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Community and Q&A

Sub-Slab Insulation and Seismic Performance

andyfrog | Posted in General Questions on

Does anyone know of how insulation under a slab affects seismic performance? And has anyone found any resources comparing different material choices, e.g. foam types compared, or foam vs glavel, etc.

I tried searching a bit before asking and I could only find a few papers and one article about a development forgoing sub slab insulation in California because the engineer wasn’t comfortable with it. There was one paper that discussed a theoretical application where a slippery layer was installed between the two layers of foam and horizontal stops were added at the edges so the whole slab could slide back and forth, but they didn’t do any testing.

There was one paper that modeled XPS but its basic conclusion was “it shouldn’t be a problem but we need field studies”.

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  1. Expert Member


    That's about all I got from the engineers I've worked with on projects. They didn't seem too concerned, but what that was based on was unclear. What they did worry about was too much insulation between the slab and stem-walls allowing repetitive lateral movement during a seismic event.

    Like all code requirements, seismic standards are just agreed to improvements which decrease but don't remove the damage that would be done in a large quake. I suspect there are a lot more remedial steps that could improve foundation performance, but are too onerous to ever gain much acceptance.

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