Can you offer advice on constructing a well-insulated concrete slab?


Can you offer advice on constructing a well-insulated concrete slab?

Asked by Garth Hawkins

I hope to build a small one-bedroom house in the future and follow many of the passive house attributes. I plan on having a well-insulated concrete slab, and most designs use several inches of foam under the slab, which I plan on doing. My question is how to insulate the perimeter of the slab to prevent heat exchange from the interior of the house to the exposed outside vertical wall of the concrete slab? Most designs recommend insulation between the soil and the outer wall of the concrete, but I'm concerned about termites in the insulation (and then the wood walls) as well as constant weather/water action on the insulation. Is there a good way to insulate the "inside" of the slab perimeter without having exposed insulation in the house floor? And how to a prevent having three concrete pours (footing, vertical "stem" walls, and living area floor)?

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Robert Hartford's picture

This part of construction can be tough, but if it is well done, the results in energy performance are quite noticeable.

I use two techniques for structural insulated slabs.

  • One is a form-left-in-place method,
  • the other is masonry retained.

Visit my site at and find your way to plan 2559S, the full detail page. At the bottom of the page, you will find some photographs of a very successful insulated slab construction that is energy efficient and structurally sound.

Drop me an email from that site and I will send you a pdf of the technique with a few more details. Please reference this comment.