I bought a house built on a concrete slab on grade. It's an older house, so no part of the slab was insulated. At some point in the past, previous owners decided to pour concrete around the entire perimeter of the house, effectively increasing the size of the foundation slab so that it protrudes two or three feet out beyond the walls (this appears to have been done at least partially for termite control).

My problem concerns how to insulate the exterior of the slab. Obviously I would have to dig to expose the slab edge that I could insulate, but would this even be possible given that the slab has effectively been extended outwards? Even if I insulated the perimeter of the extended slab with foam boards or a 12" perlite-filled trench or something, the top of it (that you can walk around the house on) will still be exposed to the exterior temperature.

Should I just cover the exposed slab surface on the floor with polyiso (R-7) foam insulation boards instead? The final floor coverings will be mostly wood, with some tile and carpet.

The walls are already insulated to about R-10 or so.

  • For termites? That's not going to be effective at all. Are there any other reasons for that slab? What's your climate? What does the thermal mass of the slab do to help (or hurt) your energy costs now?
    – Bryce
    Jan 27, 2014 at 9:06
  • The climate is high desert Albuquerque, NM. I don't know about the current energy performance since I just bought the house and am doing some work to it before I move in.
    – iLikeDirt
    Jan 27, 2014 at 15:08

1 Answer 1


Insulating out 2-3 feet won't make sense, as particularly on high desert nights far too much will be sucked right out of the extended slab to the air.

Removing the slab extension should be explored. It has no valid termite purpose, other than to provide good habitat and a staging point for termites looking to crawl up one of the inevitable cracks. French drains on the other hand help keep the soil or sandy soil dry, reducing termite habitat.

A local insulation expert will have to comment on the cost/benefit of traditional slab sidewall insulation: have them show you before & after energy statements as proof their products have a positive payback in your microclimate.


Floor coverings are fine, but in your climate you have extreme desert temperature swings, and the thermal mass of the slab is helpful in both heating and cooling seasons.

  • Thanks, Bryce, that makes sense. Removing the concrete ring around the house doesn't looks like it would be too difficult to accomplish. I worry about foam insulation right up against the (smaller) slab, though; wouldn't termites be able to go right through that? What do you think about the idea of insulating the (smaller) slab with a 12" wide trench of perlite? The perlite would permit soil drainage and act as an insulator, too (R-2.7/inch, inspectapedia.com/interiors/Perlite_Insulation.htm). Scoria is possible as well, and blends in well with typical southwestern landscaping.
    – iLikeDirt
    Jan 27, 2014 at 21:58
  • Perlite sounds interesting, but I can't give any guidance. Termites will go through everything: you need several inches of clear foundation above the top of whatever you use. The energy and termite goals are at odds here : greencomplianceplus.markenglisharchitects.com/interviews/…
    – Bryce
    Jan 27, 2014 at 22:18

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