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Community and Q&A

Polystyrene over vapor barrier?

dfiler2 | Posted in Energy Efficiency and Durability on

I’m in the process of finishing the inside of my new pole building….
Currently, between the 8′ on center columns there is batt insulation installed with a vapor barrier to the inside of the space, then metal panel from the floor up 4′, open space up the next 8′ (that will be covered with 3/4″ MDO) with horizontal nailers spaced 16″ apart, then the next 4′ up to the ceiling is covered with metal panel.

I’m thinking about adding 1-1/2″ pink foam board between the nailers on the outside of the vapor barrier.

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  1. Expert Member
    Dana Dorsett | | #1

    The vapor permeance of 1.5" pink board is about 0.6-0.8 perms, which is a bit on the tight side, but still OK as long as there is still ventilation space between the foam and siding. Unfaced 1.5" EPS would have about twice the drying capacity.

    Even though you wrote "... between the nailers on the outside of the vapor barrier..." it sounds like you're actually putting the foam on the interior side of the vapor barrier? If that's the case- the foam is on the interior side of the assembly, you'll get somewhat better performance out of 1.5" of foil-faced polyiso than you would out of pink board, and the cost may even be cheaper.

  2. wjrobinson | | #2

    No plans?
    No permit of said plans that must be followed or amended?
    No builder with appropriate knowledge?

    How can someone post in the middle of building a project the need for specifications?

    I have never ever started a project that needed plans and permits like how people post here at GBA.

    I don't get it, someone clue me in. I must be stupid.

  3. GBA Editor
    Martin Holladay | | #3

    I agree with Dana that your description is confusing. Do you plan to install the rigid foam on the interior side of the polyethylene vapor barrier, or on the exterior side?

    We don't know your location or climate zone, so it's hard to provide advice. In general, this type of building definitely needs a good air barrier as part of the wall system -- but in most climate zones, it doesn't need a polyethylene vapor barrier.

  4. GBA Editor
    Martin Holladay | | #4

    I'm happy to explain things to help you with your confusion. By now, AJ, you should know that there are many regions in the U.S. where there are no building codes and no need for building permits. I live in such a region.

    One reason that some jobs don't have a builder is that owners do the work themselves.

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