A mostly foam free cathedral ceiling question
Hello all,
First, thanks to everyone who has commented on my prior questions (Akos in particular). This is site is a remarkable resource.
I’m in the process of a major renovation of a 1930’s brick rambler in Salt Lake City (Zone 5B). Unfortunately, the roofline is a bit tortured, so I’ve been looking at cathedral option for a couple of dormers.
My current thought is an assembly much like the one Josh Salinger outlines here: https://www.greenbuildingadvisor.com/article/building-a-vaulted-high-performance-and-foam-free-roof-assembly
My only change would be to do 2-4″ of polyiso, taped seams in place of the Intello. It would be held up with 2x on the flat for a service cavity. I would also do my best to have my drywall crew make the drywall layer as air tight as we can. This would eliminate much of the inward drying capacity of the assembly, but given that Salt Lake is really dry (we actively humidify, something I know sounds crazy to folks in the south east), that seems like less of an issue. The reason for doing this is to get my ceiling insulation a bit higher than the 11 7/8’s I Joists will allow. That said, maybe the extra insulation isn’t worth the loss of drying capacity. I’m curious on this group’s take.
so overall, the assembly from exterior to interior would go: Shingles, decking, 2x on the flat for vent cavity, Tyvek, dense pack blown in for 11 7/8″ TJI depth, polyiso, 2x on flat for service cavity, drywall.
This will let me avoid spray foam in the roof and make any decking replacement or roof repair down the road substantially easier. Thanks in advance for any insight.
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Replies
jacktneslon,
What you describe sounds like it will be a well performing roof. It will have excellent drying capacity to the outside, you don't need to worry about it being able to dry inwards. The interior foam will also mean it will not be susceptible to condensation problems in the cooling season.
Malcolm,
What are your thoughts on how to best seal the screws or nails going through the 2x holding up the polyiso? Tape along the boards? Don’t sweat it too much? This is one piece that
I’m not sure of.
Jack
Jack,
I wouldn't worry about them at all. Fasteners work by friction. If the fastener was loose enough that air could pass through the gap, the fastener would fail.
Malcolm - thanks for the quick response. This is what I thought, but its nice to have someone with actual experience confirm it. I also think this may be a little easier with the crew I have to get a decent air barrier, as I have real concerns about the crew's ability to avoid puncturing a smart barrier.