I have heard varying perspectives on how important foundation and slab-on-grade insulation is. I know that Joe Lstriburek of BSC has a rule-of-thumb of 10-20-40-60 (slab-foundation wall-exterior wall-roof), but I have also heard that insulating below the frostline is unnecessary due to the fact that the soil that deep is a constant 55 degrees. I would appreciate any feedback you have about this, as I tend to err on the side of 10-20-40-60, but would like to know if you feel this is overkill with regards to the elements in contact with earth below the frostline. FYI, I am in Madison, WI, so we do get considerable seasonal temperature swings. Thank you for your time,
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You're in climate zone 6, which is relatively cold. The short answer is that you should definitely install continuous horizontal insulation under your basement slab. Such insulation is difficult to retrofit, and it will never be cheaper to install than it is during the original construction.
There are many factors that affect the soil temperature under your slab, so I'm not going to identify a temperature. But assuming your estimate of 55°F is correct, that is certainly an argument in favor of insulation. If your basement temperature is 68°F, then heat will flow continuously through your slab to the ground below all winter long. The point of the insulation is to slow down that heat flow.
There are two other reasons to insulate a basement slab:
1. To improve the comfort of the space in case it is ever finished, and
2. To reduce the chance of summertime condensation on the slab. (Unless you install continuous horizontal insulation under your slab, you really shouldn't install carpeting on a basement floor, because of the chance of moisture accumulation and mold under the carpet.)
I would stick with the 10-20-40-60 in Madison, with 7,499 hdd you will need all of this. Do not scrimp on foundation and sub-slab insulation, this really adds to the comfort of a house and will minimize temperature stratification. Pay special attention to airtightness, no more than 1.5 ach50, 1 would be better.
Kris: Also keep in mind that your ground temp below 3' or so may never be above the temp of the inside of your basement, so the heat loss is going to happen 24/365, while the losses to air are for a shorter time period. Someone in your area will have annual soil temp profiles; either local soil engineers or folks who drill for various reasons. Check them out and throw that into your decision criteria. I would bet that if you run some numbers on heat loss you'll be encouraged to install about 4 to 6" under the slab, and more outside the foundation wall. As said above, now is the time.