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WRB behind rigid foam and rain screen – is this risky?

natesc | Posted in Energy Efficiency and Durability on

Location: Central NY, zone ~4b

Zip-wall says you don’t need a gap between their product and exterior foam (let’s pretend it is vapor closed foam board, 3+” of XPS), as long as you don’t have a vapor barrier inside the wall cavity (closed cell spray foam). Water that gets trapped between the zip-wall and the foam will dry via diffusion.

Kind of confused by this product, they claim something like 12-16 perm rating on the impregnated membrane, yet the OSB itself is not nearly that vapor open. Is this misleading? What is the perm rating of the OSB & WRB combined?

I guess I can’t wrap my head around the idea that we can have (from outside in) a rain screen, then rigid foam, then WRB. Rain will still get behind the foam. It will only be able to dry via diffusion. This seems risky to me. Especially if windows are installed as innie’s.

Tyvek (I’m sure you all know) has a housewrap product that is ‘crinkled’ or ridged, that will allow whatever water that gets behind the foam and rain screen to drain. This coupled with good old plywood seems like it would be a much more robust system. The problem with this is if I want an exterior air barrier, for the best result I’m going to have to tape the plywood, as housewrap simply does not make for a good air barrier. I haven’t put pencil to paper, but this has to be significantly more expensive, even excluding labor.

One solution I pondered was to use capped roofing nails poked into foam board to create a small gap between the foam and the zip-wall to allow for drainage. The small gap will not have much impact on r-value. Is this just crazy talk?

This buildingscience article implies I shouldn’t worry so much:

“If water does get into the OSB from the outside it will be minor in nature and the OSB can dry to the interior – as long as we have cavity insulation that is vapor open and no interior vapor barrier. This works for all claddings, not just hardcoat stucco. But, you do need the gap.

I’m a little concerned that around innie windows and other openings, the amount of water that gets behind the foam might not be minor in nature.

Can anyone set me straight on this?

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, you may want to read this article: Where Does the Housewrap Go?

    If you choose to put your WRB between the wall sheathing and the rigid foam -- and many builders do -- there are many reasons why you don't have to worry about water being trapped behind the foam. The main reason is simple: Unless you have made gross construction errors, you will never have any liquid water in this location.

    Huber, the manufacturer of Zip System sheathing, reports that this sheathing has a vapor permeance of "less than 1.0 perm." So it's not 12 to 16 perms as your report (at least if you include the entire product, not just the factory-applied overlay).

    Anytime that you have a vented rainscreen gap between the rigid foam and the siding, you have included a mechanism that is able to handle water that gets past the siding. Any water that gets past the siding quickly evaporates. The water never reaches the WRB.

    If you will sleep better at night knowing that your wall has crinkly housewrap between the rigid foam and the OSB, then by all means use a crinkly housewrap. Joe Lstiburek discusses the pluses and minuses of this approach (the thermal penalty exists, but it is very small) in his article, Mind the Gap, Eh!

  2. JC72 | | #2

    My understanding is that current code already requires the taping of seams with regards to house wrap. As for foam the assumption is that the seams of any foam you apply on the outside of the house wrap are filled with foam and taped.

    Have you inquired about ZIP-R ? You can get R6 with the 1.5" product (Their foam layer is in the interior side of the OSB). The cost of this material is approx. the same as the cost of OSB, TAPE, house wrap ,and equivalent foam thickness. Your savings is hopefully realized in the labor.

    As for innie windows you'll apply peel-n-stick over the top of the foam and either have an extended sill or build out a box/picture frame within the rough opening in which you'll set the window.

  3. KeithH | | #3

    If you are losing sleep over this, did you consider a vapor open exterior insulation such as Roxul Comfortboard? Full disclosure: I want to do this type of exterior sheathing but pricing is still tough in my area. However, I suspect it is readily available in your area.

  4. natesc | | #4

    Thanks for the responses everyone.

    I just noticed I accidentally posted I am zone 4b (Which is my Grow Zone). My Department of Energy Zone is Zone 6.

    Martin / Chris- the more I am getting into the details of how to properly flash an innie window the more complicated I'm realizing it is. Connecting the window to the WRB is very simple, just like installing any window. I didn't consider how important it is to drain the sill and jam extensions back out to the siding. I've read a bunch of articles on this today, and haven't found a whole lot actual specific 'best practice' methods for this.. not sure if I am looking in the wrong places?

    For example, how should I apply peel n stick over the exposed foam that faces the window, are we basically extending the window & WRB interface out to the siding? In the Roxul install guide for ComfortBoard, they put down their rough sill membrane, then another membrane on top of the rough sill that will extend out to the siding.

    Keith - I need to get quotes on Roxul. I am still in planning stages, but am aware of the product. If the price is right, I might go for it. I can get 4" EPS 4x8 sheets delivered for $49 each.

  5. RedStag | | #5

    I plan on using Zip system sheathing, rigid foam, 3/4" strapping for rainscreen, then vinyl siding. I've read the article "where does the WRB go", but am unsure of how to flash an "outie" window even with a plywood picture frame around the opening. Is there a best practices for this?

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