Replies

1. GBA Editor
   Martin Holladay | Jan 05, 2015 02:29pm | #1

   Apollo,
   You should consult with your radon mitigation contractor, if you have one. But radon can enter a crawl space through cracks in a CMU wall as well as through the soil.

   Because you are trying to limit radon entry, you should extend the polyethylene up to the top of the walls and should secure the polyethylene to the blocks near the top of the wall in an airtight manner (using a horizontal batten attached with screws, and plenty of Tremco acoustical sealant). The rigid foam insulation can then be installed on the interior side of the polyethylene.

   For more information, see these two articles:

   Building an Unvented Crawl Space

   All About Radon

2. 
   agurkas | Jan 05, 2015 02:33pm | #2

   I actually built my radon system. Had 12 in full size basement and now it is down to 1.4 month later.

   Reason I asked about how high I can install vapor barrier was because I have seen numerous mentions in articles here warning against any kind of poly behind insulation in crawlspaces.

3. GBA Editor
   Martin Holladay | Jan 05, 2015 02:50pm | #3

   Apollo,
   Polyethylene can cause problems in walls insulated with fiberglass. But in your case, the polyethylene will help keep out the radon.

4. 
   agurkas | Jan 05, 2015 02:53pm | #4

   Thank you!. That was the confusion. I am taking out all fiberglass. Looks like mice loved that stuff with previous owners of the house.
   P.S. This site has been great help. I have done a lot of projects myself with great results, because of advice I got here.