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2 layers vs 1 of outsulation?

user-5070925 | Posted in Energy Efficiency and Durability on

Greetings from Fort Worth, TX!

We are finalizing our house layout and I have been looking into various wall systems. I’ve about settled on 2×6 advanced framing dense packed – Zip Wall or OSB with Wrap – 2″ foil faced iso tapped.

This should give me roughly a R-25 wall.

My question is; not including labor, material cost is roughly 50% higher to do two layers of 1″ vs 1 layer of 2″. If I have a WRB and air barrier under the foam is it really necessary to have two layers?

I understand the benefits of offsetting seams, but is this really a belt and suspenders situation when I would be OK with just the belt?

Any help is appreciated!

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  1. user-2310254 | | #1

    If you tape the seams of the zip or OSB layer and the foam layer, it should be fine. (That is what I did.) you have to be conscientious about your openings as well.

    You might consider using reclaimed foam instead of new ISO. It will be much more economical.

  2. Anon3 | | #2

    Have you run a BeOpt simulation yet?

  3. Dana1 | | #3

    In Austin they are getting to Net Zero Energy with 2x6/R20 walls with NO exterior insulation, and though FT. Worth is somewhat cooler in winter you can probably still get there with 1" foil faced polyiso, spending the money for the next inch on other building envelope upgrades or more rooftop PV solar, if that's your performance goal. (You probably don't have enough even at 2" to hit PassiveHouse.)

    As a rough guide to Net Zero take a look at the zone-3 row of whole-assembly R values in Table 2 of this document:

    The authors are suggesting an R20 wall is about right, and you'd be there with 1" of foil faced polyiso (taped seams). Steve is correct that reclaimed 2" polyiso (any facer type) would be cheaper than 1" virgin stock goods, and greener too, since no new materials are needed for manufacturing it.

    Mind you, that document was written when PV solar cost 3x as much as it does today and better-grade PV was 15% efficiency instead of today's ~20%, and was generic, not accounting for the higher insolation of Texas, better class mini-split heat pumps tested at HSPF 10/SEER 15 unlike today's HSPF 13/SEER 30. If you're super-careful in the design (the AF framing alone buys you another R1-R2 of whole wall R) you can probably hit Net Zero Energy with no foam, with a PV array that fits comfortably on the house, but the economics of a couple of inches of RECLAIMED polyiso are probably still going to be favorable against the upsized solar & mechanicals (TBD- do the math.)

    Bottom line, you'll probably be just fine with the belt, but tape the seams of the single-layer foam.

  4. user-5070925 | | #4

    Thank you guys for taking time to answer! Great to have my hunch confirmed by more knowledgeable minds.

    I actually have a barn full of 2" reclaimed ISO (using it on the interior of the barn). It is fiber faced and was dirt cheap.

    When I bought this material I inquired about reclaimed foil faced and the guy said they almost never see the stuff. I didn't know you could use fiber face ISO as outsulation, so I'll have to do some reading on the topic.

    Thank you again for your input!

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