Unvented Crawl Space – poly on walls
SteveR_79
| Posted in Energy Efficiency and Durability on
“Building an Unvented Crawl Space” recommends polyethylene on the crawl space walls to within 3″ of the top. In “How to insulate a Basement Wall” the recommendation is no poly anywhere on the wall. Is there a simple yes or no answer to poly on the wall of a crawl space with foam insulation?
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Replies
Poly is a problem where it will trap moisture and cause mold issues. The classic example is a batt-insulated basement wall with the foundation on one side, a stud-framed wall with batts in he middle, and poly+drywall on the interior side. An assembly like that will trap moisture from the foundation wall and soak the batts, and probably rot the framing too.
Poly sandwiched between the foundation wall and rigid foamboard shouldn’t be a problem and is commonly done when a crawlspace is both encapsulated and insulated. The foam board itself is often times a vapor barrier (such as with foil faced polyiso), although more open types (like EPS) are generally vapor retarders and not barriers at reasonable thicknesses.
If your plan is to encapsulate the crawlspace, running the poly from the floor up the walls, and then put rigid foamboard over the poly on the walls, you should be ok.
Bill
HI Steve -
If you are looking for best practice details for unvented crawlspace, you can often find climate-specific details at the Building America Solution Center: https://basc.pnnl.gov/resource-guides.
From this page, you can filter by Climate (3rd filter bar on the right) or key word, such as crawlspaces (see 4th filter bar on the right).
Peter
No poly on the walls. From BSC in 2015:
"The best insulations to use are foam based and should allow the foundation wall assembly to dry inwards. ... The greater the permeance the greater the inward drying and therefore the lower the risk of excessive moisture accumulation."
https://www.buildingscience.com/documents/information-sheets/crawlspace-insulation