Vapor barrier for open-cell spray foam?
markz
| Posted in Energy Efficiency and Durability on
Hi,
I’m trying to find out what are recommended materials to use for a vapor barrier over open-cell spray foam in an exterior wall.
Mark
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Replies
Mark,
What's your climate? You almost certainly don't need a vapor barrier, but in some climates a vapor retarder makes sense. In many climates, painted drywall is adequate to serve your needs.
Hi Martin, I'm in the northeast, mid hudson valley new york. Another bit of info to add, I pulled off siding on the house, and it has ice/water shield on the exterior sheathing, which I'm guessing is stopping any moisture or vapor from moving through that side.
Mark,
You are concerned about the wrong issue. Whether or not you have an interior vapor retarder is the least of your problems. Your exterior vapor barrier (in a cold climate) is a potential disaster.
Please describe your wall assembly (siding, sheathing, stud depth, insulation, interior finish materials).
My guess is that you will have to remove the siding and sheathing from your house to fix this problem.
Yes, I thought the exterior vapor barrier might be a problem when I discovered it.
pine siding, plywood sheathing with the ice/water shield on it, 6" wall cavity, open cell spray foam insulation, sheetrock. At this point, the sheetrock has not been primed or painted yet.
This is an old post and beam building that the previous owner completely renovated, but didn't finish completely. The whole building is up the point of sheetrock and taped. What's inside the walls was just discovered while creating a new entry door and moving a window.
Mark,
Your wall assembly is a machine to create rot. Your insulation is vapor-permeable, so your wall sheathing will accumulate moisture all winter long. The wall sheathing won't dry to the exterior, so rot is likely.
I can't think of an easy solution short of removing all the siding and sheathing on your house. If you decide to leave the Ice and Water Shield in place, one possible solution would be to install 2 inches of rigid foam on the exterior side of the Ice and Water Shield; that will keep your plywood sheathing warm enough to prevent rot.
Repairing this disaster will be very expensive.
OK, thanks, Martin. I'll pass this info along.
Just for my info, if you took the ice/water shield out of the equation of the wall, would you normally use a vapor retarder or barrier in this climate zone, and what would be good materials
Mark,
I've only heard of one case of diffusion-caused wall sheathing damage in a house with open-cell spray foam; that house had T&G boards as the interior finish material.
In your case -- and in most cold-climate homes -- painted drywall is all you need. You definitely don't want a vapor barrier -- just a vapor retarder.
Martin, I take it what to design for, is drying potential to both the interior and exterior of walls, or getting the dew point to the outside of the wall with the exterior insulation.
What about roof assemblies in this climate zone with spray foam insulation? It seems as though they'd have similar issues tho the wall we're discussing.
Mark,
They do. You raise a good question. The usual way to deal with sloped roof assemblies filled with open-cell foam is to install a vapor-retarder paint on the interior side of the cured foam.
That approach might be tried with your walls; it would certainly be less expensive than installing exterior foam. I don't have enough experience to know how risky it would be; the Ice & Water Shield on your walls makes me very nervous. If I were the contractor, I'd want a waiver of liability form from the homeowner before proceeding with the vapor-barrier paint solution.
Perhaps some other GBA readers will chime in with their opinions. Answers will depend on individuals' appetite for risk.