Need advice on spray foam under roof sheathing of low-pitched roof in Texas
ablondguy
| Posted in GBA Pro Help on
Is it ‘more advisable’ to use 2 inches of closed-cell spray foam, followed by the needed R-value inches in open-cell foam, in a non-vented, inaccessible, 18-inch truss space, on the underside of a low-pitched roof in a humid, Texas climate?
The roof will be 1/4 inch slope per foot, highly reflective, PVC, fully adhered membrane with Densdeck underneath and rigid polystyrene for creating needed slopes on top of Advantech decking/sheathing.
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Replies
Norman,
More advisable than what? I'm not sure I understand your question.
For more information on insulating low-slope roofs, see Insulating Low-Slope Residential Roofs.
Should have read "more advisable as opposed to all closed cell or all open cell"
.
In the recommended article it states "You can install a thick layer of closed-cell spray polyurethane foam on the underside of the roof sheathing.
You can install a more moderate thickness of closed-cell spray polyurethane foam on the underside of the roof sheathing, supplemented by a layer of vapor-permeable insulation below that."
I would like to know specifically if one is any more recommended than the other, e.g. pros and cons, e.g., both could achieve the same R values granted with varying thicknesses and cost of installation, as closed cell is more expensive. Some argue that closed cell makes roof leaks harder to trace but others counter that closed cell give an added layer of protection from leaks and prevents and/or reduces condensation etc.
Norman,
I don't think that there are any advantages or disadvantages in your climate between the three options you mention (all closed-cell foam, all open-cell foam, or a combination of closed-cell and open-cell foam). If I were you, I would make my decision on R-value and cost (as well as the reputation of the spray-foam contractor, of course).
In the gulf coast region there IS no wintertime "condensation" risk to the roof deck- the winters are too warm. The TX panhandle there is some, but not much.
Assuming you are nearer the gulf than the panhandle, using open cell foam is preferable from a moisture management point of view, since it allows the air conditioning to dry out the roof deck quickly, whereas with 2" of closed cell any moisture accumulation takes weeks/months to purge. Open cell is also less expensive per unit-R, and uses a 1000 x less damaging blowing agent (water, instead of HFC245fa).
Closed cell foam has to be applied in 2" lifts or less to not risk shrinkage/cracking after curing (or catching fire during installation). Open cell has to be to be applied in lifts of 5-6" (similar issues). If you are doing the full 18" it's pretty expensive either way, and probably not "worth it" in your climate. Even R50 is a lot for a TX climate, and that takes only 14-15" of open cell foam. Either way you're looking at a minimum of three passes. (There are many installers who will violate the foam manufacturer's instructions on lift thicknesses, and if it doesn't catch on fire, you're the one stuck with the sub-par result.)
If you intend to fill the whole 18", you'd do pretty well with 2.5lbs density (or denser) blown cellulose or 1.8lbsOptima / Spider / L77 fiberglass, which would usually be less expensive. With an air tight membrane roof it won't get any tighter by going with foam. A full cavity fill of damp sprayed (preferred) or dry blown cellulose is probably the price/performance leader, and would probably cost about the same or less than R30-R38 open cell foam. If that's too much dead-weight loading for the under-side gypsum, fiberglass works (but is usually more expensive.)