We are building an ICF house and want to include a root cellar. Our original idea was to create a room in one of the corners of the basement as the root cellar. We would insulate the partition walls and ceiling and strip the insulation off the ICF walls (both exterior and interior) on the bottom 4 feet of the 2 exterior walls of the root cellar. This seems like it would work fine for the root cellar, however, this got me thinking about how this would impact heat loss for the rest of the house. The root cellar itself would be insulated from the rest of the house, so the heat loss there would be minor. However, the concrete that has the insulation stripped would now be about 40 degrees (we live in a cold climate) and that concrete is connected to the rest of the concrete that is surrounded by insulation in the rest of the ICF wall. Since concrete is such a good conductor that would effectively suck heat away from the concrete in the rest of the house. I'm not really sure how to figure how far through the concrete the heat would travel from this uninsulated portion, how much loss there would be? Does anyone have any thoughts? Is it a bad idea to do this? Thanks.

I would consider keeping the ICF wall foam intact and just not insulate under the slab in the root cellar area. Put a thermal break in the slab between the root cellar and the remainder of the basement slab. With the walls and ceiling being well insulated, that uninsulated concrete slab in solid contact with the undisturbed earth should provide a good root cellar environment. And that will keep any possible condensation at the floor rather than on the walls.

Is the basement heated? IF not, and depending on how well the ceiling/main floor above is insulated and how much heat leaks down from above, the concrete in the ICF is likley to settle out near earth temp anyway with no heat in the basement to weep thru the ICF foam and keep it from being so...

I don't think you have to strip the ICF foam anywhere. I think if you insulate the top and sides other than the ICF wall, that root cellar space will also approach earth temp as long as there is nothing in there to provide any additional heat...

That way you don't have to do anything special with the waterproofing on the outside either...

Interesting ideas, thank you. Yes, half the basement will be heated. The area the root cellar will be in will not be heated, but the interior partition walls are perhaps R-2 (drywall) whereas the exterior ICF walls are R-25 so I would expect that even without heating the "unheated" part of the basement it will be fairly close in temperature to the rest of the house due to heat migrating through the walls and ceiling. I would insulate the root cellar room from the rest of the house but it seems to me like if I insulated that room to R-25 from the rest of the house the stable temperature would settle at pretty much halfway between the ground temperature and the house temperature, though I could be wrong about that. arkie6, the thermal break in the slab is an interesting idea as yes it seems like that would give good earth contact if it weren't insulated there. Do you have any idea for how to provide that thermal break in the slab? Thanks.

Put 2" of EPS or XPS foam oriented vertically directly under the interior wall partitions. Cut the height of the foam to span from earth to top of slab. Drive rebar dowels into the soil every couple of feet along side the foam and punch tie wire through the foam and twist it to the rebar to hold in place. When placing the concrete for the slab, you will have to evenly apply it both sides of the foam to keep it from shifting. I would probably use a 2x6 partition wall here for insulating value and capability to span across the 2" of foam. Secure the bottom plate of the 2x6 wall (use a treated 2x6 bottom plate) to the concrete on either side of the foam using Tapcons, i.e. blue screws. If you use an exterior type insulated door for access to this room, it will have a threshold that will span across the 2" of foam.

I'd just lay foam and flooring over the slab and I wouldn't rule out adding some AC. Ie, create a well insulated, underground, walk-in refrigerator with the waste heat going into the house (Winter).

It depends on what kind of root you will be storing e.g. apples should be refrigerated but damp.

Most will store vegetables so keeping it cool is paramount. Your onions, squash and pumpkins like it warmer so I concentrate on cold first. Your deep earth temperature is likely in the 50F. range so you have to insulate below the frost line to make the earth work for you. Root cellars are cold weather food storage spaces and use controlled air flow to maintain ambient temperatures in the 32-40F. range. If you use natural convection a vent at the top and intake over the door is typical. This can be automated or fan driven. Mine has a double-door air lock on the outside and the house-side to keep air flow under control.

There is 4' of earth over my root cellar and a minimum of six on the sides. To simulate dry-earth, about R-10 for dry soil, you will have to insulate. Mineral wool or XPS work well for damp applications. We use 2" of XPS for most walk-in coolers and for frost protection below a walk-in freezer.
Second, your carrots, beets, parsnips celery, leeks and the like are happy with 90-95% rH. My grandmothers kept them in sand. Naturally the key factor, once you have decided what you are keeping, is the outdoor climate. will have to insulate accordingly.

Third and most important, is proper ventilation. Careful design with a pitched ceiling leading to the vent will keep the air flowing and dew point under control so no spoilage occurs.

http://www.agf.gov.bc.ca/resmgmt/publist/Leaflets/FruitVeg/331-50.pdf

Thanks for all the detail.