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Polyiso interior insulation in old barn

ThreeBirdFarm | Posted in Green Building Techniques on

I am starting on the second floor remodel of a 19th c. barn, and I’m considering using polyiso panels on the interior side. I’ve never used rigid and non-rigid insulation in conjunction, and I have a few questions.

The Basics:

This will be a storage space and workroom. The barn is in Maine. The barn exterior is original horizontal siding (which I may be replacing but not until summer) over original vertical 1x sheathing (with some gaps). This is attached directly to the frame of the barn, which is open on the inside and roughly 5″ deep.

My Plan:

Install fiberglass insulation into the 5″+ wall cavity (probably fiberglass), attach 4x8x2″ polyiso with foil facer to framing (and to 2×4 studs on interior walls), install horizontal strapping and then attach my electrical boxes, panelling and shelving to the strapping.


Is 2″ polyiso the way to go? It’s most cost-effective and available for me.

Is R-20 the way to go for the fiberglass cavity insulation?

I can add pictures if that would help.

Thanks in advance!

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  1. user-2310254 | | #1

    Another poster had a similar issue a few years ago. I think Martin's replies will help you to identify a safer and more effective approach.

  2. ThreeBirdFarm | | #2

    Thanks, Steve. We're not going to go spray foam though. Would you use two layers of rigid with an air gap between exterior layer and exterior wall instead of fiberglass and rigid? Cutting all that rigid to fit the odd sizes of the cavities sounds like a PIA, but if that's a better solution...

  3. user-2310254 | | #3

    I think two layers of foam with an air gap is a waste of one layer of foam. Beyond that I don't feel competent to say exactly what you should do in this case. I'm assuming you are not going to condition this space (at least as extensively as living space).

  4. Expert Member
    Dana Dorsett | | #4

    With the siding nailed directly to the framing or over plank sheathing without a proper weather resistant barrier you need both an exterior side air barrier for the batts, and a vent space between the siding and air barrier as a capillary break. Snugging the fiberglass to the siding or unprotected planking without the air gap allows moisture that blows by the siding to wick inward, and it inhibits the drying of the siding.

    If there's some sort of rosin paper or felt layer between the planks & siding you can probably just use #15 felt as an exterior side air barrier snugged up to the plank sheathing, and side-stapled to the studs, bending it with a 1-1.5" stapling tab.

    If taking the vented approach, strips of half-inch foam board (any type) alongside each stud as spacers for the air barrier , and half inch to 1" of foam board (or half-inch asphalted fiberboard) works. This picture is a roof assembly, but the principle is the same:

    If the exterior air barrier is foam board, keep it to an inch, if foil faced, since the drying path for the batts is via the short path around the foam at the rafters. But it can be thicker if unfaced EPS (up to 3") which is sufficiently vapor permeable for the assembly to dry.

    So if it's ~5.5-6" deep and you add a half-inch air gap with a half-inch air barrier that leaves 5-5.5" for the fiber insulation. An R20 batt compressed to 5" still performs at R18.5-ish, which is decent, and won't apply enough pressure to half-inch foam board to cause it to bow out and touch the siding.

    The stud-to-stud spacing and stud thicknesses of 19th century buildings are rarely suitable for batt widths designed for milled 2x studs at 16" or 24" spacing. There will be some trimming to do- buy a purpose made batt knife, and trim the batts to be 1/4-1/5" wider than the cavity they're filling. That way there won't be performance robbing buckle/wrinkle compressions or side gap convection paths. Tuck the batts in around the full perimeter of the cavity to ensure there are no long skinny voids where the extrior air barrier meets the studs, then gently tug the batts out so that they're just proud of the stud edges, for a compression fit when the interior side foam board is applied.

    Using 2" recliamed roofing foam or factory seconds is a lot cheaper than virgin stock foil-faced polyiso. This outfit in Thorndike ME seems to have a steady supply of factory-seconds foil-faced goods:

    Reclaimed goods are usually cheaper still. Searching the local craigslist for rigid + insulation often turns up local sources, eg:

  5. ThreeBirdFarm | | #5

    This is so helpful, Dana.

    There is no felt (or anything else) between the vertical plank sheathing and the horizontal siding on the second floor (I did replace the exterior siding on the first floor and added Tyvek when I did so).

    So I'm thinking (outside to in): 1) Horizontal siding (existing), 2) vertical plank sheathing (existing), 3) .5-inch foam spacers, 4) .5-inch foil faced Polyiso board, 5) R-20, 6) 2" foil-faced polyiso, 7) strapping, 8) interior panelling.

    Does that sound like a solid plan?

    Thanks again!

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