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Rigid Foam on roof deck, clarification

Sean Cotter | Posted in Green Building Techniques on

I feel like I am getting some conflicting information or perhaps some of it has changed.

Zone 5, Spokane WA. Looking to build garage with ADU above. Older part of town and there are some restrictions to the footprint, finishing details and overall height. Max height of the primary ridge is 23′ from grade and side walls of 16′. However, that 16′ is for the primary ridgeline and the dormers cut in on the second level have freedom to, essentially, go to that 23′ line as well, but functionally and aesthetically that doesn’t make much sense.
The attached image is more or less what we’ll probably end up with, design wise (different finishes, probably no cantilever, etc).

What I am having issues with is the roof build up and insulation. Desired is the dreaded cathedral ceiling. https://www.greenbuildingadvisor.com/article/how-to-install-rigid-foam-on-top-of-roof-sheathing seems to imply that R20 on the top (ISO) and then R30 below (2×8 stick framed, Rockwool batts) would suffice, but I feel like I read somewhere that the ratio might need to be more like R26 outside and R23 inside for Zone 5? I think we’ll be allowed to stick frame 2×6 with the latter, but its more foam, longer screws, etc.

The plan is something like Risinger’s Monopoly House design, with the rafter tails as outriggers and part of a vented assembly. In this video, https://youtu.be/yTbMVdmnNlw?t=317 they go ahead and sandwich the hotroof with Zip, I plan on some venting on the foam with wood strapping (that meets up with the partially embedded outriggers).

 – is there a definitive inside/out ration for Zone 5 if it’s hot roof versus the top deck vented? This will be dark shingles (sorry) to match main building so I think vented will increase performance/life span of everything, if there is a leak it can pass through and hit the polyiso and the second deck can dry?

– is felt or foil coated polyiso suitable under the strapping, if the seams are taped? Or do I need to adhere another membrane on the foam layer, then strapping, then deck+shingles

– the first deck was going to be OSB, #15 felt, then the polyiso, but I am wondering if ply and/or another membrane should be used?

– due to cost, general framing will be 2×6, R23 rockwool and careful air detailing at the drywall level, outlets, etc. I’ll personally oversee that portion. Again, OSB for cost (though taped seams, internal foam seal where the sheathing hits the studs) – would tape ply be better?

– would true #30 felt work as the WRB on the side walls? I have seen this done online, demoed buildings with 90 felt still intact… seems like Tyvek jobs are incredibly sloppy.

Thanks

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Replies

  1. Sean Cotter | | #1

    Bump.

  2. Brian Wiley | | #2

    I’ll let other more-experienced members answer subsequent questions, but in regards to the first there is a definitive ratio for CZ5: 41% must be on the exterior. In a code-minimum r-49 roof that equates to r-20 topside.

    Those numbers are from that same article you referenced. If you go to the last image in the slideshow you can see the table with the ratios: https://www.greenbuildingadvisor.com/article/how-to-install-rigid-foam-on-top-of-roof-sheathing

  3. Expert Member
    Akos | | #3

    Top vented roof is something you need to do in heavy snow country. Unless that is the case, it makes little sense for the added cost. Either way, it will do very little for shingle life, the best way to increase the life of your roof is not to use shingles.

    Although code is R49, lot of places allow for compliance based on U factor. With a roof with continuous insulation, this works out to around R38. You can generally meet that with 2x6. Run your roof assemblies through this calculator to see:
    https://www.ekotrope.com/r-value-calculator

    You still need to maintain the 40% of assembly R value as rigid.

    Whether unvented roof or walls, you need a solid air barrier. The simplest is always taped sheathing, trying to do this from the inside with canned foam or caulk just doesn't work.

    For a roof, generic stackup is:
    -drywall
    -rafters + batts
    -roof deck with taped seams
    -rigid insulation
    -2nd roof deck screwed down
    -synthetic underlayment
    -roof

    Sometimes you put an underlayment over the deck under the foam to get the place dried in quicker. This can be any roof underlayment, I usually go for peel and stick as it holds up better to the elements (in case of peel and stick you can skip taping).

    30 lb felt can be used and works. It is a bit more fragile to install than house wrap. If you are installing it with care it should be fine, if by trade, go for house wrap. Also make sure it is a rated felt, lot of supply places only carry the non ASTM rated one.

  4. Sean Cotter | | #4

    Thanks for the replies.

    Snow load is 30lbs/sqft by code. Some jurisdictions have changed it to 40lbs. This is interior WA State, next to the Idaho panhandle.

    The city is being pretty good about the ADU, they are encouraging that sort of thing - but some requirements are set in stone (so to speak). Some design elements are required, which I think I can meet. Metal roof, standing seam or some other architecturally elegant metal roofing sounds appealing but probably in the budget. If it came to that, I would probably stick with conventional roofing and add solar.

    A conventional WRB is probably on the books since we'll have a crew of subs on some of this.

    City is requiring the minimal of R49 for the roof. They are OK with the foam over and are familiar with the 41% rule for the zone, they said min of r20 up top. Right now, my build up with used Polyiso (fingers crossed that used is available out of Portland/Seattle) puts 43% on top of a total R52.7. Given that the Polyiso has a longterm depreciation in the R value, I am thinking that the 22.7 up top will degrade to 20-something anyway. R30 batts in 2x8 framing below the foam.

    Rafter tail design allows for an air gap between the foam and the second deck if I fill in with 1x3 purlins, which I think I will do as some extra insurance.

    Would foil faced second layer of polyiso, with taped seams be OK as a WRB for the foam itself, for the vent channels?

    1. Expert Member
      Akos | | #5

      I would doublecheck the insulation needs, most codes have both perscriptive and performance options. There is very little energy saved between an R38 and an R49 roof, the latter costs a lot more of you have to go from 2x6 to 2x8 and extra foam. In almost no case is there an ROI on this type of roof going much above R30.

      Almost all reclaimed roof with be fiber faced, so will most of what you can buy for cheap from commercial roofing suppliers. Almost all roofing materials require an underlayment, foil faced foam doesn't hurt but you still have to include the underlayment.

      I've had good luck with metal tile roofs (Metstar), pretty low cost per sqft and quick to install provided you keep the roof simple.

      One benefit with lot of metal roofing (most standing seam) is that it can be installed over strapping saving you the cost of the 2nd roof deck. At the current lumber price, I wouldn't be surprised this would pay for the material cost difference, install would still be significantly more unless you can DIY.

  5. Expert Member
    Malcolm Taylor | | #6

    Sean,

    I'm currently designing a very similar ADU for a lot in a town up the coast. Buildings that look like the one you posted are essentially a 1 1/2 to full 2 storey box with roof sloping somewhere between 3/12 and 6/12. The steeper parts of the roof which gives it the lower feeling profile, are just applied overhangs. (I've attached a photo to illustrate this visual trick)

    The easiest way to build them is with either common or scissor trusses, which can be insulated without foam, or multiple layers of sheathing. That's what I'd explore before resorting to a complicated un-vented assembly.

  6. Sean Cotter | | #7

    Thanks to both for the suggestions. I will check out that meal roofing option. With lumber where it is, it seems that insulation and other materials are cheaper options if you can make it work.

    In my back and forth with the city, I approached the option of whole roof r value vs taking the application of the materials in sum and calling it R49. They seemed to allude to it needing to be 49, based on the stack up and not performance.

    Max height of the ridge is 23’. The sidewalls have a max height of 16’ - but the dormers don’t count against that. Not sure of guidance there other than they don’t want square box garages/adus in a craftsman neighborhood.

    I was just sketching out an idea with the lower pitch of the roof being facades on all four sides, was going to email the city to see if that type of buildup would meet the design code and allow the shed dormers to be pitched at a degree that gives us nice headroom inside.

    Malcom on your design there, those attic trusses? Flat ceiling on the second floor? Part of the appeal with the roof over with rigid was a cathedral ceiling in the adu, to give some volume to the space. Trusses and conventional insulation and venting makes a lot of sense $$$.

  7. Expert Member
    Malcolm Taylor | | #8

    Sean,

    It will be conventionally framed with I-joists spanning the garage, and the walls on the second floor are 7 ft high. The roof will be scissor trusses.

    I'm in much the same situation. The downtown of Ladysmith is almost all old Craftsman style buildings, and the ADU needs to match.

    In your situation I guess it comes down to their judgement call as to whether they will accept those walls as being dormers, or say they are part of the main roof.

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