Currently in the final planning stages of my new house ("raised ranch" style), and the first floor is going to be slab-on-grade. My current insulation plan has 2" of XPS under the slab across its entire area (with a vapor barrier and 4" of 2B stone beneath). However, when I went to put the compliance information into ResCheck, that application only had parameters for entering perimeter insulation (e.g. from the edge of the slab, vertically down to the footers). That leads me to believe entire-slab insulation isn't that common.
My question is, from a thermodynamic standpoint, is it pointless to insulate the field of the slab if the perimeter is insulated, or do you still get a lot of energy migration down through the stone into the dry earth below? Does most of the energy loss in a basement occur from conduction of the slab to the foundation walls to the exterior, or is there still a significant amount of vertical conduction into the soil?
Best Answer
Depends on your climate. Ground temps more than a few feet downtend to be close to the average yearly temperatures. Here in Alberta (10,000 degree heating days/yr) that means that 8 feet down it's about 7C. This makes for a cold basement floor. Eventually the soil below the floor will warm up -- but that can take years. And the equilibrium will be somewhere between room temp and normal soil temp for that depth
Usually perimeter insulation schemes are used to make an less expensive floating slab foundation. In a seriously cold climate, however, a perimeter insulated foundation will still freeze if the building is left unheated.
A first order approximation is fairly easy. If you have a 1000 sq foot basement and a 20 degree F differential between inside and ground temps then with R1 of concrete it is taking 20,000 BTU/hour. This ignores:
If you put in R10 worth of foam, this is reduced to 9% or about 1800 BTU/hour.
20 BTU/hr = 1/4 million BTU/day or about 2.5 Therms. or about a quarter of a gigajoule.
Here natural gas costs about 3-4 dollars per GJ, so to heat that slab is about $1000/year instead of $90/year with R10.
Is raising the insulation from R10 to R20 worth it? Probably not, unless you have a bunch of foam that fell off the truck. An extra R10 would save out about 90% of that $90 would would only be another 81 bucks.
Here a common practice is 4" of foam for the top 4 feet of basement, and 2 inches below that, and under the slab. Not sure how it's detailed where wall meets floor.