Closed-cell foam / Flash and batt
Pga chick
| Posted in Energy Efficiency and Durability on
I am in the process of reinsulating my home. I have a 1,000 sq. ft. flat roof. Under the roof is 6″ wide tongue and groove boards attached to a 2 x 6. It’s sloped ( -2/12 pitch).
I would like to use a “flash and batt” approach. I’m a little unclear on venting. Does it need venting? Currently I have 2 sides vents and a soffit off the back of the house.
Would 1″ to 2’of closed cell foam with R-38 batts be ok? Or would foam board be better? Under the roof is more then enough space to accomodate the insulation. I like the foam idea becuase the house is in the woods. Just concerned with moisture, bridging, etc.
Any advice?
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Replies
Gregg,
You can use a flash-and-batt approach to insulating your roof if you want, as long as the spray foam layer is thick enough to prevent moisture from condensing on the interior side of the cured foam during cold weather. If you go this route, you'll want to seal up all of your attic vents. You'll also need to insulate your short attic walls, and make sure that you maintain thermal continuity between your wall insulation and your roof insulation.
For more information on insulating a low-slope roof, see Insulating Low-Slope Residential Roofs.
I'm a litlte concerned of why I shoud seal up all my attic vents. The 2 X 6 roof deck(sloped)) will be spray foamed with an 1' to 2" closed cell foam. The flat ceiling is a seperate 2 x 6 joist that I plan on adding R-38 fiberglass batts. Between the 2 areas is a big area of open space. Shoud this remain open? Windwashing they may call it? Or is it better to push the batts up against the foam? The soffet vents are minimal. Both side of the house house have vents in the wall above the ceilng line.
Also curious of how much batt insulation is too much that will create condensation?
Gregg,
You have two choices: you can insulate the attic floor (the ceiling), or you can insulate the underside of the roof deck. If you are installing spray foam against the underside of your roof deck, that means you've made your choice. You have decided to insulate your roof. This is an unvented approach.
If you do that, you have to seal up all your vents. You also have to insulate and air seal your attic walls. If you want to combine spray foam with fiberglass batts, the batts need to be installed tight to the roof deck, touching the underside of the cured spray foam.
The minimum thickness of the spray foam layer depends on your climate. You can only use 1 inch of spray foam if you live in a mild climate (climate zones 1-3). Two inches of spray foam will also work in climate zones 1-3, as well as climate zone 4C. If you live anywhere colder, you need thicker spray foam. All of this is explained in the following article: How to Build an Insulated Cathedral Ceiling.
If you want to keep all of your insulation against the ceiling (also known as the attic floor), you can. But that means that there is no reason to install any spray foam. It also means that you have to follow strict venting requirements. This approach is explained here: Insulating Low-Slope Residential Roofs.