Double shed roof with vaulted ceilings in zone 5. How to best insulate and possibly vent 2×12 rafters?
I’m new to the site and new to designing generally. It may be that my roof system is not ideal for this climate (zone 5). Opinions on the design are welcome, though even if the aesthetic or layout disagrees with you, I’d also like your opinion on how to best insulate it. Please see the attached pdf.
I’ve read many of the articles on this site and elsewhere about insulating cathedral ceilings and skillion roofs, whether they be vented or unvented setups. The 2012 IRC rules in my county, here in Northwestern Illinois, so I know I need to get to at least R49. I also know that we have more in common climate-wise with Wisconsin 5 miles north than we do Chicago, 100 or so miles southeast.
I’ve chosen to build the roofs with 2×12s rather than the 9.5″ I-joists the local lumber supplier sized in order to increase my insulation options and cut costs a bit.
There are 3 main roofs on the house. 2 shed/skillion roofs are enclosing conditioned spaces, while the third gable is an unconditioned sunroom that terminates over the lower shed.
For the south-sloping lower shed roof, which leans into a wall, I have been thinking about going with an unvented setup – mainly because I’m concerned about snow during a bad winter, such as this one, entering a half-ridge vent. If anyone knows of a better way to vent such a roof when snow is a concern, that would be great.
If going unvented on this lower south facing roof, I’ve been nervous about installing R20 on the outside of the sheathing due to increased costs and inexperience doing it, but I’ve been racking my brain for any other way to go unvented.
Even if I did go R20 on top of the sheathing, there would be imbalance if I threw R30 or R38 batts into the cavities in order to meet code, right? Perhaps more foam to the top would be advised?
I’ve looked at cut and cobbling, r20 to the underside of the sheathing, followed by r30 fiberglass. That option would have thermal bridging and probably require additional 2× material tacked to the rafters for space before finishing.
I’ve also thought about dropping the ceiling with 2×6’s hung on hangers from the walls, running more foam between the 2×6’s and rafters to deplete thermal bridging.
As for the other conditioned, north-sloping roof, I would be inclined to vent it, building my chutes on site with two 2″ layers of rigid foam, then use R38 faced batts below.
I’m planning on finishing the ceilings of both of the shed roofs with white-washed tongue and groove pine.
I’d also appreciate advice on how to insulate the sunroom pictured on the south elevation, if at all. It will be unconditioned and outside of the home’s thermal envelope, but a portion of the roof will extend over the lower shed roof, and I’m unsure of what to do insulation-wise with any portion of that roof. I imagine the sun is going to heat up that room, causing a certain amount of moisture. Is there a specific way of enclosing the space above the shed roof to limit its impact there? Since the dormer portion will be seeing the snow rather than the shed, what factors are at play here?
Thanks in advance,
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