I might be replacing a foundation and I'm getting bids to do it. The poured foundation can have some foamboard insulation dropped in before backfilling. I've heard some energy efficiency experts say that insulating on the outside of the concrete is better because you get the benefit of the thermal mass from the concrete which helps neutralize temperature swings.

I intend to keep the basement semi-finished, but it wouldn't be that expensive for me to put some foamboard on the inside of the concrete as well. I would gain even more R-value (albeit below grade in climate zone 5), but I guess I'm losing the benefit of the thermal mass.

If you were in this situation of replacing a foundation and you could choose how to insulate it, how would you do it?

I'll let the experts weigh in on the interior vs. exterior vs. both in regards to mass, but since this is a new pour wouldn't the use of ICFs be something for consideration?

I will let the experts weigh in on this too…

I will say that there is a diminishing return on investment in using insulation for below ground level basement walls once you get below a couple feet from the ground surface level. Heat loss is proportional to the delta T between inside and outside the wall (and between the inside and ground side of the floor). Once you get below a couple feet from the ground surface level, the soil temp is relatively stable and typically in the 50s F range. However, the first couple feet of soil can approach ambient outdoor temps and this is where having wall/floor insulation can significantly reduce your heat loss.

The majority of heat loss occurs from the building’s exposed perimeter (i.e., wall or floor exposed to the ambient outdoor temps). This is why using edge and perimeter insulation is very beneficial for grade-on-slab floors. If the floor is NOT heated, relatively little heat loss occurs from the central floor area because the floor temp is somewhere between the warmer soil temp and the indoor room temp (i.e., a relatively small delta T and heat loss). So there is a diminishing return on investment in using insulation in these central floor areas when the floor is NOT heated. When the floor is heated, having adequate insulation beneath the entire floor becomes more critical because the floor temp could be in 70-85 F range (i.e., a relatively large delta T and heat loss). Of course, the devil is associated with the “relative” details and your overall design objective/budget.

A minimalist R16-R20 ICF is pretty decent for a zone-5 location, combined with ~R8-R10 of EPS under the slab. Unless your labor rates are pretty low, it's minimal (or even negative) cost savings to add rigid foam to an already-poured wall.

While keeping the thermal mass of the foundation fully inside the thermal envelope of the house is more efficient, it doesn't trump having 2x the R value, and putting R16-R20 outside the foundation presents a more difficult flashing & finishing situation on the exterior. Even with an R16 ICF you'd still have a 2" bump-out to deal with (some of which you may be able to absorb by moving the foundation in 1-2", cantilevering on the foundation sills, if the engineering analysis says you can get away with it. But moving it in 3-5" for an all-exterior approach may take more re-engineering than it's worth.

If you go with a rigid panel approach after the fact, you can use EPS on the exterior, and fire-rated Thermax iso on the inteirior. Use an EPDM or metal capillary break between the concrete & foundation sill, and consider putting a capillary break between the footing & foundation wall as well to minimize ground water wicking up the concrete to the susceptible wood, and efflorescence on the above-grade exterior.

sailaway: Insulating a foundation is not all about net-present-value on heating energy savings- its also about basement moisture/humidity control and mold potential for anything stored in a semi-conditioned basement. In zone-5 locations east of the Rockies the subsoil temps are well below the summertime outdoor dew points. Insulating both the slab and the foundation walls keeps it all a bit warmer AND drier, both winter & summer, reducing the mold potential.

Yes, excellent point Dana. Don’t know how I could have forgotten about the humidity east of the Rockies…that was one of the reasons I moved west of the Rockies as soon as I was able.