Boy, the more you think about, the more questions come up! My house will have a stairwell down to the basement. The entire house is ICF, so technically it is insulated, but not heated. There will be a door at the top of the stairs, so the stairwell will be isolated from the living space. That being said- is there any value in insulating the walls that face the interior? What about the door? It won't be a typical sealed exterior door, but a basic interior door. The temperature range in the basement will generally run between 55 and 75, although it could vary more in extreme weather. Does anyone address this, or is it not worth it?

Okay, so basement and upper floors are all ICF and the basement appears to be unheated. What is the insulation between the basement and the main floor? What would make the basement temperature get to 75F?

I've been told that I don't need to insulate the main floor because the basement is insulated, but I will probably put up batts and double 1/2" drywall (for fire resistance). Here in SC, uninsulated basements run about 55 degrees in the winter; 75 in the heat of summer. The basement is a walk-out, so not fully in the ground, and I have no idea yet what effect the ICF construction will have on basement temperatures. The concrete slab is not insulted- I hope I don't regret that later on. I will eventually finish the basement, and use a mini-split for HVAC, but that could be years from now- or never.

I would put a better door on the basement access if you don't intend to heat/cool that area for the foreseeable future. Something like an exterior insulated metal or fiberglass door - the same type of door you would put between the house and the garage.

You can run heat loss calculations on your basement for various times of the year to predict what temperatures you will see down there. It's really just a six-sided box.