In the basement of our new build we will be pouring a footing all the way down the middle to support a load bearing wall. For the rest of the basement we will be putting 2" of insulation under the slab, however, I realized that for the load bearing wall footing there will be no insulation there. Is this a thermal bridge to the ground under the house that partially defeats the insulation? Is there a way to mitigate this? Or is this very minor and I shouldn't worry about it? Thanks.

I'd recommend putting something there, even if it's 1", except directly under the lally columns. Same on the wall footings, and a vertical piece (2" thick if possible) between the floor and the slab. the idea is to have the slab separated from the ground and the concrete walls which are in contact with the ground. The more you can isolate the slab, the closer it will stay to room temperature.

Posted By Bob I on 09 Oct 2015 04:51 PM
I'd recommend putting something there, even if it's 1", except directly under the lally columns. Same on the wall footings, and a vertical piece (2" thick if possible) between the floor and the slab. the idea is to have the slab separated from the ground and the concrete walls which are in contact with the ground. The more you can isolate the slab, the closer it will stay to room temperature.

Thanks Bob. I'm not sure how this would be done, or maybe we're talking about different kinds of walls. There's going to be a 2x6 load bearing wall built. Is it possible to put the bearing wall over foam? I thought this wasn't allowed but please correct me if I'm wrong. Thanks.

I don't know what is allowed where you are, but you could conceivably leave intermittent holes for direct support, perhaps every 6" or 12" x the wall width. The smaller the thermal bridge the better.

Is the floor heated, or just a plain slab? Is the slab poured over the footing? What kind of soil are you on? Footings are usualy poured neat over undisturbed soil or compacted approved fill.

The load bearing wall is going to cover a good chunk of the footing. IF the floor is not heated, the downward loss thru that center footing is not going to be a lot, You could put foam alongside each side of the center footing below your slab foam which would help direct any heat path down thru the center of the footing, which is the part covered by the wall...

I had the same dilemma when I did my slab. I have no insulation under the thickened footing areas. Looking back I wish I had consulted an engineer and done it differently. What if instead of a 4" slab with 8" thickened footings the entire slab was 6" thick with continuous insulation and upgraded rebar to handle the bearing loads at those locations? I don't know if something like this is even feasible or if any engineer would ever go for it. I think it's strange people say not to insulate under the footings because the insulation will compress. So then you have insulated and non-insulated areas compressing differently under the same slab. Seems like you would be better off to insulate the whole slab continuous and get even compression under the entire slab. I have heard people (passivehaus builders) having to fight with inspectors and building departments over the use of foam under footings

There are foams that can handle more PSI than many soils. So a "no foam under footings" doesn't make sense. Maybe with some pea gravel to allow the foam to sit evenly on not perfectly flat/smooth soil.

HP Home and jonr's solutions are both valid. The sad thing is you might need an engineer to convince the builder and/or the building inspector that it's OK, and that might cost more than the heat loss. But it's not just the heat loss--it's also a surface on which you might get condensation in the summer.