Thermal break in the foundation

Submitted by Andrea on March 31, 2011

Sigh... every time I think I have some detail of our house planned, the rug gets pulled out from under me. This time it's the foundation plan, which had been nicely settled since September. The problem is that we're on a sloping lot, but it doesn't slope steeply enough to have a walk-out basement/garage. So we decided to do a slab on top of a 4-foot frost wall. We were going to follow one of the standard approaches, but Ted lost sleep thinking about where the dew point was going to hit inside of the wall (condensation + studs + tightly-sealed house = mold), so we needed to tweak things. Marc suggested a material he'd recently heard about called FoamGlas. It's a fairly nifty product: a strong, insulating block with good compressive strength. Just the thing to provide the thermal break in our foundation, which meant the dew point would hit inside the exterior rigid foam insulation where there's no risk of mold. Here's what it was going to look like [click image for larger version]: Both Marc and our structural engineer approved, so I felt good about the plan. Ted, however, was a little skeptical, since it's a fairly new product in the US. We called the technical contact at Pittsburgh Corning today to ask some questions, and it turns out that FoamGlas has a cousin in Europe called Perinsul which is designed for our exact purpose and has a much longer track record. Perinsul has higher compressive strength than FoamGlas, and it's also pre-coated with an impervious seal. We could have sealed the FoamGlas ourselves with some asphaltic mastic, but the lower strength and the lack of a history for our application make it a non-starter for Ted. So now we're back to the drawing board (i.e. Google searches & SketchUp). Pittsburgh Corning is willing to import some Perinsul for us from Belgium, and I asked them for a price quote, but it seems absurd to have foundation blocks shipped overseas (even lightweight ones). The only upside is that it might convince them to start manufacturing these babies in the US. We asked Marc for some alternatives, and he proposed autoclaved aerated concrete (AAC). AAC is swell stuff (we were familiar with it in Arizona under the name E-Crete), but it's not manufactured in the Northeast. I spoke to a helpful sales rep today who will send me a quote from a factory in Florida, but it's also not an ideal solution. Marc also suggested some bad-ass industrial blocks made by General Plastics, but he acknowledged that they're very expensive and one look at the site confirms that they're total overkill. The General Plastics site was hilarious, BTW. It had an interactive product finder with choices like "Is this an application where you are trying to keep something hot (but under 250 degrees F)?" or "Is this an application where you are trying to keep something cold (+40 degrees F to -40 degrees F)?" And that's just from the section on Thermal Insulation — there are other question trees as well. It's like a "Choose Your Own Adventure" for evil scientists! Planning this house has been a crazy amount of work for me, but fortunately I can rest once all the specs and plans are finished. Oh, wait, maybe not...

Submitted by Nick Barnes (not verified) on April 1, 2011

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What goes outside the 4" rigid foam, on the outside of the house? Some sort of wooden cladding?
(I think you told me this before)

Submitted by Andrea on April 1, 2011

In reply to by Nick Barnes (not verified)

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It'll either be wooden clapboard or fiber-cement faux clapboard.

Submitted by Dave (not verified) on May 5, 2011

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So why don't you run the rigid insulation down the exterior of the foundation to the footing instead and leave out the foamglas? Termites?
Why 6 inches of rigid insulation inside the frost wall if you have subslab insulation and the foamglas? Looks redundant to me.

Submitted by Matt Holop (not verified) on August 10, 2011

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Appreciate your use of Foamglas in your project. Our approval/certification as a Passive House component has started to take off here in the US. We are not limited to use as a thermal bridge at the perimeter. We are acceptable below grade and as an exterior insulation. Thanks for pushing corporate to include this product as a stock item here in the US or hopefully manufactured here.

? If we were to manufacture/stock Foamglas as a perimeter (thermal bridge) component, what widths and thickness would you recommend?

Thanks in advance...

Matt, thanks for your interest. We wound up not going with Foamglas both because of limited availability in the U.S., and because the cost of building the foundation was starting to look extreme because of the slope of the lot. However, I think it's an excellent product for cases where a slab foundation like the one we've depicted above really does make sense.

When we were considering it for this application, the size we would most likely have wanted it in would have been 8"x6"x??, where ?? is whatever length is easy to deliver but not so short as to be a hassle to assemble. I think we had been talking about 18" sections when we were considering ordering the lower-strength U.S.-made version of Foamglas. The reason for the 8" width is that that's the size frost wall our structural engineer recommended. The reason for the 6" is just to maintain parity with the thickness of the foam insulation under the slab. A different height would have been okay too, just less convenient.

The understanding that people have been using this product in foundations in Europe was definitely a consideration for us in preferring the European version of the product. When we spoke to the gentleman in the U.S. who answered our questions about the U.S. version of the product, he didn't know of cases where the U.S. product had been used in an application like ours. He was pretty confident that it would work just fine, and I am sure he was correct, but it's much easier to have confidence in something when you know that it's been used the way you intend to use it. The prospect of being the first person to try a new foundation technology is a bit daunting, as I'm sure you can imagine. :)