Questions about a Lstiburek article on slab insulation

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Questions about a Lstiburek article on slab insulation

Attached is a figure from a Joseph Lstiburek article on slab insulation, and copied below are two sections of text from the article. I'd be grateful for answers to a few questions.

The entire article can be accessed at

1) "You make the assembly out of three pieces – a footing, a stem wall and the “flat plate” slab part. Then you insulate it under it and on the inside of the perimeter making sure you uncouple the plate from the stem wall at its edge (Figure 1). This edge thing is a big deal – the thermal bridging that occurs if you don’t do it costs you more than half the thermal resistance of the whole deal."

Why does a slab lose so much heat at the edge as compared to the underside?

In various places the edge insulation is referred to a a "bond break": what does that mean, exactly, and what is the significance of calling it that, rather than referring to it simply as "insulation?'

Lstiburek cites with approval a project in Chicago that features a 2" bond break. Would a thinner one suffice in Zone 4C?

2) "Do you really need to insulate the stem wall if you insulate the edge of the slab and insulate under the slab at the perimeter? Yes and no. Yes in climate zones 4 and higher, no in climate zones
3 and lower. Is this based on a hygrothermal analysis? No. Is this based on an energy payback analysis? No. Is this based on minimizing your carbon “footprint”? No. Get serious. It is based on something that is real. We found that in climate zones 4 and higher if you didn’t do it people felt uncomfortable. It is not a good idea to annoy your clients. Especially if they are old – remember you will get old too – sooner than you expect."

How does the stemwall insulation keep the slab warmer? Does it need to go all the way to the footing?

3) Regarding the stemwall from a structural point of view: it appears that the placement of 2" slab insulation results in the 2x6 sill being bolted to a stem that is only 3/1/2" wide. Is that enough?

bsi059_figure_01_web.jpg42.09 KB
Asked by Ray Sten
Posted Tue, 02/19/2013 - 16:07
Edited Wed, 02/20/2013 - 05:09