Liquid Applied Impermeable Vabor Barrier vs Closed Cell Foam for Unvented Roof Assembly
bradesp
| Posted in General Questions on
I’m reasonably well versed in the latest codes and best practices for unvented roof assemblies including the ratio of impermeable insulation vs permeable insulation. My question is this… would a well detailed/ applied liquid vapor barrier on the underside of the roof deck and rafters along with rockwool to fill the entire rafter bay be a viable alternative to spraying closed cell foam? If the role of the Closed Cell Foam on the underside of the roof deck is to prevent moisture from moving from the house into the roof, wouldn’t this alternative approach to controlling moisture in the roof assembly work just as well?
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"would a well detailed/ applied liquid vapor barrier on the underside of the roof deck and rafters along with rockwool to fill the entire rafter bay be a viable alternative to spraying closed cell foam?"
No. You're describing a wrong-side vapor barrier. The first condensing surface (e.g. the sheathing or the interior surface of closed cell foam) must be kept warm enough to limit condensation in an unvented assembly, hence the minimum R values in the code specified for air impermeable insulation.
Hmm... Well, I think what your saying is that to avoid moisture from accumulating on the underside of the roof decking there much be a dense insulation that is mechancialy "tight" against the wood... to shield the wood from the temperature differential...
I don't agree with your statement that I have described a "wrong -side" vapor barrier, when in fact, 2-4" of closed cell foam sprayed to the underside of roof sheathing IS a non-permeable vapor barrier.
I'm not sure which code applies where you design or build, but if it's the IRC check out section R806.5. Then read the article below for precedent.
https://www.buildingscience.com/documents/building-science-insights/bsi-100-hybrid-assemblies
The assembly you've described is a code violation in climate zones greater than 3 wherever the IRC is mandated, and in 1 to 3 you would require a vapor diffusion vent at the ridge to make it work.
> a well detailed/ applied liquid vapor barrier on the underside of the roof deck
This vapor barrier will be cold (in typical GBA climates), frequently below the condensation point - meaning it's on the wrong side.
> 2-4" of closed cell foam sprayed to the underside of roof sheathing IS a non-permeable vapor barrier.
About .2 perms - which is low but isn't non-permeable.
Why not apply a zero perm liquid vapor barrier to the underside of the sheathing and then spray closed cell foam... redundant from a non-permeability, but provides an extra measure of safety.
While both the liquid applied barrier and close cell foam are vapor barriers/ retarders on the underside of the roof, the difference is where the condensing surface is. As the CCspf is also the insulation, the first condensing surface will be on the interior face of the foam. Since it's on the interior side of the insulation, that surface will be warm with no / minimal risk of condensation (depending on the thickness and assembly construction).
With your proposed alternative of a fibrous material, the first condensing surface will be the roof deck which is cold. Humid interior air can move through the insulation and reach that surface, potentially causing condensation. You didn't mention what climate zone you're in, but this is a risky setup if you're anywhere with cold weather.
"[...] provides an extra measure of safety."
How?
Material compatibility would need to be vetted and the budget is increased. "Redundant" vapor-impermeable materials have an impeccable tendency to increase risk in building assemblies. In an assembly including closed cell spray foam, it becomes a solution in search of a problem.
I'm in climate zone 4A in one hour north of Raleigh, NC
Cost of redunancy is a non -issue.. .the structure is a simple 12x16 studio / office with simple shed roof.
With SPF in the rafters, the liquid applied membrane is not doing anything. Extra step and cost for no benefit.
If the studio won't be occupied full time and there are no major moisture sources (showers/cooking), you can probably get away with fluffy insulation in the rafters with a diffusion vent at the ridge. Much cheaper than SPF for a small space like that.
https://www.buildingscience.com/sites/default/files/document/ba-1511_field_testing_of_unvented_roof_with_fibrous_insulation_tiles_and_vapor_diffusion_venting.pdf
You would still need a decent warm side air barrier, taped OSB/CDX/drywall.