Cathedral Roof: To Vent or Not To Vent?
I’m building a vacation house on some rural property in Montana. It can get very cold in the winter and very hot in the summer. And we have a fair amount of wind in our area. The walls of the structure are going to be built using Faswall ICF blocks. The roof will have two sections with a single pitch roof design separated by a middle section with a more conventional two-pitch roof.
For the two single pitch sections, I’m planning on using 14″ TJIs to span roughly 20′ with a cathedral roof design. I can use 4″ of closed-cell polyurethane foam and 7″ of Roxul Comfortbatt to get close to an R-60 roof. But I’ve heard some people say they’d NEVER spray foam to the underside of their roof decking, anticipating what a nightmare it would be if the roof ever had to be torn off.
So I’m wondering why a guy couldn’t set the TJI joists then fit some 1/4″ plywood between the trusses attached to the upper web of the joist, spray your foam onto that material, fill in the remaining thickness of the web with the Roxul, and then the interior ceiling material. The actual roof decking itself could then be laid down and there would be about a 1.5″ open gap (the thickness of the flange) between the 1/4″ filler plywood with the foam on the interior side and the 5/8″ roof decking.
This way I could have a vented space below my roof decking, cover it with 30# felt, and put down my metal roofing. It seems like a pretty clean way of insulating the cathedral portions of my roof while protecting my roof decking and, should I have to ever replace or repair the roof, I could remove the decking without having to rip out all the sprayed foam insulation.
Does this make sense or am I missing something big?
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Replies
David,
Your suggested approach will work. You're not the first person to think of it, and I know others who have done it. I think it's a good plan.
-- Martin Holladay
Note that 4" of closed-cell foam ordinarily comes with a substantial climate impact, due to the blowing agents used. You can avoid that by specifying one of the new closed cell foams blown with more benign "HFO" blowing agents: Lapolla 4G or Demilec "Heatloc XT.
But any closed-cell foam will degrade in R-value over time, so your foam-to-fluff R-value ratio will degrade and eventually you have less condensation protection. And your overall insulation will degrade. And that's after spending a lot of money on thick spray foam. So you might consider some other ways to achieve the same result. One would be to use the same baffle configuration, but with 18" TJIs, so you get the same R-value, but with all mineral wool (or cellulose), and you get a long-term stable R value. Another would be to put polyiso boards on the bottom side, between the TJIs and the ceiling drywall.
Thank you for the prompt response. Now that I have some assurance the plan is feasible, I'm wondering about materials. Any preferred material to use between the TJIs to provide the substrate for the closed-cell foam installation. And I'm assuming I don't want anything coating the material on the upper "vented" side so as to allow any moisture to dissipate. As for the roof decking, I was thinking about using the 5/8" Zip System roof sheathing, then roofing felt, then my metal roofing with a Cor-A-Vent product on each end to allow air movement through the open channels between the TJI flanges.
All in all, I think it should work well in our area which has big temp swings but is pretty dry most of the year. Any additional insights you might offer would be greatly appreciated.
And, hey, I just ordered your new book. My wife says my next project should be building a house using all of the home improvement and building books I have around here.
Again, thanks for your advice.