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Vapor barrier placement — basement sub-slab insulation

Tom Cross | Posted in Energy Efficiency and Durability on

475 High Performance Building Supply has just released a new book – High Performance 2X Framing. It recommends that the vapor barrier for basement sub slab insulation (EPS or mineral wool) be placed under the insulation and in contact with the compacted rock fill. This differs from a Roxul mineral wool technical bulletin, the Hammer & Hand Best Practices Manual and other articles I have read that place the vapor barrier on top of the insulation and in contact with the concrete slab. I questioned 475 on this and they responded that they prioritize protecting the insulation. I question their recommendation and would like opinions on this.

In addition in the book, 475 recommends barriers over both the CDX sheathing and an additional barrier over the mineral wool outside insulation – 2×6 advance framing with mineral wool batts, CDX sheathing, WRB, 2″ mineral wool board, another WRB, 1×4 battens and fiber cement siding. I question the second WRB on top of the mineral wool board. Any opinions on this are appreciated.

We plan to build in climate zone 4 – Virginia or Kentucky.

Tom

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Tom,
    Concerning the polyethylene under slabs: I disagree with the recommendations made by 475. Here is a link to an article that explains my reasoning: Polyethylene Under Concrete Slabs.

    If you ignore my advice and follow the advice given by 475, you won't be the first builder to go that route. But you had better pray that there is no rain between the time when you install the poly and the time that the concrete is placed.

    According to Ken Levenson (see Comment #2), the e-Book does not recommend the use of two WRBs.

  2. Ken Levenson | | #2

    Tom and Martin,
    The eBook does not recommend two WRBs - that would be silly and wasteful. Please have a closer look at the drawings and let me know if you have any further questions. Therefore Martin, you don't need to disagree with a phantom recommendation.
    As for the vapor barrier on grade - our position is that dry insulation is better performing insulation so with the proper installation, enclosing the insulation helps insure long-term performance.

  3. GBA Editor
    Martin Holladay | | #3

    Ken,
    Thanks for your comments. I have edited my earlier comment in light of the information you provided.

    I stand by my recommendation for the placement of polyethylene under concrete, for the reasons laid out in the article I linked to.

  4. Tom Cross | | #4

    Martin,

    Thanks so much for your response and link to the article. What I appreciate so much is your thorough research and objective approach to building science. I will put the vapor barrier over the sub slab insulation and in contact with the concrete slab. The information is compelling for that approach.

    I was incorrect on the two WRBs comment. The 475 drawings do recommend an interior air barrier layer over the stud wall and an exterior WRB over the exterior insulation. I again question the recommendation of putting the WRB over the exterior insulation. Normal practice is to air seal the CDX plywood with tapes/caulk/liquid-appled material and place the WRB over the air sealed CDX plywood then the mineral wool board, battens and siding. Comments on this?

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