Spray foam covers entire 2×4 on roof truss…bad?
I had my roof deck sprayed with spray foam (to the underside of the truss). My roof trusses were designed at 16″ OC and constructed with 2×4’s. They needed to spray 4″ of foam to get the code requirement for insulated value, so they covered the 2×4’s completely on the roof deck.
Now I’d never really looked at and or thought about it till after they were done…(should have had the trusses designed with 2×6’s) but the 2×4’s are fully encapsulated with foam. I’m worried they are going to rot because they can not breath. Is is not a big “no-no” to encapsulate wooden studs?
I’ve purposely used #30 roofing felt instead of synthetic roof membrane, as most roofing membranes are a vapor barrier and at least the roof can breath from the top.
I live in Northern Canada where its DRY DRY DRY all the time, so no humidity issues.
Let me know,
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Replies
Far from being a problem, encapsulating the rafter-elements of the trusses is best practice. The vapor retardency of closed cell foam is about the same as that of sawn lumber. An encapsulated 2x4 under 4" of foam is at no greater (and arguably less) risk than 4x10 beams/rafters would be.
It's the humidity of the indoors, not the outdoors that creates the risk in a cold climate. In an occupied building that's maintained at 20C with 25% relative humidity, the dew point of your conditioned space air is about -0.5C. Any structural wood that is below that temperature for extended periods would be accumulating moisture, with NO hope of drying toward the inteirior. But with 4" of closed cell foam between the cold wood and the humid conditioned space, the rate of accumulation is extremely slow due to the ~0.25-0.3 perm vapor retardency of that much foam. (Traditional 6 mil polyethylene comes in at about 0.05 perms).
The #30 felt is also low permeance, but variable depending on it's moisture content. A layup of #30 felt + asphalt shingle comes in at about 0.1 perms which is even lower permeance than 4" of closed cell foam. Your local climate actually matters, as well as the stackup from the felt out toward the exterior, since that all determines whether the roof deck just keeps accumulating moisture, or whether there is a sufficient drying season through the foam toward the interior, since that's the higher-permeance (though still low permeance) side of the roof deck.
So no worries then? I recall hearing a wood stud will rot if encapsulated in spray foam?
Its very dry here, especially in the summer. Winter is also dry and -40c. Sound like its ok as is then.
I appreciate your technical reply :-)
It could still rot even at 0.25 perms if vapor pressure is always toward the wood, and it's drying season is short & shallow enough that it never dries out, which is why the exact climate still matters.
What is your location?