Vent over roof over parallel cord roof trusses
I’m designing a home in northeast Wisconsin, IECC climate zone 6. I’m considering a 12:12 sloped parallel truss for roof trusses to allow dense packed cellulose and provide a cathedral ceiling for the second floor. The truss will span 26 feet. My target is R 65. I understand that ventilation baffles are not vapor open, so any moisture that gets into the cellulose from inside will not dry to the outside (?). Therefore, I’m considering a vent over roof. My roof assembly would be raised seam metal roof, roofing membrane, 5/8 inch plywood sheathing, 2×4 on edge for a 1-1/2 inch ventilation channel, 5/8 inch under sheathing, 18 to 20 inch parallel cord roof truss and a gypsum ceiling with class II vapor paint (air barrier), 1-1/2 inch service cavity, and gysum ceiling.
I’ll have PV panels on the roof. The 12:12 slope provides space for upstairs bedrooms and helps shed snow off the PV panels.
Is this a reasonable roof assembly? I’m open to any comments or suggestions.
Also, can insulation contractors install dense packed cellulose into a truss system without penetrating the ceiling (inject from the eave end of the truss), so I don’t have to place and patch holes in the gypsum (ai barrier)?
Thank you very much for your thoughts.
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Replies
You can make vent baffles out of vapor open materials such as 1/4" waferboard. You also typically have some limited amount of drying even with conventional baffles, since they aren't perfectly sealed, and some moisture can typically migrate through the framing materials into the vent channel as long as the amount of moisture that sneaks up into the assembly is pretty minor. My preference here would be to use the waferboard though and do site built vent channels.
I don't think you'd be able to dense pack from the ends for such a large span. You might find Spider insulation is a good fit here though, although it's fiberglass and not cellulose. Spider is sticky and can be applied to open ceilings, so you could use that and then drywall could go up after.
Bill
+1 on Bill's comments. It is hard to dense pack cellulose in that sort of ceiling for a bunch of reasons. It is very unlikely that it could be done successfully from just the eaves. If you go with any sort of blown-in insulation, it gets done from multiple points on the inside prior to drywall. If your trusses are more than 16" on center, the insulation netting will sag enough to make drywall installation a nightmare. Cross strapping can help with the sagging and can also provide a thin chase for electric to reduce penetrations.
Steve,
Building codes preclude having permeable insulation against the underside of the sheathing in your climate zone.
A variant of the roof you describe that works well, is to use house-wrap instead of the lower roof sheathing. That allows good drying into the vent channel, and saves a lot of materials and expense.
https://www.finehomebuilding.com/2021/04/28/a-new-take-on-insulating-a-roof
https://www.greenbuildingadvisor.com/article/building-a-vaulted-high-performance-and-foam-free-roof-assembly
Ha! I agree this one isn't allowed. hopefully that other discussion gets picked up, as you say.
freyr-design,
I came on a response from Martin on that subject (which of course I now can't find again...) that agreed with you it was code compliant, but not used because it was too risky. I'll poke Randy and see if he has any thoughts.
I've used the system Josh described in those articles on several occasions, and will be using it on a current project. I always get pushback from builders but those who do it say it's not that hard. I've done it; it's not that hard. And it works very well. It's code-compliant with the IRC. They only require sheathing to be fastened directly to framing at walls.
SteveBanks, what you describe is a viable method; I did it once using 7/16" ZIP sheathing on the interior, 12" bays of dense-packed cellulose and 5/8" ZIP on the exterior. We used 2x4 furring run diagonally without another layer of sheathing, but in the end the roofers said don't do that again.
The main downside is that sheet goods aren't very vapor-open; even plywood will slow down vapor movement.
It's possible to insulate deep, long bays from one end but it takes a lot of experience and testing to do it reliably well. I would not rely on most installers to be able to do it well. I always plan for a full Insulweb membrane for blowing behind, then add any airtight layers against that.
thanks so much for the excellent responses. I really appreciate it!
I ran into this product last year at IBS:
https://advancedbuildingproducts.com/product/insulair/
It's designed to create a space for roofs with closed cell spray foam at the roof deck. I think it would work with dense packed cellulose, but you may want to check with the manufacturer first.
Such steep roof over 26' you have a lot of height near the peak.
One option is to frame up a mini attic that is a couple of feet high (more height, easier to insulate). Don't need any thing fancy, pretty much a collar ties between the two legs of the trusses to form the attic space. You can make it accessible through a larger gable went on either side of the house.
You would still need those permeable baffles (thin OSB/CDX/MDF or house wrap) but now you can loose fill the whole thing from the mini attic. Since it is loose fill it will settle a bit, so you have to add additional insulation in the mini attic to make up for this settling, which is easy enough as there is lot of space there. The loose fill also won't put as much pressure on the baffles or the ceiling.
Easier than an over-roof and none of the issues of trying to dense pack trusses.
Akos,
Also solves the complications around the ridge beam that always occur when you take the roof to a peak. That's how my house is framed, and I like the resulting small flat areas.