Self-Adhered WRB on Board Sheathing
Hey all,
Would any of you have any knowledge of how a self adhered WRB performs when placed over board sheathing? Specifically, I am considering these materials for new construction.
Am I overthinking things when I imagine that there could be issues over the long term, with the gaps between boards / potential sharp corners, affecting long term air tightness? It occurs that the width of the gaps between boards could change throughout various seasons, I’m having trouble finding any info as to weather or not tears in the membrane could result.
Plywood is of course an option, I just happen to be planning on building in an area with a number of local sawmills (central VT).
Thanks!
Ben
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Replies
Check out this Q&A thread answering the same question. Henry Blueskin wrap is a recurring theme.
Ben,
A lot depends on how dry the board sheathing is. The typical advice to use a self-adhered WRB over board sheathing comes in the context of dealing with an old house with dry lumber, where the WRB will also serve as an air-barrier.
In new construction board sheathing can shrink appreciably (The once tight 1"x8"s I used on two dormers now have 3/4" gaps between them) and it's rare for the WRB to perform the function of a primary air barrier. If I was planning to use board sheathing, I'd apply a regular sheet WRB like Tyvek Commercial.
Thank you both for the info!
It appears to me, on the linked Q&A thread, that the house that is being remodeled had 3/4” (ish) gaps in between boards. When reading about the new construction dormers that developed a similar issue, it occurs that my concerns might be justified.
In new construction, with a mechanically fastened WRB, if it is better to have a second air barrier, is an interior variable perm membrane the way to go? If so, In your experience have you seen any benefit of building a service cavity to reduce puncturing the inner membrane?
Thanks again, your help is much appreciated
Ben
Ben,
I think an interior air-barrier, whether a smart membrane or air-tight drywall, is easier to detail as the primary air-barrier than the WRB, and less prone to damage both initially and over time.
Before deciding to include a service cavity, work out what will actually be run in each wall. You may find there is nothing but one or two outlets - which may be easier to just detail in an airtight manner, rather than build a whole wall.
There's been concern, mentioned when I proposed Tyvek+board sheathing, that mechanical house wrap won't make board sheathing airtight. I'm still skeptical of that.. Mindel and Morse Builders & Robert Swinburne in Southern VT seem to use alot of board sheathing and taped proclima Mento WRB. I know some of their houses are certified Passiv hause but I'm not sure if they use a interior airbarrier for air sealing instead of the exterior WRB.
Kylie, how about getting Mindel and Morse to share some tips on building with green local lumber?
I will bet it is not green, just local. Board lumber has to be very dry and the edges need to be smooth if you want a good fit and low shrinkage. My experience.
This is all great information. Sounds like one would have to commit early, get the lumber sooner than later, and put it on stickers. I appreciate the viewpoint of all the effort being put into building the service cavity, for the potential of it not really doing much.
I really enjoyed Robert Swinburne’s presentation on BS and Beer about building a Pretty Good House. It was the first real world example I had come across of a high performance build with board sheathing. (If the podcast is looking for topic material, I for one, would drink the appropriate beverage while watching an episode dedicated to this subject)
Ben
I would not use it over rough sawn green lumber.