Hello all,

I will be building my home on a full basement. The bedrock is at 2 feet below ground so my basement will mostly be above grade.

I was planning on insulating from the outside, but since i cannot insulate under the footing, this would make a thermal leak at the footing. I have decided to insulate from the inside because of that.

I will have a concrete floor and will use passive solar to heat it. since the floor will be mostly at ground level and I will use it as thermal mass, i suppose i would be better to insulate under the slab. how much? I am in a more than 9000 heating degree day...

would it be better and more effective to do just horizontal perimeter insulation for the slab while still insulating the perimeter wall? maybe 4ft horizontally at the slab level on the outside?

thanks


Alexis

Most of the time you can get away with thermal bridges and such in a basement because it it so far underground, but in your case you are very close to the surface. I would make a foam board and taped plastic sheeting "bed" under the slab and up the edges of the slab, so that the slab isn't actually in contact with the ground or the exterior walls. Then you just insulate your block walls to the same level as your upper floor exterior walls. This will not work if you have a lot of ground heave in your area, since the ground under the slab could push it up or down relative to the block walls. The best way to do it is to insulate on top of the slab, but then your would lose all your thermal mass and you wouldn't be able to heat it, which is one of the parameters you gave us to work in.

Build an all ICF house and put 4" of high density extruded polystyrene (XPS) under the basement slab. Only insulate the footings if you need shallow frost footings.

Combined with your passive solar, this would be a very energy efficient, comfortable home. If you are concerned about the thermal bridge from the footing to the basement slab, make your walls 4" taller and put the XPS on the footing, then with ICFs no thermal bridging. We also do a wick resistant footing, which lessens moisture wicking from the footings into the walls. Just waterproof the top of the footing, before you do the ICF walls. I don't know how much good it does, but LEED thinks it is important.

thanks for your answer,

i understand that i need to insulated the slab, but can i insulate the basement from the outside? It seems that, if for example, i build an icf basement, i will only be benefiting from the interior insulation because after the first layer, you get to the concrete wich is directly in contact with the footing, wich are directly in contact with the bedrock...

Bingo! Congrats! Your best bet is to add insulation on the inside.

Of course, opinions vary on this, but I believe that ORNL would agree.

You will get benefit from the outside insulation on an ICF basement. The bedrock will not reach the same temperature extremes as the outside air also the thermal bridging will diminish the higher up on the wall.

Hi aardvarcus,

Like in your case, a significant portion of my basement is above grade. When we were deciding whether to go ICF (and how much) I pretty much reasoned the same way as Brad above. We live in southern Ontario, Canada, where there is quite a bit of frost in the winter and it gets really humid and hot in the summer. We decided to go ICF for the entire house, not just the basement. You can read about it more at www.AllIcfHouse.com. We did not "insulate" the footing, but we will have in-floor heating, with which you end up installing insulation below the basement slab. Regardless of whether in-floor heating is on, the insulation is always there. So, as you suggest, my only "leak" downwards is through one layer of ICF foam, then through the exterior concrete walls, then through the footing, and into the ground. But my [engineering] gut feel was that this is insignificant compared to the leakage through windows and doors. Furthermore, unless you are on a very steep slope, ground temperature at the footing will for most regions be lot more favorable than the temperature of the outside air.

Vlado
www.AllIcfHouse.com