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Community and Q&A

Slab vapor barrier termination

shedworkshop | Posted in General Questions on

Where should a sub-slab vapor barrier be terminated?

I see different detail drawings from Building America Solution Center with the vapor barrier terminating below grade and above grade (see attached images).

I’d prefer to terminate above grade so I can extend my vapor barrier up over my below-grade vertical insulation and forms.

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  1. Expert Member
    Joshua Salinger | | #1

    It really depends on if you are using it as a vapor barrier or using it to double as your vapor barrier and air barrier. Often it will run up and over the foundation and under the sill plate to connect with the air barrier. Of course, this all depends on the details of the specific assembly and where the air barrier is located.

    A vapor barrier stops vapor in a linear relationship. That is to say that if one covers 90% of the surface with a Class 1 vapor barrier, 90% of the vapor will be stopped. This is why it isn't as critical to cover all areas with the vapor barrier. This is different than the air barrier, where even a small percentage left uncovered can lead to huge air leaks.

    1. shedworkshop | | #2

      Got it, thank you Joshua! I found this detail from JLC that looks very well thought out. Between it and your advice, I think I'm going to run the vapor barrier up the vertical face of the slab and terminate it at the z-flashing. That way the Polyguard Term barrier is adhered to the sheathing, slab, and z-flashing.

  2. Expert Member


    Note that both the thickened slabs you posted are very similar in shape to what we settled on in your previous thread.

    1. shedworkshop | | #4

      So it is :) Very reassuring to see.

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