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Polyiso board for pole barn insulation

Dpembert | Posted in Energy Efficiency and Durability on

Climate zone 4 ( Kansas ). I want to insulate my pole barn walls with 3.25″ iso board I bought used without the foil facing for $8/ea. The plan is to screw it to the interior of the girts leaving the 1 1/2″ gap between it and the outside metal as an air gap/rain screen, then sealing it to the vertical posts and floor with foam or caulk, taping all seams. I also want to add interior girts to the posts and install at least 1/2″ osb as an interior finish. Maybe 3/4″ t&g if budget allows. Prime and paint interior. Ceiling will be osb also with blown in insulation. I want the building to be as sound proof as possible due to my woodworking hobby so I don’t bother my neighbors. With the extra 3″ gap inside the wall I’m also thinking about adding 3 1/2″ fiberglass rolls between the interior osb and the iso board as extra insulation and for sound absorption. So from outside to inside its metal siding, 1 1/2″ gap, 3 1/4″ iso board, maybe 3 1/2″ fiberglass then osb. Do you see anything wrong with this? I know air infiltration is my no. 1 enemy. Will I have problems with moisture with no vapor barrier on the inside or outside? Any recommendations? I will be heating and cooling as needed when I am out there with a used hvac unit from a house.

Great site!
Dan

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Dan,
    Q. "I know air infiltration is my no. 1 enemy."

    A. You're right about that.

    Q. "Will I have problems with moisture with no vapor barrier on the inside or outside?"

    A. You won't have problems if you can limit air flow. The thick rigid foam is already a vapor retarder.

    Q. "Any recommendations?"

    A. The wall assembly would be improved if you start by stapling housewrap to the interior side of the girts. Strive to make the housewrap as airtight as possible. (Tape the housewrap perimeter).

    Installing a layer of fiberglass batts on the interior side of the rigid foam is fine. The key to making this work is airtightness. Seal everything.

  2. Dpembert | | #2

    I thought about the house wrap but was unsure if I could seal it up to make it worth it. Maybe brush some mastic sealant like "Total Wall Total Stop RA Pre-Mixed Rubberized Weather Resistant Barrier" on all the seams and joints of the metal? Says its a moisture barrier and vapor permeable. Or should I let that outside layer breathe freely? Should I set the foam board on a treated ledger board to keep it off the concrete or can I tape the bottom edge with something and set it directly on the concrete? Your thoughts on adding plastic to ceiling as vapor barrier since there is none in the walls

    Thanks again
    Dan

  3. GBA Editor
    Martin Holladay | | #3

    Dan,
    Your ceiling doesn't need a vapor barrier. It needs an air barrier, however (for example, ordinary drywall). For more information, see Do I Need a Vapor Retarder?

    Your questions about the housewrap are a little confusing.

    Q. "Maybe brush some mastic sealant? ...Or should I let that outside layer breathe freely?"

    A. Housewrap stops water entry. It's also an air barrier. It's not supposed to be a vapor barrier. It's supposed to be vapor-permeable. Ordinary housewrap tape at the seams will work.

    Q. "Should I set the foam board on a treated ledger board to keep it off the concrete or can I tape the bottom edge with something and set it directly on the concrete?"

    A. A high quality tape like Siga Wigluv (available from 475 High Performance Building Supply) will bond housewrap to concrete.

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