Breaking thermal bridge of dry stacked wall from foundation
hypno7oad
| Posted in Green Building Techniques on
I planning to build a house with a basement soon, and looking into dry stacked concrete block walls with exterior foam board insulation. I’m looking into dry stacked wall technique due to cost, durability, thermal mass, and DIY friendliness.
The issue I can’t seem to wrap my head around though is the connection between the foundation/footers and block wall. It seems to me that this would be a huge thermal bridge, and since concrete only has an r-value of <1, I would assume that all the heat capacity of the concrete would quickly be sapped down through the footings. Are my concerns valid? Are there any ways to break this thermal bridge?
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Replies
Robert,
You may be interested in reading this article: Foam Under Footings.
After reading the link, its explained that it is OK to place foam under the footings. However in diagram 1 of the link, would it be feasible (and easier) to replace or add to, the capillary membrane between the footing and the base of the foundation wall with a layer of EPS (high compression or not). Thus isolating the moisture and temperature of the footings from the foundation wall, which itself (say ICF) would be waterproofed and thermally protected from the soil?
Sal,
Your suggestion -- installing a layer of rigid foam between the concrete footer and the concrete basement wall -- has been tried at one project I know of, Roger Normand's custom home in Maine. You can read details here: Construction Begins — and We Encounter a Few Snafus.
Roger Normand wrote, "Our architect, Chris Briley, cleverly added a 2-inch thick layer of high-density XPS installed at the bottom of the first course of block. The XPS creates a thermal break between the footer and the foundation. ... The foundation crew mistakenly installed 2 inches of regular XPS with a compression resistance of 25 psi rather than the required high-density XPS. Oops. No big deal, though, as only the first course had been loosely laid on the footers. But we also needed clarification on which of three available types of high density XPS was required. Owens-Corning makes Foamular 400, 600, and 1000 versions, with compression strengths of 40, 60, and 100 psi. The answer was we needed the Foamular 600."
If you decide to go this route, make sure to have your plan approved by a structural engineer, as well as by your local building inspector.
What is the reason for placing the foam between the top of the footing and bottom of the wall, as opposed to placing it between the ground and the bottom of the footing?
I am curious as to this approach as well. Isn't the whole point of the footing to provide a more stable base for the foundation wall that is connected to it. Without the key from footing to foundation wall, will the loads properly transfer? This just sounds scary to me...
Instead having structure supported on thinner section under foundation wall, it seems to me to make more sense to surround the larger footing with the insulation.