Foam Board Insulation Between Trusses
tpechtel
| Posted in General Questions on
I live in Zone 5 (michigan).
New construction.
I was wondering if I could put 2 inch board foam between my trusses and then seal it with spray foam. I could then put r 38 batts above that in the attic space to get a total of r 48 insulation with a better air seal.
I am worried about moisture though.
would this work?
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Replies
tpechtel,
What are you trying to gain by adding the foam that you wouldn't have with just all batt or blown insulation?
I thought that the board foam will create a better seal if I spray foam the edges of the board foam. I would then have a flat surface to lay the batts going the opposite way. Then it become a like a flash and batt.
tpechtel,
leaving aside whether it' worth it, practically it faces some problems. To cut, place and foam the boards so they are adequately supported suspended above the ceiling drywall, and flush with the bottom chord of the trusses is a huge, very difficult job. Once that's done, the batt insulation can't be tightly laid in the opposite direction because of the truss webs.
Unfortuately this is one of those proposed assemblies you never see done because there are no appreciable gains from a lot of additional effort and materials.
I think the idea is the foam plus spray can air-seals the ceiling pretty good
Deleted
tpechtel,
If you have a ventilated attic and are installing the insulation on the "floor," I don't think there is any advantage to this approach. Rigid foam is relatively expensive and installing it using the cut and cobble method is labor intensive. If it were my house, I would focus on air sealing the attic and then install R-48 (or even R-60) of blown cellulose or blown fiberglass.
One FWIW, the cut and cobble method has kind of fallen out of favor among the GBA community. Apparently, it is hard to maintain the seal between the foam and its surround framing. As the framing moves, it tends to break the seal and allow conditioned air to condense on the exterior sheathing. (At least that is the way I am interpreting the feedback from the building science experts.)
"I would focus on air sealing the attic"
Seems to me the 2" board and spray can is addressing the air sealing, if owner is willing to do the labor. I think the question is whether 2" of foam directly above the ceiling drywall is a good or bad thing vis a vis moisture.
If moisture migrates through sheetrock might it condense between sheetrock and foam with nowhere to dry? Perhaps not an issue with R50 (R12 foam plus R38 batt) above it.
In any case it's an interesting approach to reaching R50 in the attic with better air sealing directly above the ceiling than batts could offer
KT,
The drywall ceiling is already a good air-barrier. Its only weakness is the penetrations for services, and over interior walls. That's where to concentrate, not on the truss bays where a secondary barrier won't make any difference.
The foam wouldn't be a problem from a moisture perspective. The interior face will be warm enough that there will be no condensation.
Has anyone had problems with spraying wall cavities with foam. After reading this, it makes me wonder if the walls may break the seal as well.
If you have a vented assembly, you could use rigid foam on the underside of the trusses to give you a layer of continous insulation that would act as an air barrier, and also a vapor barrier depending on the type of rigid foam you use. If you were to do this, I would use 1/2" polyiso since the batts above would be providing most of the insulating R value for the assemly. If you have a particular R value target you need to hit, then it would make sense to use additional rigid foam to get you to where you need to be. I would tape the seams of the polyiso to make a good air barrier.
Note that drywall is also a good air barrier, so you don't really need the extra step of the interior side rigid foam here. I would NOT use spray foam here at all, it doesn't gain you anything -- taping the seams of the rigid foam accomplishes the same thing and does at least as a good a job as a layer of spray foam would.
If this is an UNvented roof assembly, then you have moisture migration issues to worry about and I'd use sprayfoam against the underside of the roof sheathing instead of batts, and I'd NOT use interior side rigid foam -- especially anything that would act as a vapor barrier.
More detail about your entire assembly would help us to provide more detailed advice.
Bill
The roof is vented.
I was going to use 2" formular (XPS) because I read somewhere that moisture will not penetrate through an r value of 7.6 worth of foam. The 2" XPS is r-10. In my mind this worked out. But my mind could be wrong.
Vapor permeance has little to do with R value, and more to do with the structure of the foam. XPS IS the least vapor permeable of the rigid foams if you go by the foam only, but when you add a facer, then foil-faced polyiso is the least vapor permeable -- because of the foil facer.
2" polyiso is R13. I would use polyiso here. I would not use XPS (least green, and expensive). You could use Type II EPS, which would be a bit more vapor open, but it's also a bit more difficult to work with.
I strongly advise that you use polyiso here if you build your proposed assembly.
Bill
I got a good deal on the XPS that I was gonna use for the walls. Would you avoid the XPS for walls and ceiling because of the money or performance?
What are y0u planning to do with it in the walls? Where foam does make a real difference is continuously either inside or out, or just over the studs to limit thermal bridging. Putting in the cavities usually doesn't buy you much.
I try to pick the most suitable materials for the application as a cost/benefit. XPS is best underground in that regard, since it is least prone to absorbing water. Polyiso is best for R/inch and is greener, so it makes sense where you have to maximize R value in a limited space. EPS is cheapest, and also pretty green, so it's best where you're on a budget or need maximum vapor permeance.
In your case, if you got a good deal on the XPS there is no reason you can't use it UNLESS you have an unvented assembly, in which case the very low vapor permeance might be a problem.
I would not use the XPS in wall cavities. I WOULD use it as exterior continuous insulation, which is where "exterior rigid foam" really helps with wall performance. You're better off with batts in the stud cavities of the wall, which are much easier to install.
Bill
Thanks, great info.