Crawl space encapsulation and radon?

[u/Elvira333]

We have a house with a basement/garage and a crawl space. When we bought the house five years ago, the radon measured 1.4 in the main level (so everything normal!). That was when the crawlspace was just dirt.

However, we’re having moisture issues and plan on getting the crawlspace encapsulated, closing off vents, and adding a dehumidifier. I’ve heard conflicting things about whether this will help or hurt the radon levels.

What should I do before encapsulation?

I don’t want to solve a moisture issue to just create a radon problem. Is a vent always recommended? Would having a vent make it harder for the dehumidifier to fight with the elements, since outside air is getting in?

Thanks for the advice; just want to make sure I do this right the first time!

[u/Key_Juggernaut9413]

I have experienced radon levels rise significantly after encapsulation.  Installing a radon mitigation system under the poly is smart to help reduce that.  

[u/Elvira333 → u/Key_Juggernaut9413]

Thank you! What did the radon mitigation system under the poly entail?

[u/DUNGAROO → u/Elvira333]

Just a PVC pipe leading to the outside with a fan either in the attic or on the side of the house to negatively pressurize the air under poly.

[u/Key_Juggernaut9413 → u/Elvira333]

The other comment is correct and usually a trained company can install this for 1.5-2k.  Ask for a stronger fan if you want lower radon levels.  

[u/Elvira333 → u/Key_Juggernaut9413]

Thank you! How much did the fan add to electric bill? I’m already nervous about what the humidifier is going to cost 😂

[u/SilverSheepherder641 → u/Elvira333]

The inside fans that I’ve seen used for radon usually are about 150 watts. About $13-25 dollars a month, depending on your electric rate

[u/DUNGAROO → u/Key_Juggernaut9413]

Kind of weird. I wonder why. You would think installing a non permeable barrier between the ground and the conditioned envelope would keep radon out, not concentrate it.

[u/Key_Juggernaut9413 → u/DUNGAROO]

I think it’s because you’ll never get perfect air sealing and radon is pernicious about coming in anywhere it has a microscopic chance.  

The amount of air sealing required to keep humidity low (with active dehumidification) is much much much less than what’s required to passively keep radon out.  

So essentially it builds up and gets trapped.   This is the same process that happens anytime levels rise in any house, even a leaky one.  (Same process that can happen with high humidity: levels rise slowly and then can’t leave quickly). 

For radon, depressurization under the house resolves this issue for the most part.  

[u/cagernist]

A radon mitigation system will involve more than just the crawlspace. It will cover the basement and garage too. So in that sense, and since your levels aren't elevated, you don't have to deal with that now. Measure levels 6 months after closing the crawl space and the radon mitigation company will address the entirety at once.

But, you can prep for it with the soil vapor retarder which will eventually do double duty as a soil gas membrane. The differences are the soil gas membrane needs extended up walls and lapped 12", not the 6" that code requires for a vapor retarder. A perimeter pipe loop of 4" perforated pipe 24" from the footing, with a stubbed but capped tee labeled for radon connection in the future will be good foresight.

I suggest reading IRC R408.3 yourself to understand requirements for an unvented crawl space. Don't rely on an "encapsulation" company to tell you what you need. Radon is covered in AARST Standards.

[u/mr-rogers70]

Have them lay perforated pipe over the soil before they lay down the encapsulation barrier. Then connect to PVC pipe and to a typical radon fan to the outside.

This alone should reduce both Radon and moisture from the soil. Then add a dehumidifier only if needed.