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Double shed roof with vaulted ceilings in zone 5. How to best insulate and possibly vent 2×12 rafters?

user-7364466 | Posted in Green Building Techniques on


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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  1. Expert Member
    AKOS TOTH | | #1


    Part of my roof section is similar to yours. I couldn't come up with an easy way to vent the shed section bellow a clerestory either. It collects a lot of snow, I would definitely recommend against any half ridge type venting there.

    For the roof I went with exterior rigid insulation but I also have an air tight ceiling underneath. If you want to go with T&G, I would recommend either getting a layer of drywall underneath it or going with a larger ratio of exterior/interior insulation to avoid condensation (see for details

    For the upper shed, you can vent it but to get R49, you will need more depth. My suggestion is to go with batts between the joist, than a layer of rigid insulation underneath to get the needed R value. With taped seams the rigid insulation can also serve as your air control layer, so you can put the T&G directly on it.

    Cut and cobble is a HUGE waste of time. The end result is an assembly with not much better R value then using just mineral wool batts and it is not air tight enough to allow for an unvented roof. Stay away from it.

    The best way I've found to do exterior rigid with metal roofs is to lay down peel and stick on the roofing, install 2x purlins on edge with the rigid foam in between the purlins.

    This makes the purlin spacing slightly above 24" OC but that still works for most standing seam roofs.

    The way I installed this is to put down the one purlin, put down the 2' wide foam beside (I put a bead of expending foam around the edge of the foam) and then the next purlin. This way you don't have to measure anything or cut the foam.

    This saves adding another layer of plywood above the foam and dealing with long screws through rigid insulation. The R value is less because of the thermal bridging , but much quicker and cheaper to install.

    1. user-7364466 | | #4

      Thank you Akos. I will definitely go with an unvented setup. It is true that purlins would seem to contradict the "continuous" requirement of the 2012 IRC.

      I've read on this site how important ratios of externally mounted rigid board to interior fiberglass batts is, but I still don't quite understand. I think mainly because of Dana's math in some of the comments as they relate to the area I live. In this part of zone 5, our winters more closely resemble Wisconsin (zone 6) than the rest of Illinois. Yet, we have very hot and humid summers, of which we broke all kinds of records for rain last year. I'm wondering if r20 on top plus r38 in the cavities would be sufficient or outright dumb. Obviously more would be better, but my budget won't allow for it. Would upping the rigid to r25 be better or worse for the ratio in this climate?

      Thanks again,


      1. Expert Member
        AKOS TOTH | | #6


        There are details about the correct ratio here:

        Based on your climate, somewhere between 40% to 50% of your R value should be above your sheathing. One thing to be mindful of is that you still need an air tight ceiling under this roof. Your T&G would not be good enough air barrier, either install a layer of 1/2" taped drywall or OSB with the seams taped underneath. The more rigid you put on the outside the better the assembly will perform. Just be mindfull of the fasteners, whatever thickness you go with, you eventually have to put a long screw through it and hit a rafter. This gets tricky above 4".

        If your spans allow, might be worth it to go with something less than a 2x12, a 2x12 holds too much fluffy insulation (or requires an excessive amount of exterior rigid to have the correct ratio).

        I'm not familiar with US codes details, but there is an assembly U value based alternative when you go with continuous external insulation which would let you use less than R49 center of cavity. You can search this forum for details.

  2. Jon_R | | #2

    > install 2x purlins on edge with the rigid foam in between

    Note that some later codes changed to "continuous", evidently preventing putting the foam between purlins. Might be a good idea to adhere to newer code even when you don't have to.

    "Alternatively, sufficient continuous insulation shall be installed directly above the structural roof sheathing".

    1. user-7364466 | | #5

      Thanks for your input, Jon.

  3. Expert Member
    Peter Yost | | #3

    Hi Chris -

    Thank you. It's not often that someone uses a construction term that I have never heard of and one that made me crack out my 20-pound Random House unabridged dictionary because the internet definitions did not in any real way address derivation of the term. Of course, I am referring to the term skillion; who else had to go look that one up?

    What a strange term. Here is the same Random House dictionary word-for-word entry for this term:

    "noun, Australian. a lean-to serving as a room or shed (1860 - 65 alter. of skilling, original dialect of S. England. ME skyling: sense suggests kinship with dial. scale hut, shed but phonetic development obscure."

    Now that is one mysterious word; I love it.


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