Attic Floor with Closed Cell on Undersurface Ventilated Attic
nikojo
| Posted in General Questions on
Planning for ventilated attic above second level.
Mechanicals would be in closet on second level. Ducting would be within ventilated attic. Would seal ducting with closed cell as needed.
I understand ideally would be best to have conditioned attic. I don’t want to use closed cell on undersurface of roof and don’t want to use rigid foam outside roof. Also would prefer to not use blown/batt in attic.
Plan would be to lay down plywood over truss bottom cord to have attic floor. Then blow closed cell along bottom of plywood to insulated/seal.
Drywall would then intall at end as second floor ceiling.
Any problems with this??
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Why don’t you want ccSPF on underside of roof sheathing if you are using anyways?
In new construction there are lots of ways to run ductwork in the conditioned space. You can just frame the ceiling a foot higher and drop the ceiling. Or you can use open trusses.
One of the big advantages of a ventilated attic is that you can run insulation in a horizontal layer across the attic floor, which allows you to use the least expensive insulation. You're losing that advantage by using spray foam.
nicojo,
- What kind of trusses are you using? Typically they have webs about every six feet or so, which makes any floor very difficult to install or make continuous.
- Does 3" of closed cell foam meet code where you are?
- Beyond your preference, I don't think using foam on the attic floor makes any sense.
Thanks for responses.
I don't like idea of closed cell on undersurface of roof sheathing as I worry 'if' there is a leak it will be more difficult to identify and will be more difficult to mitigate if closed cell needs to be removed.
Agree cheaper insulation can be used over the roof attic but like the idea of closed cell in terms of air/vapor control/sealing.
Agree trusses may make installing plywood more difficult. Would strap down from the plywood and bottom of truss to make up enough depth to apply needed thickness of closed cell, ie 9-12 inches of depth and spray enough for R49.
Only challenge might be installing lights/cans as needed.
A variable permeance membrane and a bit of tape will do everything you're expecting closed cell foam to do at a fraction of the cost and a fraction of the environmental impact.
In New England we furr ceilings as a matter of course (called "strapping"); I typically install a variable permeance membrane on the bottom of the truss, then run 2x strapping and 1/2" drywall, which works well with LED wafer lights. But it doesn't leave space for mechanical systems.
On a current project, instead of strapping we're using 8" T-studs as ceiling joists and will run mechanical through them, keeping all mechanicals below the air control, vapor control and thermal control layers.
It feels a bit like the worst of both worlds. If you want a vented attic you are able to use cheaper, greener insulation. If you want an insulated attic you use spray foam, as it’s an easy vapor/air seal. You should just use either your gypsum ceiling and paint or a smart membrane as you air vapor barrier or else enclose your attic with spray foam. The not noticing a leak does not seem like a good reason to not use spray foam on roof deck, and if you do get a leak on roof deck remediation would be from the roof side anyways.
What if they laid down rigid foam over the trusses and then plywood. Then do the needed amount of closed cell from underneath and onto the rigid foam? That would limit need/use of closed cell.