Method to use permeable insulation at cathedral ceiling or sealed attic?
The oft repeated rule here is that closed cell spray foam is required at cathedral ceilings and the underside of unvented attics. The reasoning is open cell spray foam, or cellulose, or other loose insulation is vapor permeable, and will thus allow moisture from the house to migrate through to the sheathing, and cause the sheathing to rot. However, the big downsides to closed cell foam are the environmental concern, and the cost.
This article:
https://buildingscience.com/documents/insights/bsi-088-venting-vapor
seems to indicate that in climate zones 1-3, you can use vapor permeable insulation if you build a vapor vent. The article specifically references cellulose & fiberglass. Would this vapor vent revise the recommendations here for Climate zones 1-3? Would this vapor vent allow you to use open cell spray foam at a cathedral ceiling or underside of an unvented attic?
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Replies
C.L.,
Joe Lstiburek has been promoting his idea of "vapor vents" (more commonly called "diffusion vents") for the last few years. To the best of my knowledge, this method of venting is not yet recognized by any building code.
Lstiburek's initial excitement about diffusion vents had to be dialed back somewhat after a field test in Chicago yielded disappointing results. Data from warmer climates have been more encouraging, but this is still (in my view) an experimental approach. I wouldn't advise the use Lstiburek's approach until more field testing confirms initial findings.
As far as I know, Lstiburek has never advocated the diffusion vent approach for roof assemblies insulated with open-cell spray foam.
For more information on diffusion vents and the Chicago field study, see A Researcher Looks at Insulated Roof Assemblies.
We install a vent channel in the rafters or truss if we have a cathedral ceiling or are insulating the slopes such as in a cape style home. We air seal the vent channel and typically use dense packed cellulose to fill the bays. I can't see why using open cell foam would be a problem there, but it is not necessary & will cost more than is necessary to spend.
You can create a vented roof assembly with either open-cell foam or cellulose if you install site-built vent channels or AccuVent-style cathedral vents. Depending on truss design and target R value, open cell may actually be cheaper than cellulose.
When the labor and materials associated with venting are taken into account, a lot of the price differential compared to an unvented roof with closed-cell will disappear. If you go with closed-cell, be sure to find an installer who uses fourth-generation blowing agents, which have drastically reduced global warming impacts compared to earlier HFCs.
Some other options are a mix of closed cell spray foam and cellulose or a raised heel scissor truss (allowing a vaulted ceiling and a vented attic with cellulose).