Double Air Barrier
Designing my own passive house inspired home in 4A. The kitchen and living area will have scissor trusses. I recently found and attached a roof assembly that uses a double air barrier and scissor truss am an wondering if this is a problem. Specifically as you move towards the peak and there is to cellulose on the underside of the roof sheathing and the roof is airtight. Not sure if this is a concern or if I am overthinking this.
The second attachment is my modified version with materials that I have access to. Should I remove the roof Zip and use plywood with a highly permeable peel and stick to increase the ability to dry to the exterior or is this not a concern?
Finally, I am assuming that I should (or it would at least not cause a problem to) install and vapor diffusion port at the ridge so that any trapped moisture can get out into the 2×4 ventilation space with no resistance. Is this a correct assumption?
Thanks in advance!
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Replies
Mick,
Unfortunately your assembly won't work or meet code with sheathing under the vent space. You can't use permeable insulation without either a vent space below, or continuous insulation above.
Code also doesn't allow the use of vapour-diffusion ports except in climate zones 1, 2 and 3
What does work is the same assembly without the lower sheathing. The moisture can move through the permeable membrane into the vent space. Josh Salinger has a good blog on those types of roofs, which perform very well: https://www.greenbuildingadvisor.com/article/building-a-vaulted-high-performance-and-foam-free-roof-assembly
Malcolm,
Thank you for your reply and alternative idea.
I thought the same thing until I saw this video. Is this bad advice or is there something different about the assembly Steve is suggesting that I am missing? https://www.youtube.com/watch?v=SooIofGUJdc
Assuming I built the assembly that Josh showed in the video how would I attach the roof sheathing? would nailing to the 2x4s with 2" nails be enough or would I need to nail through the plywood and 2x4 into the roof truss?
Mick,
There are two downsides.
One is code related. IRC R806.2 appears to preclude having permeable insulation in contact with the roof sheathing - unless you have an unvented assemble with continuous insulation above.
The other is practical. With Steve's roof any moisture has to diffuse through the roof sheathing before the vent space can remove it. Plywood is between 2 and 10 perms depending on how damp it is. Zip is only 3. A permeable membrane like Josh uses is between 30 and 70 perms, so you get considerably more drying.
Steve's roof also means you need a second basically redundant layer of sheathing.
I think the minimum nail length for attaching sheathing under your code is 2.5", so they would penetrate the top chord of the trusses slightly.
Malcolm,
Thank you for your response and knowledgeable advise.
Mick, I have designed one project using your proposed assembly, with diagonal 2X strapping over 5/8" Zip and standing-seam metal over that. At the interior we used 7/16" Zip sheathing as air control and vapor control, because the budget was extremely tight and the interior had to be semi-finished. That system cost less than using the variable permeance membrane and drywall I had first spec'd at the interior. The builder is the most diligent I know; he achieved 0.14 ACH50, with mostly double-hung windows, there is a whole-house ERV, etc. so I'm not worried about it, but I would not do it that way again.
I have built and/or designed roofs using the Sarking membrane system that Josh shows in the video that Malcolm posted, and that's my preferred way to design vaulted ceilings/roofs. I have had a lot of pushback from builders who have not built that system before, that installing the exterior membrane directly over the sheathing is too challenging, but I've done it myself and it's really not bad at all, just a few extra steps compared to the traditional approach.
Michael,
"I have designed one project using your proposed assembly"
I've advised Mick that his proposed assembly doesn't meet code because it is an un-vented assembly with permeable insulation against the sheathing, and no exterior insulation. Have I got that wrong?