Replies

1. 
   user-2310254 | Jul 20, 2016 11:32pm | #1

   Martin will correct me if necessary, but I don't think you need to seal the poly to the side walls. After the slab is cured, you need to seal the perimeter to provide an air barrier between the home/basement and the underlying soils.

2. GBA Editor
   Martin Holladay | Jul 21, 2016 06:39am | #2

   Adam,
   I agree with Steve Knapp. The purpose of the polyethylene is to act as a vapor barrier. It doesn't need to connect to anything. In fact, there is no need to fold it up around the sides of the slab.

   Once the concrete slab has cured, you should seal the perimeter of the slab with high-quality caulk, to complete the air seal.

3. 
   JC72 | Jul 21, 2016 08:42am | #3

   Martin,

   I've wondered about this as well. Almost all of the drawings I've seen show poly going underneath and up the outer side of the slab terminating at ground level. If the slab isn't insulated what keeps water from making its way between the poly and the edge of the slab at ground level?

   From a layman's perspective I would think that the poly would degrade at ground level thereby allowing dirt/debris to create a gap which will then allow moisture to make its way into the concrete.

   Is it assumed that the backfill will keep it in place?

4. 
   JTyler | Jul 21, 2016 08:45am | #4

   Does anyone have a recommendation for how to handle air sealing this location when a rigid foam thermal break is used between the slab and the walls? Would it be effective to apply two beads of caulk or a sealant/adhesive - one at the joint between slab and foam, and one at the joint between the foam and the wall?

   Edit: I should add some specifics of my situation. I installed an XPS rim over my sub slab insulation to isolate the slab. My concrete contractor was not well versed in insulated slabs, and poured a thick slab that left the XPS rim under about an inch of concrete. My plan is to cut around the perimeter and remove the concrete to expose the foam rim and thermally isolate the slab, then air seal with two beads of sikaflex 221, then continue the insulation install up the interior of the basement walls. Does this sound like the best approach for a quality end product, or should I consider using over poured concrete and a single bead of caulk to create a possibly more reliable perimeter air seal? My apologies for hijacking the thread - hopefully this is closely related enough to the original poster's question.

5. 
   JC72 | Jul 21, 2016 08:48am | #5

   Jim,

   Something like this?

   http://buildingscience.com.678elmp02.blackmesh.com/sites/default/files/migrate/jpg/BSI059_Figure_01_web.jpg

   http://buildingscience.com.678elmp02.blackmesh.com/sites/default/files/migrate/jpg/BSI059_Figure_04_web2.jpg

6. 
   JTyler | Jul 21, 2016 09:42am | #6

   Chris- I like the idea of a vapor-open tape or membrane over the foam to bridge the gap between wall and slab. This seems more reliable than sealing between dissimilar materials, foam and concrete, with caulk. Thank you for the help!

7. 
   alumniu | Jul 21, 2016 10:28am | #7

   Thank you Steve and Martin. This makes it even easier that I was expecting.

   No worries on hijacking Jim. I got my question answered already.

8. 
   alumniu | Jul 21, 2016 10:38am | #8

   This may be a nonsense question, but got me thinking...
   Is there any application where poly would extend from sub slab, and be wrapped up the entire basement wall? Any water infiltration through wall cracks would presumably channel down behind the poly into the soil below the slab.

   I assume this would be ill advised because this promote would promote mold growth behind the plastic?

   Is there a similar concern with using thicker (2~2.5") XPS insulation on the walls, that vapor permeance of the XPS would also cause the walls to not be able to dry inward?

9. GBA Editor
   Martin Holladay | Jul 21, 2016 10:47am | #9

   Adam,
   Wrapping the poly up the walls is a standard approach used by some contractors who transform vented crawl spaces into sealed crawl spaces. It's done less often for basements.

   I'm an agnostic on the practice. It seems to work OK -- but I think that wall poly in that location is unnecessary.

10. 
    JTyler | Jul 21, 2016 01:23pm | #10

    Adam - it is ok to install an impermeable material like poly or thick XPS against your concrete wall. It is also ok if your concrete wall can't dry to the interior; moisture will in no way harm the concrete. Mold needs food to grow, and neither concrete nor poly is on the menu. An impermeable material in this location keeps the moisture contained to the concrete and away from other materials in your basement that could feed mold spores.