Monopoly Framing and Overhangs
Hello,
I’m in Northern Minnesota, so climate zone 7. I’m taking GB’s advice and going with R30 exterior roof deck insulation for the cathedral ceiling in my cabin. Will probably use rockwool on inside between the trusses.
Starting from the bottom, it will be ZIP Sheathing, 6” of XPS, then topped with OSB and shingled. I’m trying to avoid wood in areas where insulation should be, so as to avoid potential moisture in those areas.
I’m going with a rainscreen behind the siding anyway so I thought why not use 2×4’s which could also potentially act as the structure for the overhangs, as my rough illustration shows.
Any thoughts, concerns or improvements on this approach?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
2x4's are kind of overkill for rainscreen. You could just have a 1x3 rainscreen, and then have only the wall support of the overhang be a short piece of 2x4. My intuition is what you really want is a horizontal 2x4 ledger but I don't have the details thought out.
How is your roof sheathing attached? R30=5 inches of polyiso? I'd be thinking 1-1/2" of continuous and then an overroof made out of 2x4's on 24" centers with another 3-1/2" of insulation between them. That gives you rafter tails and something to nail the sheathing to. I calculate that the thermal bridging would reduce the R-value to 27.5.
Thanks for the reply. The continuous ledger makes sense. 1x3 makes sense too but was thinking maybe the cost difference wouldn't be much.
It will be a total of 6" xps. Was thinking 3 layers of 2". Was planning on attaching the sheathing using 9" sip screws.
Would urge you to price out polyiso instead of XPS if you haven't ordered it yet. It is higher R-value per inch, reducing your total assembly height and lower GWP. In my area at least it is cost competitive with XPS.
Also, a stronger rafter tail would be supported by the slope of the roof, roughly 2/3 of it on the roof and then 1/3 of it would be the overhang. I've attached screenshots from one of Risinger's videos on the roof install at his own house. He uses a horizontal ledger on top of the furring strips, which is nice because it keeps the rainscreen vent channel open continuously from bottom to top of wall.
Thank you for the reply. I had planned on going with polyiso, but I live in climate zone 7. Many days below 32 and even well below zero. That is when I really need that R-value and my understanding is, polyiso doesn't maintain in those conditions.
Are there other benefits I'm missing?
You should take a look at this very unscientific look at the burn characteristics of insulation. I can’t believe he is just burning them in the open but pretty entertaining nonetheless. Enjoy!
https://youtu.be/8NC79e0oztM?si=EWqj8Wm_4W4nSODJ
Polyiso's R-value does drop with low temperatures but only the outer few inches at most are affected. XPS' R-value drops over time, to around R-4.2/in eventually, not the R-5/in it's advertised as.
With Polyiso, if you use Fiberglass Faced, is that used on all layers or only certain ones?
I have never seen polyiso that had different facings on the two sides. It might exist but it's not common.