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Another wall assembly ?

user-7445053 | Posted in General Questions on

Greetings,
I am a home builder in southern Maine (Zone 6) and over the past year I have read many articles and watched countless videos trying to educate myself on building more energy efficient structures.  The building sciences really intrigue me.
I am about to build a new home for myself and while I don’t mind being a guinea pig for some new methods or products, I really don’t want to run into a situation years from now where the “technology or science” hadn’t come far enough and I have issues.  I think a lot of builders feel the same way up in New England where the old tried and true is a guarantee not to get a callback even though the methods used are not the best practices.
It’s difficult to hear, “it should be fine” when figuring out wall assemblies.   I have heard Joe Lstiburek say so many time, “don’t do stupid stuff”!  It’s true but some information is contradictory from one professional to another and I want to get it right!  So here is my question with side notes as to why or why not I want to use that product…Thanks in advance.

I would like to construct a wall assembly that uses;
-2×6 studs (I am currently communicating with the company of TStud but it will be a long shot because of shipping charges but we will see and that would change this entire thread! lol)
-blown in celluose (still cost effective but will fill those difficult areas that batts dont reach well)
-1.5″ Exterior foam (Polyiso doesn’t work well in zone 6, so zip R is out.  1.5″ seems to be a nice fit of insulation value and I would be able to fur out the gable ends and window bucks with 2x material so i dont have to use foam there.  I can use a reasonable length galv. roofing nail to penetrate the studs without is being a 4″ nail if say i used 2.5″ of foam or greater.  I do realize that the 1.5″ of foam is not the 11.25 minimum but a recently article in GBA mentioned that it can still work)
-Certainteeds Membrain (I am thinking this will allow the lesser R-value foam to work because any moisture buildup will dry to the interior.
-I am using vinyl siding
-My windows will be OUTIE 

So, if these above items work together, my Big question is this: Water and air sealing, what type and where?
I am hesitant to use a WRB that doesn’t self seal like blueskin vp100 etc. but they aren’t approved over foam. When driving siding nails, I really want to seal up those penetrations.  So if i don’t use any barrier between plywood and foam and I use a product like TYVEK on the outside of the foam, which seems to be the consensus online when using outie windows, all the nail penetrations from the vinyl siding just eliminated its air and water sealing.  Yes, the foam can be taped and sealed to be the air barrier but isn’t there a chance that moisture can build up between the foam and the sheathing?  I have looked for the answer all over and no one has really mentioned it.  If moisture leaks past the foam (ex: through a nail from siding or a missed penetration or vapor from the interior moves through the osb/plywood and hits the foam, isn’t that a place for mold/rot?  I really like the idea of zip R with a fluid applied product but I feel it’s reduction of R value in the cold isn’t a great product to choose.  Also, I am skeptical of not having some type of WRB over the foam and using just foam to seal air and water.  I struggle to trust tapes by themselves over the foam.  Sorry for long post and thank you.
Doug

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Replies

  1. Jon R | | #1

    Use foam with > 1 perms. It's best to have air barriers on all six sides of your cellulose. Taped plywood can be one of them.

  2. AlexPoi | | #2

    If you decide to go with a wrb behind the foam, it's still possible to integrate it with outie windows. You just need to build a plywood or 1x4 box to extend the window enclosure. Just like in this video https://youtu.be/-jolGhGbd18

  3. Bill Dietze | | #3

    Doug,
    For zone 6, 1.5" of foam is much less than recommended here: https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing

    Bill

  4. GBA Editor
    Brian Pontolilo | | #4

    Hey Doug,

    As Alex pointed out, you don't necessarily have to flash outie windows to a WRB installed over the foam. There are a few variations for each type of window install. Innies seem to me to be less complicated to flash, and either way, your building some jamb extensions on one side of the wall of the other. If you haven't seen these article, you may find them helpful:

    https://www.greenbuildingadvisor.com/article/innie-windows-or-outie-windows
    https://www.buildingscience.com/documents/insights/bsi-085-windows-can-be-a-pain

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