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Community and Q&A

Correct Orientation for Foil-Faced Polyiso Over Drywall

michaelbluejay | Posted in Green Building Techniques on

I plan to install 1-1/4″ polyiso foamboard over existing drywall, then 3/4″ furring strips, then 1/2″ of drywall. Should the foil face towards the interior or the exterior?

If it matters, this is for Austin, TX 78722 where we need more cooling than heating, and I’ll be using two layers of polyiso (1/2″ + 3/4″), because I can’t find 1″ polyiso locally. I do plan to tape the edges to prevent degradation of the foamboard. Thank you for your help.

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  1. GBA Editor
    Martin Holladay | | #1

    If your brand of polyiso has foil on only one side -- many brands have foil on both sides -- then the foil should face the air space. (The foil is a radiant barrier. If the radiant barrier faces an air space, then the air space will add between R-1 and R-2 to the R-value of your wall assembly. If the foil doesn't face an air space, it won't add any R-value. Of course, the polyiso has an R-value independent of the R-value of the air space.)

    In your case, that means that the foil should face the interior of your home.

  2. michaelbluejay | | #2

    Thank you very much.

    On a related note, I'm trying to figure out why furring strips seem to be recommended for this type of install, when thin plywood + thicker insulation would get more R-value for the same depth. One page of stats I found gives R1 for air gaps of 1/2 to 4", maybe the addition of the foil adds a little more, let's call it R-1.5 for a 3/4" gap:

    1" polyiso (R6) + 3/4" furring (R1.5) + 1/2" drywall (R-0.45) = 2.25", R-7.95

    1.75" polyiso (R10.5) +1/4" ply/OSB (R-0.3) + 1/4" drywall (R-0.2) = 2.25", R-11

    Or if 1/4" ply + 1/4" drywall isn't strong enough:

    1.5" polyiso (R9) + 3/8" ply (R-0.5) + 3/8" drywall (R-0.3) = 2.25", R-9.8

  3. GBA Editor
    Martin Holladay | | #3

    If you are adding rigid foam to the interior side of your studs, you can skip the furring strips if you want, and install the drywall with long screws that extend through the rigid foam to the studs. I've done it.

    Most drywallers hate this approach, though, because the foam is a little squishy. You may end up with a few more screw pops than usual, so budget some time for drywall repair.

    But you gain the 3/4 inch by skipping the furring strips.

  4. michaelbluejay | | #4

    As I noted, I'm installing over existing drywall (which I don't want to remove because it has asbestos).

    My question was, why do people go with furring strips when thin plywood + thicker insulation provides more R-value for the same amount of depth added to the wall?

    Also, is 1/4" drywall over 1/4" ply over 1" foam over 1/2" drywall strong enough? If not, what's the minimum drywall thickness and plywood thickness I should be looking at?

  5. GBA Editor
    Martin Holladay | | #5

    What's the plywood for? Are you thinking of installing plywood paneling as a finish material? Or do you want to use plywood to hold the drywall screws?

    Thin plywood won't hold drywall screws -- I would think you would need at least 1/2 inch plywood for the screws to hold.

    And plywood is unnecessary -- assuming that your wall has studs. The studs can hold the drywall screws.

  6. michaelbluejay | | #6

    I'm not thinking of using the plywood as a finish material. My proposed stackup, as listed above, is existing drywall, foamboard, ply, new drywall.

    The plywood would be to (1) provide a firmer backing for ultra-thin drywall than furring strips would, and (2) allow a higher R-value for the same amount of depth added to the existing wall.

    So again, I see furring strips specified in various how-to articles, but not thin plywood, even though the use of thin plywood should provide a higher R-value for the same amount of depth added to the wall. That makes me think that I'm missing something, and that there's some reason I should use furring strips and not plywood. Am I indeed missing something, or is plywood as I specified in Reply #2 above indeed a better alternative to furring strips?

    Also, assuming both the plywood and the new drywall are both screwed through to the studs, is 1/4" ply + 1/4" drywall strong enough, or should I be looking for something thicker for one, or the other, or both? What's the minimum thickness for the ply and for the drywall?

    Thank you very much for your help.

  7. user-2310254 | | #7

    On exterior applications, you want furring strip to create an airspace between the cladding and the foam. This helps the cladding to dry (at least that is my understanding). You can install the drywall directly to the foam. I would use 1/2-inch (5/8-inch if the foam is a bit wavy). That's likely to be the simpler and more cost-effective approach.

  8. michaelbluejay | | #8

    I installed the 1/4" ply over the insulation (screwed through to the studs), and 1/4" drywall on top of the ply (screwed only into the ply). It seems to be sufficient. We'll see.

  9. bueller | | #9

    What if the air space that the radiant side faced is the “air gap” for the soffit to ridge venting? Creating a baffle, with more faced insulation on top of that in a cathedral ceiling application. would that assembly work? Be effective? Not cause rot or mold?

    1. Expert Member
      BILL WICHERS | | #10

      That will work just fine and will gain you a little bit during the summer cooling season. It’s similar to tacking up a radiant barrier under the rafters to keep the attic a little cooler in the summer. During the winter heating season you probably won’t see any benefit.


      1. bueller | | #11

        Great thanks Bill!

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