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Vented over roof on parallel chord truss

WLSchebaum | Posted in Green Building Techniques on

I’m about to commence a project and I’ve got all the details dialed in up to the roof and I’m leaning towards the following.

16″ parallel chord truss, 2/12 pitch, 16″ O.C with no overhangs. I would sheath it in 3/8″ cdx with all joints taped and then WRB on top, either felt or triflex. Then on top of each truss, I would put a parallel 2×4 either on edge or the flat, which would then fly out over the edge of the truss, creating my venting channel and soffit overhang. On top of the 2×4 purlin would then be probably 5/8″ zip system. I’m leaning towards just sheetrocking the interior side of the truss but worry about having it blown off by insulators. Otherwise I would probably go with 1″ polyiso and then strapping.
The logic being that I get a nice solid roof cavity to dense pack with cellulose and not have to go through the trouble of dealing with adding in baffles or what not to get the vented space.
Any thoughts and/or suggestions? I’ve attached my drawings as they stand…

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  1. user-2310254 | | #1

    A shed roof with dense-packed cellulose and no ventilation might be risky. Would you be willing to consider using exterior rigid foam or a combination of foam on the outside and cellulose on the inside?

  2. WLSchebaum | | #2

    Steve, I guess my original post wasn't clear enough. I am venting the roof with either a 3 1/2" or 1 1/2" space on top of the truss. The 2x4 over roof wont be insulated. The drawing I attached shows the detail.

  3. Expert Member

    - Have you thought of substituting a robust permeable membrane for the CDX and felt? It would save a lot of time and money.

    Not directly related to your question, but I'd get the truss manufacturer to extend the level bearing points the ends of the trusses so you didn't have to bevel the top plates and angle cut your studs for the inner-wall.

  4. WLSchebaum | | #4

    Malcom, what did you have in mind for the "robust permeable membrane"? I do like the idea of having the level plates extended, that would be a huge labor savor for sure. For the midspan walls, which there are a few, I could just add a small rip to the top plate, 9.5 degrees over 3.5" isn't all that much of a rip.

  5. Jon_R | | #5

    Gypsum/lots of cellulose/plywood/vent sounds similar to a double wall.

    If you still want to do it, consider adding a smart vapor retarder and another air barrier layer to the interior side and monitor indoor humidity and pressure in the Winter.

    On the other hand, omit the plywood and it sounds similar to a traditional vented attic - a well proven design.

  6. Expert Member

    I'd use Commercial Tyvek, but I'll defer to other posters with greater dense-packing experience as to what might be more appropriate.

    Many double-wall builders like to attach the top plates together with a plywood gasket, both for ease of construction and to provide blocking for the dense-pack. You show the drywall extending over the inner-walls (and the interior partitions?). That makes the sequencing of construction a lot more complicated. Either way, as you say, it isn't that big a slope. Might not even be worth ripping the plates or sloping the studs.

  7. GBA Editor
    Martin Holladay | | #7

    Steve and Malcolm are right -- the layer in this assembly to worry about is the plywood on the exterior side of the dense-packed cellulose. The question is: will the plywood be permeable enough to keep the assembly safe?

    My guess is that it will be fine. But if you are worried, you could use a vapor-permeable membrane rather than the plywood, as Malcolm suggested. The usual choice is Solitex Mento membrane from 475 High Performance Building Products.

    You'd probably need to install the 2x4s (on the flat) over the Solitex Mento before blowing the cellulose insulation, to keep the membrane in place. This approach raises buildability issues -- it's tricky to work on a roof with a membrane but no sheathing.

    You could also use old-fashioned board sheathing (covered by a layer of asphalt felt or vapor-permeable roofing underlayment) instead of the plywood -- it's much more vapor-permeable (although less airtight) than plywood.

  8. WLSchebaum | | #8

    Thanks all for the response. I've done the double wall before with the ply top plate which is great for all kinds of reasons, I would most likely go this route again, especially if i get the truss builder to extend the level heels of the truss. I'm not too worried about the inner layer of the ply, especially if its 3/8 CDX. I could not tape the seams which would help in the perm aspect. As I mentioned, I'm not opposed to 1" of polyiso, taped and strapped on the inside. Martin, as you mentioned, the buildability aspect is a big factor overall that I've been considering. I thought about 2' parallel chord trusses and still aiming for R60 but to get the venting i want I would then be relying on extensive netting up in the truss bays or some form of complicated baffling system.

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