Vapor sealing with open cell foam insulation

PerpetuallyRepairing

I have a cathedral ceiling that has plywood air channels against the roof. I sprayed 15" of open cell foam insulation onto the air channels. There is a drywall ceiling but the foam is exposed inside the wall cavities. I am concerned about vapor transmission through the foam at the wall cavities & condensing on the plywood air channels. I am in Zone 5. This is not a problem with air leakage, just moisture transmission.

I removed the drywall on the walls to expose the foam at the top of the wall cavities. It is looking quite difficult to air seal using drywall and joint compound because the geometry is so complex. Is there something I could spray on the foam to seal it? There are 50 linear feet of wall cavities. Those 2 part spray foam kits such as "Foam it Green" FOAM IT 12 Patch & Repair Kit might be good for this application. It is a high density foam so it will act as a vapor barrier.

Will this work? Are there better products? Am I being overly concerned about something that won't be a problem? Its been like this for two winters & I have not seen any issues. Thanks

Windows on Wash

Vapor barrier paint.

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Hates my roof

Wouldnt the air channels whisk away any moisture that finds its way in there?

Windows on Wash

It should, however, you should try to minimize the air/moisture transfer through the ceiling/foam.

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Gary in WA

Zone 5, you don't want a vapor barrier with a vented roof system.

Gary

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Windows on Wash

It is not necessary with a vented ceiling, however, he should still minimize any moisture/bulk air transfer.

A vapor barrier paint is not necessary in his case as noted by the lack of issue so far and the air tightness of the foam will stop the bulk transfer.

I was answering the question, not the basis of whether or not it is necessary.

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Gary in WA

WW is correct, install drywall as the air barrier.

From what I understand, Zone 5 is the cut-off point whether or not to vapor barrier an assembly (ventilated without exterior foam board). General guides from BSC:
“8. Zone 5 requires a Class III (or lower) vapor retarder on the interior surface of insulation in ventilated insulated roof or attic assemblies.” From: http://www.buildingscience.com/docum...vapor-barriers

“In addition to an air barrier at the ceiling line, a Class II vapor retarder (see sidebar) should be installed in Climate Zones 6 or higher (see Map 1).
Class I vapor retarders (i.e. vapor barriers – see sidebar) can be installed in vented attic assemblies in Climate Zones 6 or higher (see Map 1) but should be avoided in other climate zones as top side condensation can occur in summer months during air conditioning periods.
No interior attic assembly side vapor control is required or recommended in climate zones other than Climate Zones 6 or higher (see Map 1) for vented attic assemblies (note the distinction, this is not the case for unvented attic assemblies as will be discussed later). With vented attic assemblies moisture that diffuses into the attic space from the conditioned space is vented to the exterior by attic ventilation.” From: http://www.buildingscience.com/docum...on?full_view=1
Your roof is similar to a slab or a wall—fig.#4: http://www.buildingscience.com/docum...e-perfect-wall

Using a link from the end of that article, in your Zone 5, you have similar HDD to Chicago or Boston. Notice fig #3a; http://www.buildingscience.com/docum...nd-wall-design

The drywall is enough, another low perm vapor retarder/barrier as vapor barrier paint (0.45 perms) on one side and plywood (0.75 perms) air channel would give you a v.b./r. on both sides of the assembly. Not recommended for your Zone.

Be sure to air seal the drywall as WW said. This will stop more moisture potential (-100 times more as page 10 shows) than by diffusion; http://www.buildingscience.com/docum...wall-approach/

Gary

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