Vapour / radon barriers below slab floors

There are conflicting thoughts on this kicking around the web, but we will share what we know as best as we can. However, please be sure to check with your local building inspector regardless to ensure you meet any regional qualifications to build.  

4 inches of EPS rigid insulation can meet code requirements as a vapour barrier only if it is overlapped, to make sure moisture does not pass through the butted joints if you are only installing single sheets.

Some building certification programs such as Passive House will have higher standards than building code and will require that you add a slab vapor barrier, and to achieve LEED for Homes Certification a Radon barrier radon evacuation stack is mandatory.

To continue about radon – exposure to radon gas in homes is the second greatest cause of lung cancer. There is no way to test radon levels in a home before it is built and no way to install a barrier after a slab is poured, so I’d certainly install a radon barrier it if I were you since you are building in a high-risk area.

Of the relatively few builders that seem to be are aware of radon gas health issues, many will just use a standard 6-mil poly vapor barriers as a radon barrier, which is better than nothing, but I’d want a thicker one for my home.

There is a barrier called Radon Block, which is first of all a much thicker and puncture-resistant membrane, and manufacturers claim it is 100 times less permeable compared to standard poly of the same thickness.

If a Radon Block barrier is more than you can afford in your budget, I would at least consider a thicker poly membrane of 10 or 15 mil. With only 6-mil it is almost a foregone conclusion that you will puncture it by walking on it, dropping tools, piercing it with rebar and reinforcement mesh, mesh chairs etc, so to ensure protection against radon I would choose the added protection of a thicker membrane myself. And for installation, if you can, lay the barrier down BETWEEN layers of foam, that way you walk on it once and then cover it. The only issue with that is that you need a strategy to remove or drain any water if it rains before you pour concrete. 

In our LEED V4 Platinum Edelweiss Demonstration house we installed a 10-mil underslab vapor barrier from WR Meadows, we found it to be extremely puncture resistant and our final radon test showed levels under 20 becquerels, far lower than the 200 becquerel level that Health Canada considers unsafe for longterm exposure. 

As a final thought as well, remember that radon isn't only going to enter homes fromt the soil, but radon gas in water is another potential source that should be considered to ensure safe indoor air quality in a home.

We always recommend testing for radon gas contamination in homes as it is a significant health risk if levels are elevated. For sure you will not get accurate reading a gutted house, but presumably you aren’t leaving it like that. Are you in a high risk area? You can see here for radon gas maps to see if you’re in a high risk area. Either you could finish the upstairs and then test it, or, just run with the assumption that you may have it and take the pretty affordable steps ahead of time just in case, read more here – 

How to remove radon gas from basements and crawl spaces

Does the house have a basement or is it a crawl space? And what is the floor, is it dirt or poured concrete? I would assume it is not insulated nor would it have a vapor barrier, so just doing a vapor barrier itself puts you halfway to a passive radon stack, in that 6 mil poly vapor barriers are standard fare for radon barriers.

For homes in very high risk areas the more prudent builders may use a radon block membrane for added protection, but at the least a 6 mil poly will go a very long way. 

The thing is, as you realize, it is virtually impossible to know ahead of time if a leaky house, or an empty lot where a house will soon sit, will have high radon levels. So best is simply to run with the assumption that it will and put in a membrane. You can see our page here on how to renovate basements so they won't develop mold, that will show you how to insulate a floor and install a radon barrier membrane at the same time. 

I hope that helps, feel free to ask additional questions with a more details – current state of basement (stone, block or concrete foundation), ceiling height, if you intend to finish it as living space, and even the climate zone you live in, all this will help better determine what your best path forward is for a healthy and energy efficient home, but an affordably built one too!