Smart vapor retarder details
As I look at how to air seal a cathedral ceiling to be constructed with scissor trusses (Zone 6A), using a smart vapor retarder (SVR) seems to be a natural choice.
Ceiling
All the videos I see show various strapping applications with 1x or 2x lumber, and I am wondering why drywall is not installed directly onto the SVR surface.
I can see the benefits of providing sturdiness if the product is used as a base for blown-in insulation (as our is planned)–is there a reason beyond that?
Walls
I have also seen videos where SVR is draped over exterior walls and the trusses are set atop. Seems like a good way to create a continuous air barrier. However, we will be using ZipR and are confident we can create a good air barrier with careful detailing on that assembly. It seems like applying the SVR to the bottom of the truss (ceiling) and continuing to the walls below the top plates would suffice. In my case, is having the SVR running over the top plates serving a purpose?
Thanks for your insights.
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Replies
Ceilings: strapping serves several purposes: As you mention, it can provide additional mechanical support for blown-in insulation. It can also provide space for running wires below the SVR to reduce penetrations through the air/vapor barrier. It is also just "the way things are done" in much of the northeast and possibly other regions.
Wall top plate sealing: In your question, you recognize that many articles show the SVR draped over the top of the walls in order to provide better air sealing for this difficult transition area. The builders and designers who use this technique have developed it as a "best practice" based on experience. It is very difficult to make the transition from exterior to interior air barriers without using either the SVR or one of the various self-adhesive membranes. You are certainly welcome to use other techniques for this transition and you can certainly get good results with careful detailing. There is no further benefit that I am aware of to this technique.
Peter, thank you for the reply. It seems like a relatively simple thing to drape the SVR before setting the trusses for a flat ceiling.
With scissor trusses/vaulted ceiling, I see 2 options: 1) drape the SVR over the exterior wall with enough length to overlap (staple and tape) with the SVR that gets attached to the trusses, or 2) calculate the distance to the ceiling peak on each side, drape these long pieces over the exterior walls and attach to the trusses all the way to the peak. Do you favor one way over the other, or is there a third way I am not considering?
Thanks again.