Cathedral ceiling insulation question
I’m in the process of replacing the 100+ year old slate on my home with EcoStar synthetic slate. The original slate was installed on traditional skip sheathing – true 1”x5” boards with a 1” gap between. Somewhere along the way, batt insulation was installed, along with a plastic vapor barrier and drywall in part of the attic. The roof is 12/12 pitch. Not a good situation but at least the system could breathe outward. The new installation required a full plywood overlay, which would eliminate this ability, so I decided to create ventilation channels over the skip sheathing by placing 2x4s flat on top of the old sheathing, directly over the true 2”x5” rafters. ¾” plywood was laid over those, with appropriate underlayment on which the EcoStar will be installed. Not that it’s particularly relevant (just a source of pride) but I’m also having the long-gone stop gutters rebuilt as part of this project. Here’s the problem/question – how best to insulate this thing, as clearly the 5” of deteriorated fiberglass will not suffice and is probably unhealthy in its current state. Unfortunately, insulating over the roof was not an option, owing to the constraints of living in an historic district. (Just getting the 2-1/4” added thickness was a heavy lift!) Ideally, I’d use high-density foam, but the spaces in the skip sheathing would allow the foam to block the ventilation channel and blocking the spaces would be a fool’s errand. So far, cut and cobble (a 2” layer and a 3” layer of foam) to fully fill the cavity seems the best option. An airtight R25 would be a lot better than what’s there, but I’m still wondering if this is the best solution. Lastly, this area is an attic, that is mostly used for storage, but it does need to be semi-conditioned.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Hi James -
Sounds as though you just need a sheet good stop of some sort to receive/be the substrate for the spray foam?
Peter
You can staple house wrap along the bottom of the skip sheathing and spray foam over it. Since the roof is vented, you can use open cell foam.
Cut and cobble is generally a huge waste of time only to end up with a roof that is marginally better than high density batts.
Thanks. I had considered this but was wondering if the foam would stick to the house wrap.
Spray foam is very similar to Gorilla Glue- it sticks to just about anything. Even if it doesn't adhere to the housewrap in an a very aggressive a way, once it's in place it will stay there- it's fairly rigid. The housewrap serves to stop the droplets before they expand, flowing/expanding into adjacent expanding droplets to form a solid block perfectly fitting the enclosing space.
Spray foam stick pretty well to the backside of Tyvek. It sticks, but not well to the front.
About the only thing spray foam doesn't stick to is 6 mil poly.
I've had no issues with it in the walls of a garage. To make sure, it would be good to check with the spray foamer before starting.