Venting a cathedral ceiling with 24″ of cellulose
I am building a Passive House with a cathedral ceiling. I used parallel cord trusses with 27″ between the top and bottom cords. I will put OSB on the bottom of the trusses as my air barrier and to hold the weight of 24″ of loose fill cellulose insulation. I want to vent the roof, but the only baffles I’ve seen are made of thin vinyl, polystyrene or cardboard. I am afraid that they will be crushed by weight of the cellulose, which would eliminate the air channel. The roof pitch is 10:12 and is 20 feet long, so the baffles at the bottom will be under a lot of pressure.
Does anyone know of a stronger, more rigid vent baffle that could withstand such pressures?
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According to Bill Hulstrunk, an expert on cellulose installation issues, AccuVent baffles work well with cellulose. You can read more here: How to Install Cellulose Insulation.
If you are still worried that AccuVent baffles will be too flimsy, in spite of Hulstrunk's opinion, nothing is preventing you from installing site-built baffles out of plywood. Builders make site-built baffles all the time.
Here is how we're dealing with that issue. It is a similar system except that we're using 16" wood I-joists for our roof trusses (a.k.a., TJIs). In summary, we're using 7/16" OSB on the bottom of the I-joists, then Tyvek stretched across the top of them. Then a 2x4 is nailed "on the flat" to the I-joist top flange. Then plywood is used as the roof decking on top of the 2x4's. This should create a 1" to 1.5" ventilation gap . We have a "clear story" style roof so it is very simple and the ventilation should work pretty well. Our pitch is 4:12 and 6:12.
When you say "crushed by weight of the cellulose", I assume you mean towards the bottom of the trusses due to your roof pitch? Otherwise, the baffles should be on top of the cellulose so it would seem that most of the pressure (weight) is not on the baffles.
In any case, the solution that we are using should work well in your case. I would expect that there is some labor savings versus building baffles on-site. In my project I always tried to avoid tasks that framers would view as laborious and/or tedious because that would increase the bid (even more!). One of the biggest areas for griping from framers has been building the separate overhang structure... but I digress.
Chris, nice detail. If you would love to see the rest of the details you use in building, foundation, sill, etc.
Sure, I can email the plans to you if you like. Email me at crispy [at] ymail [dot] com and I can send you the PDF. It's quite large though, something on the order of 140 MB. The ideas are not that revolutionary though, I have just chosen a mish-mash of ideas that others have presented. :)
Dropbox should do it... will get back to you Chris...thank you...