Heating a basement in climate zone 6 A
We live in climate zone 6A and plan on constructing a 1,700 sq ft rectangular ranch with r40 double stud wall framing and r60 raised heel truss roof. The windows will be intus brand at around r 7-8. The foundation will be a traditional 8 ft basement with walk-out on one end and rigid foam insulation under the slab as well as on the outside walls of the foundation. We are hoping to heat the home with 2 high efficiency ductless heat pumps for the upstairs but have begun to wonder how the basement factors into our calculations of the thermal envelope. We were originally not planning to heat the basement as it will be for storage. However, since ground temperature is lower than the house, it will definitely be colder than the main floor. We are not sure if the upstairs heat will passively heat the basement, and if so will we need to add the extra 1,700 sq ft into our calculations for the heat pump loads. Or should we insulate between the upstairs and the basement, and if so, how much is recommended to maintain the tight thermal envelop of the house and maximize the efficiency of the heat pumps. Thank you for any thoughts!\
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First of all, I hope that you can update your profile so that your screen name is properly displayed. Here is a link to an article that explains what you need to do: Advice to GBA readers with goofy screen names.
Second, your basement won't freeze. Whether or not you want to heat it depends on how it will be used.
Here is a link to an article by Alex Wilson discussing basement temperatures: Heat-Pump Water Heaters in Cold Climates. Alex's basement has no heating appliance. In fact, is has "negative heat" -- a heat-pump water heater that acts as an air conditioner. Still, Alex's basement in Vermont didn't freeze.
With the basement walls & slab insulated there shouldn't be much of an issue, as long as there isn't insulation in the basement ceiling isolating it from the fully-conditioned space above.
The heat load of the partially below-grade 1700' walk-out basement isn't going to be as high as a fully above grade 1700', but it's worth calculating what that load would be, at both a comfortable indoor temperature, but also at something lower, say 45F, to get a better sense of it. You will always have some heat loss through the first floor into the basement, but with an insulated foundation it's not going to be much.
Thank you for all of this!