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Community and Q&A

Insulating a metal barn

deecoy | Posted in Energy Efficiency and Durability on

I have read a lot about condensation problems when insulating metal barns, as well as your blogs on bubble wrap scams and subsequent commentary.

I am considering installing a rigid moisture resistant insulation board (polystyrene) against the wood girts of the metal barn, leaving an approx 2″ air space between the edge of insulation and the metal siding. Despite all the negative comments about foil faced bubble wrap, I am considering installing possibly a single bubble wrap product with foil on both sides flush with the 8″ wood framing timbers, (with supplemental furring strips), to create both a moisture barrier, reflective foil to keep the heat inside the barn, and importantly, to create a second air pocket of approx 4-5″ between the bubble wrap product and the rigid board insulation. Since I can purchase the bubble wrap for about $0.20/sqft. (about the same cost of just a radiant foil w/o any R value and perhaps less strength as a siding material), I see the primary benefit of the bubble wrap with foil as a radiant and moisture barrier….not its insulation benefit. I estimate that this assembly will produce an R value of approx 19-20 (7 for each air pocket, 1 for the bubble wrap and 5 for the 1″ rigid insulation board.

Your thoughts/comments/suggestions will be greatly appreciated.

Thanks, Brian

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  1. GBA Editor
    Martin Holladay | | #1

    There are two issues here. One issue is: how will you establish an air barrier (or, ideally, two air barriers)?

    I can't tell whether you plan to install the 1-inch polystyrene as a continuous layer, or whether you plan to cut it into rectangles and insert it between the girts. In either case, you need to plan a way to seal air leaks.

    The second issue: you have grossly overestimated the R-value of an air space.
    According to most reference tables (here's one), the R-value of an air space that is between 1/2 inch and 4 inches thick is about R-1. Increasing the depth of the air space doesn't increase the R-value.

    The R-value of an air space can be increased from about R-1 to R-2 or perhaps R-2.5 if you have a shiny radiant barrier facing the air space. But you are still nowhere near R-7, the number you cite.

  2. deecoy | | #2

    Thanks Martin.
    Planning on using tongue and grove polystyrene to reduce air leaks, applied over the girts for a continuous layer. As my primary concern is keeping the building warm in winter and shoulder seasons (rather than keeping it cool in the summer), I haven't planned on using a reflective foil face on outside of the rigid board facing the metal siding.....will the foil actually raise R value in the winter by reflecting out the cold?? Also, I've been told that the outside air cavity between rigid insulation and metal siding should be vented to minimize any condensation build up on inside metal siding....correct?

    Or, do you feel I would be better off by simply using spray foam over both girts and exposed metal siding instead of the rigid board, and not worry about potential condensation on inside of metal siding? (Located in Idaho with very low humidity)

    Thx again.

  3. GBA Editor
    Martin Holladay | | #3

    The R-value of your planned assembly is quite low, so you may want to think of installing thicker rigid foam, or a thicker foam sandwich panel (like nailbase).

    Tongue-and-groove foam joints aren't airtight. To create an air barrier, you will need to tape the foam seams with high quality tape, or install a high quality European membrane or taped Tyvek.

    If you vent the air space between the rigid foam and the metal siding, then you can't count the R-value of the air space, because it's basically exterior air.

    Spray foam will provide you a higher R-value than the assembly you first suggested, especially if the spray foam is thick enough.

  4. deecoy | | #4

    Thanks again.

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