Basement slab insulation
Building a cabin in Montana (climate zone 6) which will have an unfinished “basement” functioning as a mechanical room containing electrical panel, water cistern, pump, pressure tank, storage, etc. Code requires footings be minimum of 48″ deep so I’l probably make them a little deeper and start my foundation wall at 5′ of depth to gain a little headroom in the basement area.
So here’s the question. I’ll have a direct vent propane heater to provide some heat in the winter to prevent freeze-ups but I don’t expect to heat the basement area much above 50 or 55 degrees. Does it make any sense to insulate under the slab when soil temps at that depth are generally in the 40-50 degree range year-round?
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Replies
David,
A layer of horizontal rigid foam under your basement slab may not save you enough energy to justify its installation cost. But this type of insulation is installed for moisture control, not energy savings.
If you keep the slab warm during the summer, you're less likely to get moisture accumulation on the slab, and your basement won't be as musty, moldy, and stinky.
David- If you are going to the trouble and expense of a 5' tall crawl space, with a slab, why not put in a full basement?
I wasn't clear in my initial posting. The basement area will have about 7' of headroom as the main floor will sit about 2-3' proud of ground level. And regardless of whether I utilized insulation boar under the slab, I would put down a heavy vapor barrier before pouring the slab.
Stephen,
Seven feet of headroom is obviously better than 5 feet. But Stephen has a good point -- you are so close to creating a usable space, why choose a low ceiling? I would aim for 8 feet of clearance to the bottom of the floor joists if I were you.
Your description doesn't change my advice on the rigid foam. The energy savings may be low, but the moisture performance improvements make the horizontal rigid foam valuable. It won't cost much -- and if you omit the foam, you may regret it. It's very hard to retrofit the foam if you change your mind in 3 years.