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Community and Q&A

OSB above rigid foam?

Roger Rogers | Posted in Energy Efficiency and Durability on

I’m wondering what the engineers out there would think about this wall enclosure stack: 2×6 wall | 1″ or 2″ polyiso | OSB. The 2×6 wall cavity would contain blown in cellulose/fiberglass, maybe with a flash coat of poly as well.

The idea being that I will have a standard OSB exterior, strong enough to attach conventional stucco, but also have a good thermal break/envelope. The one worry I have is the durability of sandwiching the rigid foam against the studs, but when thinking about it more, it seems a lot like a SIP wall, except that we have 2×6 construction.

I don’t like standard SIPs generally, because I just don’t trust two layers of OSB for longevity.

Note: I earlier asked a question about stucco over rigid foam: https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/19029/envelope-construction-va-stucco, and another poster asked a similar question (https://www.greenbuildingadvisor.com/community/forum/green-building-techniques/17635/stucco-lath-over-4-foam) that received answers that concern me with using traditional stucco over too much rigid foam, hence this separate thread.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Roger,
    Disagvantages:
    1. The OSB is unable to brace the wall, so you need to include an alternative bracing method like T-profile steel strapping.

    2. The foam doesn't keep the OSB warm, so the OSB ends up suffering the "cold OSB" problem. Cold = wet.

    3. I'd want to consult an engineer to OK the OSB fasteners before I went ahead and attached stucco to the OSB,

  2. aj builder | | #2

    OSB. Invented when sheathing could dry, like under vinyl siding and asphalt shingles. Lots of air leaks drying it on one or both sides.

    Now today we have this push to add a layer of less permeable whatever near it and we are stopping air leaks now that aided drying.

    For many reasons, OSB is not a useful product in superinslated super air sealed assemblies. OSB is terrible at dealing with rot. The absolute worst product in that regard.

    I know, the price is amazingly inexpensive. But you are just buying a disaster waiting to happen.

    It's like smoking, knowing that every pack you buy is buying you a future with the likelyhood of added health problems.

    Adults realize tomorrow will soon enough be today.

    OSB is fine for insulated assemblies, like a detached shed or garage. But still it's crappy stuff.

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