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How many control layers is too many?

jonny_h | Posted in Green Building Techniques on

Everyone loves questions about wall stackups, so here goes!

Existing 2×4 frame wall with dense-pack cellulose, 1x board sheathing.  Plan is to remove existing layers of siding and add fully-adhered membrane (Solitex Adhero) as primary air barrier, 3″ total of polyiso continuous insulation, and siding over 3/4″ furring strips.  Flangeless windows to be installed as in-betweenies (see my post here: 

My original plan has been insulating with 2 layers of new, foil-faced polyiso, taped seams, with the outer surface of the foam acting as the primary water barrier.  (I know the Adhero is also a water barrier, but given my window positioning using it as the primary water barrier makes all the flashing details very tricky, and I’d much prefer for the primary water barrier to be right at the back of the rainscreen gap.)

I’ve tracked down some reclaimed 3″ roofing polyiso, which I’ll be using on my roof.  It’s felt-faced, so it wouldn’t be suitable as a taped water barrier, but it’s like half the price of new, so I’m considering it for the walls as well.  This could be done a couple different ways:
-One layer of 3″ reclaimed polyiso, plus 1/2″ of new foil-faced and taped polyiso.
-One layer of 3″ reclaimed polyiso, plus a layer of regular old Tyvek housewrap (this is the lowest cost solution).

I *think* any of these work from a vapor profile perspective — there’s always enough exterior:interior insulation ratio for my area (zone 5), and actually the felt-faced polyiso is likely more vapor permeable than foil faced anyway.  Something just feels weird to be about effectively having two membrane control layers — the fully adhered layer behind the insulation, and regular housewrap outside the insulation.  Other than installation difficulty, though, am I missing anything here?  Or would I be better off sticking with new polyiso for the walls and keeping the reclaimed on the roof?

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  1. Jon_R | | #1

    I'd use only reclaimed foam + Tyvek . IMO, this will have better bulk water performance and better water vapor performance.

    General principle - don't add vapor barriers where they aren't required.

    1. jonny_h | | #2

      Well, that's one vote for and zero votes against! Going to pick up the reclaimed foam on Tuesday! Certainly will be the lowest cost solution, and it's nice that I won't have to buy much newly-manufactured foam (wish that golab had their wood fiber boards ready though!) Nice to try to be slightly more green (despite how indefensible this project actually is....)

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