insulating a cathedral ceiling
Hi, I’m building my home in Whitefield Maine, zone 6. The framing is 2×6 walls and 2×8 rafters (rough sawn true by). Walls and roof have a continuous layer of 2″ XPS, followed by 1/2″ and 5/8″ CDX plywood respectively. Then Tyvek on the exterior walls, and tar paper on the roof deck. I then have vertical strapping and horizontal strapping, and finally a standing seam roof with vent baffles at the eave and the ridge. Now I want to insulate a cathedral ceiling from the inside. I was going to dense pack the depth of my rafters and move on at r-38. Then I thought it easy enough to gusset down an extra 7″ and shoot for r-60 where it matters most. If I did gusset down, would I potentially be bringing the dew point into the cellulose cavity? Let’s assume I would have a vapor variable control membrane behind the drywall. I would appreciate any help. Thanks!
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Jose, I'm nearby in Palermo. It sounds like you're planning to vent the space ABOVE the XPS, is that correct? XPS is not very vapor-permeable so it's not a safe or code-compliant assembly. For that kind of roof system, here in climate zone 6 you need a bare minimum of 50% of the total R-value to be in the foam layer on a roof to provide dewpoint control. More info here: https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
Hi Michael, thank you for taking a moment to write a response. To clarify, the XPS on the roof is directly on the rafters, and then there's plywood on top. My plan was to vent the sheathing with two courses of strapping from the top side, (in case it ever got wet) beneath the metal roof. I had originally designed the roof to be "hot" or unvented in the conventional way, without baffles between the rafters. And my original plan was to spray foam to achieve air tightness, R-60, and prevent condensation on the inside. Seems like I should stick to that plan! ... I was recently given a copy of Pretty Good House. I'm grateful to have such a thoughtful and comprehensive guide to building my home.
Unfortunately that's still not a code-compliant assembly, or safe from risk of moisture accumulation. There is a chance that your particular situation will be ok, but a much larger chance that you will find that it results in condensation when it's cold outside. If you choose to use spray foam instead, you can save some money and carbon emissions by doing a flash-and-batt assembly, with at least 50% of the total R-value in the foam layer. Or do what I do and just vent the roof. If this is new construction or an addition, the Maine state code (MUBEC) requires R-49 in the roof, though using the U-factor alternative you can get away with a true R-38 as long as you're also accounting for reduced R-value ("thermal bridging") of the framing members.
Michael, I agree that cellulose in the rafter bay alone is out of the question. In the article you referred to above it looks like #4 to flash and batt is my best option at this point. With the goal being to get another 3"-4" of closed cell foam in the rafter bays against the existing 2" XPS before packing in cellulose hopefully to R-60.
Jose,
Is this home already under construction, or are you just now in the planning phase? I'll warn you that putting the XPS on the rafters directly with the sheathing on top is not compliant with any APA structural panel manufacturers' recommendations. If it's currently in design phase I'd swap the order of those two layers, and the assembly will perform better thermally and structurally.
How much that matters is up to you, but in any case, 2" of XPS and R38 already gets you to something like R40 effective. I'm not sure it would be worth your time spending more money insulating it there, as it would to do a great job on your air barrier.
Hi Kyle, thanks for weighing in. I am thankful to be dried in at this point, despite the situation I'm in. I agree with you, and If I could do it over, I would make many changes. At this point the challenge is to move forward with what we have, while still mitigating as much risk of moisture as possible.