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Community and Q&A

Which way should the foil facing on polyiso rigid foam face?

BuildingNewb | Posted in General Questions on

Correct me if I’m wrong, but shouldn’t the foil facing on polyiso face the interior in climate zone 5 to help retain heat? It will be used on basement rim joists. The heating season is obviously longer than the cooling season.

Furthermore, should I leave an air gap between the foam and the wood or should I place the foam directly against the wood before spray foaming the perimeter?

Lastly, should I seal the perimeter of the rim joists with the one part foam prior to installing the rigid foam and spraying around the perimeter of that?

I was thinking of not sealing the perimeter prior to installing the rigid foam to give any moisture entering the cavity an avenue to escape. Although no moisture should really be entering the cavity from the inside and to the best of my knowledge, polyiso is vapor-impermeable (not sure about the 1 part foam but I think it is vapor-permeable).

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  1. BuildingNewb | | #1

    Sorry for the mistyped words. I'm having overlapping issues on my phone.

  2. GBA Editor
    Martin Holladay | | #2

    Building Newb,
    You asked this before about the top plates of your walls. Remember when you wanted to leave holes and cracks there, "to let the moisture escape"? And remember what Dana and I told you: "Airtightness is always good, and air leaks increase rather than decrease the chance of condensation problems"?

  3. BuildingNewb | | #3

    I get it and fully understood it when not foaming in another piece of insulation. I just didn't know if i was in essence forming a sort of double vapor barrier by air sealing the rim joist and then foaming in another piece of rigid foam in front of it. Thanks for the clarification.

    As for the foil facing, that should face the interior, correct?

  4. NormanWB | | #4

    If you want it to serve as a radiant barrier, the foil must face an air space of at least 1/2".

  5. BuildingNewb | | #5

    So if I'd like the basement to stay warm, I'd face the foil toward the interior?

  6. Expert Member
    Dana Dorsett | | #6

    The foil facer adds about R1 to the performance if it's facing an air gap, and it doesn't matter which side. If the facer is in contact with the band joist or fiber insulation on the interior that modest advantage goes away.

  7. GBA Editor
    Martin Holladay | | #7

    Building Newb,
    To keep a building warm, you need something with R-value (insulation). The higher the R-value (and the lower the rate of air leakage), the better.

    As Dana explained, the foil facing on rigid foam has the effect of raising the R-value of an adjacent air space if there is an adjacent air space. It doesn't matter which side of the air space (or the foam board) is the "hot" side for this to work. R-value is R-value -- it works in both directions.

    In most cases, people installing rigid foam against a rim joist install it tight to the rim joist, without an air space. There are several reasons for this, but the most important one concerns air leakage. If there is any air leakage in that air space, the insulation performance is greatly degraded by the air space. Tight against the rim joist is safer.

    If there is aluminum foil on just one side of the polyiso, here's my advice: install the polyiso tight to the rim joist, with the shiny side facing toward the interior of the building.

    For more information on this issue, see Insulating rim joists.

  8. BuildingNewb | | #8

    Thanks. There is a film on the other side as well but it doesn't look reflective. Perhaps it's just a vapor barrier?

  9. GBA Editor
    Martin Holladay | | #9

    Polyiso always has some type of facing on both sides. Frankly, for residential use, the type of facing doesn't matter much.

  10. BuildingNewb | | #10

    Thanks for your help.

  11. dantheman69 | | #11

    Hi. I am also building a shed. I am just an hour north of Buffalo NY. We have about 4 full months of winter. My backyard shed is for storage and does not have heating. It does however contain an electrical/junction box for my steam sauna located 6 feet next to the shed. The shed is 40 sq ft.

    I am also a super newbie. I am using Durafoam EPS Rigid insulation on the interior. Just so that I burn my shed down - can I install the rigid foam with the foil side facing in to moderate the temperature and keep it dry? I am not planning to use a vapour barrier, is that ok? Am I at risk of anything? I am not storing anything flammable or perishable. I would appreciate any advice. Thanks

  12. Expert Member
    NICK KEENAN | | #12

    You'll probably get more and better responses if you start a new thread.

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