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Can I use two layers of Tyvek in a wall assembly without moisture problems?

Gary Dick | Posted in Green Building Techniques on

I am building a house which has a REMOTE wall system in it (foam “outsulation”). Although I passed on the relevant information to the builder, who is usually on top of the details, he managed to mess up with the window bucks.

The wall structure is 2×4 studs (with R13 cavity insulation), plywood sheathing, Tyvek Drain Wrap (wrinkled stuff) 2 layers of EPS foam of R10 each sealed with Siga Wigluv tape, then 3/4″ plywood furring strips followed by wood siding.

The problem is I purchased Tyvek Flex wrap to be wrapped around the “outie” window bucks (per the REMOTE manual) but he forgot to install it before placing the 4″ of foam. So, at present, if water were to get back into the foam it will run down to the headers and sides of the window openings, contacting the bare wood.

We could cut out the foam around all the windows to get to the Tyvek and then place the Flex Wrap. That’s going to be messy, probably screw up the tight and carefully staggered foam joints and I think will need lots of spray foam cans to fix the gaps when putting it back. It will also need a bunch more Wigluv tape.

I read about using foam itself as the WRB and am reluctant to do that (because of potential board shrinkage leading to gaps / tape coming unstuck etc). I was thinking that the fastest way to get back to a continuous WRB would be to rewrap the house in Tyvek Drain Wrap on top of the foam (effectively abandoning the existing layer at the sheathing). Then we can use Flex Wrap to tie the window bucks to the new Drain Wrap. I know you are not supposed to create a membrane sandwich – but will trapping the foam between two layers of breathable Tyvek membrane be likely to reduce permeance enough to cause an issue?

The house is in climate zone 6 in SW Colorado. Many thanks for any advice on this or alternative solutions……. Gary

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Replies

  1. Dana1 | | #1

    The Tyvek is at least an order of magnitude more vapor-open than any type of rigid foam. The reduction in total vapor permebility of having Tyvek on both sides of the foam is "in the noise", smaller than the statsitical variation between sheets of plywood from different lots (or even the same lot.

    Type-II EPS is less than 1.5 perms @ R10, less than 0.75 perms @ R20. Two layers of Tyvek in combination are still over 10 perms.

    The plywood itself is about 1 perm when bone dry, maybe 6-7 perms when pond-soaked wet.

  2. Gary Dick | | #2

    Thanks for your help - if it's just "noise" I think adding another layer of Tyvek to get a continuous WRB and flashings is the way to go. I am going to discuss with the GC later this morning

  3. Aron_Robinson | | #3

    Gary..how did this work out for you.? I currently have a similar problem. I'm going back to have new siding installed after defective certainties cement board failure. Will be having windows taken out and having second layer housewrap installed over foam shearing with layer housewrap under the foam and over sob. Sanwichednlike yours. Did you install 2nd layer wrap over uuour foam and didn't work well?

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