Is a vapor barrier necessary under a concrete slab with foam insulation and if so, where does go?
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Is a vapor barrier necessary under a concrete slab with foam insulation and if so, where does go?
Grady 22
Aug. 17, 2019Last 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?
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 ofEPS rigid insulation can meet code requirements as a vapour barrier只有当它是重叠的，以确保潮湿不会通过对接接头，如果你只安装单张。
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, andto 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.
如果Radon Block屏障超出了你的预算，我至少会考虑更厚的聚膜10或15密尔，只有6密尔，你会通过走在上面、掉落工具、用钢筋和加固网、网椅等来刺穿它，这几乎是一个不可避免的结论，所以为了确保对氡的保护，我自己会选择更厚的膜来增加保护。为了安装，如果可以的话，在泡沫层之间铺设屏障，这样你就可以在上面走一次，然后盖上它。唯一的问题是，如果下雨了，你需要一个策略来清除或排水，然后再浇筑混凝土。
In ourLEED V4 Platinum Edelweiss Demonstrationhouse we installed a10-mil underslab vapor barrierfrom 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, butradon gas in wateris another potential source that should be considered toensure safe indoor air quality in a home．
Ok Mike, thanks for the great response and suggestions. What I'm getting from this is that I should maybe consider using a double thickness of 2" foam insulation instead of the 4" and overlap the joints, and what we could do to protect the membrane from punctures from the rebar is to install it between those 2 layers of foam - with the added benefit of making sure that at least 2" of foam has zero possibility of getting moisture back in once the concrete slab has dried out because it will be sat inside the membrane. Thoughts please?
Hi Les,
这就是我要做的，除了我可能会把它提高到6英寸，我住在加拿大渥太华附近，我们的冬天可不轻松。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 aslab on grade form kit called Legalett, and they don't install vapour barriers ever unless for radon gas prevention or to achievePassive House certification．也就是说，4英寸的绝缘材料是可以的，但如果你想要加强一点，6英寸可能不会太过分。Here is aDIY slab on grade installation videoif you like, and if you are interested let me know, Legalett distributes all over North America.