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Community and Q&A

SIPS with Aluma Flash Plus WRB

perry77 | Posted in General Questions on

I’m going to be building a house in Climate zone 2. The home will be slab on grade with SIP walls and roof. The question is or rather has anyone used Aluma Flash Plus on SIP walls ? I’m also considering Delta Vent SA. I’d also be using a rainscreen and wood siding.

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  1. Expert Member
    RICHARD EVANS | | #1


    What does the SIP manufacturer say about using a Class I vapor retarder on the exterior?

    If water ever gets behind the Aluma Flash, then the exterior OSB must dry to the interior. To do so, it will have to dry through 5" of EPS (which is not very vapor open) and also through low-perm interior OSB- all at a higher rate than it is being wetted. This is a concern with any wall of course but with SIPS, the OSB is load bearing.

    I'd follow the manufacturer guidelines. In the absence of that, I would prefer an assembly that allows the SIPs to dry to the exterior AND interior. A rain screen gap and wood siding (as you wrote) is probably sufficient to ward off inward solar vapor drive (just stay away from vinyl wall-paper and interior poly). Ultimately, The Aluma flash may create more problems than it solves. I imagine it isn't cheap either. 15# felt and 1x3 strapping may be all you need.

  2. Zdesign | | #2

    SIPS always need to dry to the outside and prevent any interior moisture from entering into them. Vapor closed tape on the interior seams, Vapor open tape on the outside seams and open cell spray foam for the gaps from the exterior prior to taping the seams.

  3. perry77 | | #3

    Thanks guys. All great information. As this house is likely to see a lot of rain and humid conditions I like the idea of the Aluma Flash Plus but wasn’t sure in the science as using it on sips you have essentially OSB sandwiched between 2 vapor impermeable barriers the EPS foam and the Aluma Flash.

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