Hi There Folks,

I came across this crawlspace vapor barrier installed by a BPI contractor. 

Location:  Bay Area of California- near the ocean.  

Environmental Factors:  The area receives a lot of rain, and is notorious for rising groundwater.  

Health Factors:  The customers have respiratory issues, and these were communicated to the contractor.

Material:  6mm visqueen, red tape, nails

Concerns:  The material stops several inches short of the foundation walls. Some seams are taped with red tape.  Held to ground with 8 inch nails. 

I have a lot of respect for this particular contractor, but I have grave questions about how he approaches barrier installation in crawl spaces.

He has responded that this approach is adequate for unconditioned crawlspaces.  

I would appreciate insights, and direction to standards. 

Tags: barrier, crawl, space, vapor

Views: 10532


Replies to This Discussion



My understanding is that the vapor barrier should extend to the foundation so that no moisture comes from the soil into the house.  As the house is air sealed, this moisture could get trapped in the house and cause problems.  I know from personal experience, before I became an energy auditor, that even a relatively dry crawl space can trap moisture in a house causing mold and mildew to encroach.  (I had it coming up the walls after putting on an insulated roof.)  Though I live in humid New England, I think the same should apply in California.  Extend the plastic to and up the foundation, adhere with spray foam to the foundation, and tape all floor seams with a highly temperature resistant tape (not duct tape).  If the foundation cannot be reached for sealing, due to space restrictions, extend it as close as possible.

My experience with sealed, conditioned crawl spaces, you need to have a 20 mil poly-reinforced liner, with 0 perm rating, sealed with approved tape or mastic on the joints and to the wall. Good liners have an antimicrobial additive that will not support mold, mildew or fungus growth. The walls need to be insulated with 2”+ rigid foam and open cell foam on the floor joists.

As an Integrated system, you should include a drainage system, a sump pump and a dehumidifier. Also some type of supply ventilation at a rate of 1cfm/150sf minimum. I’m sure you can Google your local market for crawl space specialists.

You could also search BuildingScienceDotCom or EnergyStarDotGov for information and details on sealed crawl spaces.

Sorry, I should have said: open cell foam on the RIM of the floor joist.

Actually if there is any chance of water impacting it, it should be closed cell

On the contrary, if there is any wicking through the concrete or any moisture leaks on the rim joist, the open cell will allow any wood and the wall assembly to dry.

No!  Open cell on a wet surface will act like a sponge and the moisture will stay in contact with the wood.

John, Drying is the whole idea... with a closed cell it'll never get a chance to dry out and it'll rot; or maybe we need to agree to disagree on the building science of it.

So following with that logic, your basement walls should also be OC? Sorry but you still have numerous ways for said area to dry out without soakiing everything else & it isn't going to rot just because it is "wet"

Sorry but you might want to check up on the building science aspects of it & that does not include old Lienyne marketing brochures

1.  If it is wet enough that CC will not allow it to dry and OC soaks it up like a sponge, then the Crawl needs to be stripped back to dirt and the stem walls to concrete and a proper drainage system put in, the Class 1 Vapor Retarder replaced and the Foam re-installed.

2.  The concept of OC V CC in a crawl probably deserves its own thread. In addition to the moisture issues there are other problems with the OC in this application.

This is an unconditioned, vented crawl space.

Sounds like your "BPI" Contractor installed a dust barrier not a vapor barrier. The barrier needs to be completely sealed with 9 - 12 inch overlaps, on the sheets. The idea is to prevent moisture from passing upward into the house and moisture finds it way out of places at any opportunity - even the nail holes. Water also seeps in through the stem walls, especially if the homeowner's sprinkler system is watering close to the foundation of the house (we dried out the crawl space of a house by replacing sprinkler heads so that they didn't come within two feet of the house), so the barrier should include the stem wall too and any penetrations. How's the venting? How about bypasses from the crawl space up into the house and/or attic space?


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