbuilding (I believe it was a small barn originally; it has that kind of tall, roomy upper loft). It's about 40 by 20 feet. It's got a concrete foundation and the lower part is divided into two equal sized rooms.

I'm thinking about taking one of the lower rooms and making it a little more hospitable to hang out in and do projects. It would mean patching some wall, adding real doors, and insulating and sheetrocking it.

I want to keep the floor as bare concrete so I can anchor machine tools etc to it for ultimate rigidity. But if it's sharing a concrete foundation with the other room, is heating it in the winter going to be practical? (This house is in the US Pacific Northwest; it gets fairly cold.) Is the floor going to be a giant heat sink?

Are there are any other things I'm not thinking about here, where moisture or heat concerns might bite me if I'm just insulating and heating one room in this larger structure?

  • 1
    What's "fairly cold", on average? 20F and -20F are quite different.
    – isherwood
    Mar 8, 2019 at 21:32
  • How about covering parts of the floor with wood and only leaving those places open where you'll put your machines? Covering half the floor got to be better than covering nothing at all.
    – Mast
    Mar 9, 2019 at 4:51

1 Answer 1


I'd have no concerns about doing what you describe. I've seen it done many times here in frigid Minnesota. Build your walls (using treated lumber and suitable nails where it contacts the slab), insulate the walls and ceiling, and line the entire thing with 4 mil poly sheeting.

Yes, you'll lose a bit of heat through the floor, but since your heat source is above the floor it won't absorb much anyway. The cost inefficiency is negligible unless you're extrapolating out a few decades. The primary sink isn't the connection to the unheated slab area anyway--it's the ground. The old barn slab almost certainly doesn't have insulation under it.

You could always overlay the floor between the walls with foam and plywood or one of the ready-made subfloor products if you wanted maximum energy savings.

Have fun with your project space. I'm envious. Just don't asphyxiate yourself with homebrew heating equipment.

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