Optimal insulation for slab on grade
I’ve seen a number of recommendations for insulation for slabs, but few seem to differentiate between basement slabs, and slabs on grade. (Or for that matter, between slabs in a storage basement, versus slabs that people walk on.) For instance, Building Science in their BA-1005 report has a single column for sub-slab insulation on page 10.
My intuition says that a slab on grade should see somewhat greater sub-slab temperature swings than a slab poured below the frost line. Wing insulation probably mitigates the effect somewhat. However, I’m left wondering if I should be installing more insulation than the R-7.5 that Building Science recommends for Zone 5. Any thoughts?
–John
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Wing insulation does mitigate the effect, and greatly decreases the temperature swings. If you have radiant heat in the floor, you certainly want more, as the temperature difference between the slab and the ground is now greater. And a 70 degree living room needs more than a 50 degree basement for storage. The report you mention is for minimum R-values, and you may want to beef it up a bit,say to R-10 to 15, as the labor would be roughly the same, if it's for a living space.
John,
One answer to your question can be found in the GBA article, Can Foam Insulation Be Too Thick?
That article quotes building scientist John Straube, who wrote, “The cost of insulation becomes more than the cost of generating energy for the walls in a typical house in a 7,200-HDD climate at about R-60 (using the Building Science Corporation approach), and slabs [on grade] at about R-20 to R-25, depending the cost of placing EPS (which costs around 10 cents per R per square foot). Basements have less heat loss [than slabs on grade], so the cut-off point is more like R-15 to R-20 for a basement slab. Heating a slab with radiant tubes increases the temperature of the slab from around 68°F or so to 80°F or so on average, so the insulation levels need to be increased by about 50% over this for radiantly heated slabs.”
That assessment was made in 2009, when many experts thought that energy prices would increase fairly substantially in the future. Lowering costs for wind-generated electricity, PV-generated electricity, natural gas, and oil now lead some experts to foresee a low-cost energy future. Pick your own prediction.
The ground under a building is different from the ground under bare ground. The temperature under the building is somewhere between the building temperature and the deep ground temperature. That's true whether you are talking about under a basement or under a slab on grade, once you get away from the edges, or if you have wing insulation. So the depth doesn't matter; what does matter is:
1) Is it a storage space that you can keep at 50-60 F, or a living space you want at 70?
2) Do you want walk barefoot and sit of the floor without feeling cold?
#1 matters more for a slab than you might think. If the sub-slab temperature if 50 F, and you change the conditioned temperature from 60 F to 70 F, you double the heat loss into the slab, or you need double the insulation you need for the same heat loss.
If you have a source for reclaimed foam at low cost it is easy and cheap to go ahead with more than you strictly need.
One exception to the idea that depth doesn't matter is if the soil type and dampness is different 8' deep than a few feet deep, in which case the thermal resistance of the ground itself might be different, perhaps higher for a slab on grade because the soil might be drier. But it's hard to know the soil thermal properties precisely, and better to put in enough foam to be confident it will work well than to count on the thermal resistance of the soil.