More Q&A Spotlight
In a recent Q&A post, atburi writes that he is learning about building science while planning a two-story house with a cathedral ceiling in climate zone 7A. In thinking about the roof assembly, his two priorities are a high R-value and longevity. He’s come up with two different options for the assembly, and would like the community’s advice. Here are his ideas:
Option 1:
– Interior decking
– Strapping (for interior service cavity)
– Vapor barrier (Intello Plus or 6 mil. poly)
– TJI roof joists 16 in. deep @ 16 in. OC (mineral wool batts or blown-in cellulose)
– Strapping (2×6 laid parallel to the eaves)
– Sheathing (1/2-in. OSB, taped)
– WRB (Solitex Mento Plus)
– Metal roofing
Option 2:
– Interior decking
– Strapping (for interior service cavity)
– Vapor barrier (Intello Plus or 6 mil. poly)
– TJI roof joists 16 in. deep @ 16 in. OC (mineral wool batts or blown-in cellulose)
– Sheathing (1/2-in. OSB, taped)
– WRB (Solitex Mento Plus)
– Exterior insulation (2-in. EPS)
– Strapping (2×6 laid parallel to the eaves)
– Metal roofing
He also had two more detailed questions concerning his options:
1. For insulation within the TJIs, would blown-in cellulose (likely loose packed) be better than two layers of Rockwool? (Cellulose installation equipment is available through a local building supply store but very few builders use cellulose in the region.)
2. For strapping, I’m guessing that boards laid parallel to the eave will restrict the ventilation channels from the eave to the ridge vent, so should there be two layers of strapping (vertical below and horizontal on top) or diagonal strapping instead?
So, what is atburi’s best path to a sturdy and well-insulated roof? That is the topic for this Q&A Spotlight.
Consider thermal bridging
Rockies63 kicks off the conversation by suggesting atburi contact the Cold Climate Housing Research Center for advice.
The second comment comes from matthew25, who likes Option…
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5 Comments
I think there must have been some miscommunication between atburi and Westform Metals. Although they suggest a solid substrate (and I agree it's preferable in most situations too), they allow their snap-lock panels to be installed over strapping, and they seal fine. My own house used them that way.
What about SIP roofs? They clearly pass the high R-value criterion. Is there a concern about durability? Cost?
I recently did 2x12 douglas fir rafters 16OC with "R30" Rockwool, 7/16 ZIP sheathing (roll the tape!) with 5-1/2" of EPS foam board over it, 2x4 strapping fastened to the rafters through the foam board, and a 5/8 OSB roof deck for shingling - I would have skipped the 5/8 OSB had they gone with metal roofing. So it effectively put R28-ish on each side of the structural roof sheathing, minimizing condensation opportunities in both the hot and cold seasons, while still properly ventilating the roofing deck - and no interior vapor retarder required. This is on a 28x40 cape-style home in climate zone 6a with a ground snow load of 95 PSF.
A note about polyiso because I saw it was recommended: I wouldn't use it in zones 4a or above, as the higher insulation value per inch is only under warmer temperatures and it can actually have a lower r-value than EPS or XPS in colder temperatures. Polyiso and XPS also off-gas over time and lose some of their insulation value as a result, making EPS the more stable and cost-effective choice in my eyes (but still only as continuous exterior insulation, I don't trust any foam inside the building envelope.)
What about a roof system here in California where we have strict rules about roof venting and many of us designer prefer to fill the cathedral ceiling with a combination of high-density foam and fiberglass insulation with no roof venting. What is your suggestions or thoughts on this type of issue.
Thanks,
Paul
I think from a performance standpoint, most folks would be very happy to do an unvented roof with sufficient rigid insulation above the sheathing for the climate zone. It's usually cost that pushes people towards a vented cathedral ceilings. Option 3 in this article is one example: https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
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