Vapor issues with an unvented cathedral ceiling A Frame
I am currently building a 1900 sq ft A frame in zone 3C (Coastal). The metal roof is an unvented 24/12 pitch, with what is locally known as “flash and batt”: 2″ of closed cell spray foam to bring the underside of the roof deck below the dew point, with unfaced R-30 batt insulation in the rest of the bay. The rafters are 11 7/8″ TJIs. I have two questions: my insulator says we don’t need to run the spray foam down the OSB web of the TJI, but I think it is just as vulnerable to moisture as the underside of the roof deck. Is it a waste of money to spray foam the OSB web?
My second question regards vapor ceiling the ceiling. The ceiling will be partly sheetrock, but there is a large area that will be 5/8″ ACX ply. We are not planning on putting sheetrock behind the ply, due to the cost. So the question becomes, what is the best way to prevent vapor intrusion at the ACX ply seams? I plan on running caulk up all the vertical joints and putting a rabbet on the long edge of the ply and caulking that seam. Will that be enough? Or should I run horizontal blocking and put sill seal behind all joints?
Sorry to be so long winded, but my particular situation doesn’t fit the answers I have found on-line.
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The spray foam is your air/vapor barrier and as you stated, your condensation control layer. You shouldn't have to worry about the ceiling air barrier in this case since any air that gets up to the spray foam will not meet a cold surface that is below dew point.
I doubt you're going to have any thermal bridging issues on the OSB in CZ3, but I'll let the experts chime in on this.
A correction to Damien's comment: it is VERY important to have air- and vapor-control on the interior of a flash-and-batt or flash-and-fill assembly, at least if you're anywhere near code limits for insulation ratios. After writing this article long ago https://www.finehomebuilding.com/2011/01/20/why-flash-and-batt-makes-sense I got many complaints for years afterward that people were getting heavy condensation when not covering the interior. The ratios for foam vs. fill include vapor control in the form of drywall with latex paint, and a reasonable level of air control is implied, to limit the amount of moisture getting into the cavities, because even with those elements in place, some condensation will occur and the idea is to keep it within a safe level. If you want total condensation control with no interior finishes, you will need at least twice as much foam as the IRC currently requires. In cold climates, that's often roughly 2/3 of the R-value on the exterior or in the foam layer of flash-and-batt installations. I don't know what it would be in CZ3 but it's likely similar to cover worst-case conditions.
Regarding your roof, there is a point within the depth of your web where condensation can occur, where it reaches around 45°F when it's cold out. Because the 2018 IRC requires R-5 foam insulation in an unvented, R-38 assembly, that means that most (but not all) of the condensation will be avoided if 13% of the R-value is in the foam layer. Your 2" foam will age to about R-11, or 32% of your total R-value.
I can't quantify it right now but I believe that having 2.5 times the required depth of foam for condensation control, that your web will be pretty well protected by the foam as well and that additional foam on the webs should not be necessary. It depends on your average cold temperature.
It is important in a flash-and-batt assembly to limit air flow into the framing cavity, as you know. AC ply is airtight on its own but the seams are vulnerable. Here in northern New England we almost always "strap" ceilings with 1x material before installing the interior finish; those straps can be aligned with plywood seams to provide backing for permanently flexible sealant such as Pro Clima Orcon. Or you can use gaskets available from here or other sources: https://www.conservationtechnology.com/building_gaskets.html. Without strapping, I would run solid blocking between rafters at joint locations.
Thank you! You've addressed my concerns precisely! I will plan on adding blocking with gaskets.
Michael,
Thanks. Fascinating and useful stuff! Somehow with the migration of air barriers to the exterior in many assemblies, the role of interior air sealing of cavities gets lost. I've been a bit confused as to how important they were. Glad of the clarification.
Malcolm and cynsharon, you're welcome. From my deep files of "lessons learned the hard way."
I like to say that you need one really good air control layer in a wall or ceiling assembly, but as long as vapor is managed, more air control layers won't hurt and usually help. When foam is the only insulation it's less important because the interior side of the foam will be close to the indoor temperature. But with flash-and-batt, you're keeping the face of the foam cool (in winter) but without an air control layer, air can flow freely through the fluffy insulation.