Closed-cell spray foam vs. high density batts
We’re currently in the planning/permitting phases of a new home in the Southeast. We’ve hired a green building specialist to give us some specific recommendations and model our home to give us an approximate hers rating. He has advised that we use high density fiberglass batts in our mostly cathederalized ceilings upstairs. Our contractor (traditional) has recommended closed cell spray foam because he thinks it will help with the structure and have less air permeability. There’s a definite cost difference. Please help steer us in the right direction. FYI it’s a basic gable roof with two dormers and 1×10 rafters. Also we’re having our hvac system in the “attic” unused part of the upstairs. Am I correct in understanding that we should NOT have vented soffits and should encapsulate our entire roof/attic?
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Megan,
Either fiberglass batts or closed-cell spray foam can be used to create an insulated cathedral ceiling. Most green builders avoid the use of closed-cell spray foam unless its use is absolutely necessary, because closed-cell spray foam has a blowing agent with a very high global warming potential. (In other words, it is environmentally unfriendly.)
If you decide to insulate your cathedral ceilings with fiberglass batts, you need to include a ventilation channel between the top of your fiberglass batts and the underside of your roof sheathing. In other words, you will need soffit vents and a ridge vent.
An alternative approach that still uses fiberglass batts would involve installing rigid foam above the roof sheathing and fiberglass batts between the rafters. If you go this route, the roof assembly would not require a ventilation channel.
All of these options, and more, are explained in this article: How to Build an Insulated Cathedral Ceiling.
Thanks as always Martin.
I've read the "How to Build an Insulated Cathedral Ceiling" probably 3 or 4 times before posting my question. It's packed full of information and not sure I'm completely understanding it all.
I've read your articles and also Joe Ls (i think) from BSC on the rigid foam on the roof decking. From what I understand that would require another layer of sheathing (so two layers) on the roof. While it might be the best, it is likely more cost-prohibitive for us.
Since our forced air ducts and an air handler will be in attic space it seemed we should seal the attic and not vent it (no soffit or ridge vents). If we ran ducts through a vented attic, we would be in violation of BSC's "10 dumb things to do in the South" [http://www.joelstiburek.com/topten/south.htm] (please pay no attention to #1) :) This would require some closed cell spray foam (I've read the article on open cell being risky).
To reduce the use of the closed cell, would it make sense to put R19 worth of closed cell on the ceiling then stuff the rest with R19 fiberglass? I've not read any article on a hybrid approach (we have 10" rafters) so that gives us 9 and a quarter inch? to get the R19 closed cell + R19 fiberglass? More worried about water movement since that is always tricky to understand it seems.
Thanks again,
Megan
Megan,
I think you may still have a few misconceptions. I suggest that you read this article: All About Attic Venting.
In that article, I wrote, "If you plan to install insulation between your rafters, building codes require that the attic be sealed (unvented). The code allows you to install a ventilation channel between the underside of the roof sheathing and the top of the insulation installed between the rafters if you want, but this type of attic can't have any vent openings that allow outdoor air to mix with the air in the attic."
You wrote, "Since our forced air ducts and an air handler will be in attic space, it seemed we should seal the attic and not vent it (no soffit or ridge vents)."
If you want to create an unvented conditioned attic -- and I think it's a good idea to do so if you have HVAC equipment up there -- you definitely don't want any vents that allow outdoor air to mix with the air in your attic. But it's still perfectly OK to have soffit and ridge vents, as long as these openings serve a ventilation channel between the top of your insulation and the underside of the roof sheathing. The air in this ventilation channel doesn't mix with the air in your attic.
In your case, you can install fluffy insulation like fiberglass batts or cellulose between your rafters if you want. You just have to make sure that you include a ventilation channel under the roof sheathing.
You wrote, "Would it make sense to put R-19 worth of closed cell on the ceiling then stuff the rest with R-19 fiberglass? I've not read any article on a hybrid approach."
In fact, my article (How to Build an Insulated Cathedral Ceiling) explains how to insulate a cathedral ceiling using this "hybrid" approach. In that article, I wrote that one of three ways to build an unvented cathedral ceiling was this way: "Install a layer of closed-cell spray foam against the underside of the roof sheathing, and fill the rest of the rafter cavity with an air-permeable insulation."
More details are provided in the article. Your plan to use R-19 of spray foam will work if you live in Climate Zone 4 or anywhere warmer. If you boost the spray foam layer up to R-20, your plan will even work in Climate Zone 5.