Is a vapor barrier necessary under a concrete slab with foam insulation and if so, where does go?

Grady 22
Updated: Aug. 4, 2020

We're soon building a slab on grade foundation for our new cabin in Montana and I've decided to sit it on a 4" EPS foam insulation so the slab will retain some passive solar heat from the south facing windows in winter, avoiding having to put too much heat in to keep stuff from freezing. There's a bed of compacted gravel already down for the slab to sit on, so my question is, should I put a vapor barrier down before or after the EPS foam insulation and as there's a chance of radon what specification of vapor barrier do I need, plus can this do the job without buying something actually called "Radon barrier" and pushing our costs up? 

Responses (3)

Mike Reynolds 4 years ago

Hi Les, 

That's how I would do it, except I'd boost it up to 6 inches likely, I live near Ottawa, Canada and our winters ain't no picnic. We also have high risk of radon in this area. 

Due to changes in elevation, there can be a lot of temperature variance even within a climate zone, so I can't speak to the 'idea' amount necessarily where you are but we just installed a slab with 6 inches of EPS. It is from a slab on grade form kit called Legalett, and they don't install vapour barriers ever unless for radon gas prevention or to achieve Passive House certification. All that to say,  4 inches of insulation is fine, but six likely wouldn't be overkill where you are if you wanted to beef it up a little.  Here is a DIY slab on grade installation video if you like, and if you are interested let me know, Legalett distributes all over North America.