Retrofit roofing assemblies
Tyler E ~
Retrofitting an old 1950’s cape that has seen better days in Maine (zone 6). I want to go with a high performance roof assembly, but have some questions on which would be a better way to get there based on this situation. Any suggestions on which would work better and why would be very helpful!
Current House: 2×8 rafters, 1×10 original sheathing boards, under-lament, shingles. I will strip back to original sheeting boards, I have reservations the original sheathing will be in good enough shape to support the requirements for method #1 thus I am leaning towards method #2.
Method #1: 2×8 rafter, 1×10 original sheathing, ProClima Mento 5000, 2 layers of 2″ polyiso (R26) vertical/horizontal, OSB, ice and water 100%, roofing
Method #2: 2×8 rafter, 1×10 original sheathing, Zip system sheathing with tapped/liquid flashed seems, ice and water 100%, 2 layers of 2″ polyiso (R26) vertical/horizontal with strapping, air gap, metal roof
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Replies
Hi Tyler.
As you know, part of the answer will depend on the condition of your roof sheathing and whether it is necessary to add another layer. I am not sure why you would use ZIP and then cover it with a self-adhering membrane. Seem like an unnecessary redundancy, unless I am missing something. You could save money and use regular OBS or plywood covered by the membrane.
I also think it is impossible to evaluate this plan without knowing what is going on below the roof deck. Are the rafter bays insulated now or are you planning to insulate them as part of the process? Are these cavities air sealed at the eaves? Is there a vapor barrier above the ceiling. This type of roof needs to be able to dry inward. Finally, R-26 is just enough outboard insulation, but an R-26 roof in your area is not a high-performance roof.
If you provide some more details, you'll get more helpful responses.
I meant to include this link, so here it is. Should be helpful: How to Build an Insulated Cathedral Ceiling
Brian,
Thanks for the feedback, sorry I left out the underside, we discussed doing blown in DP cellulose or batts which would get us to R53 / R50 respectively. Based on my calculations in my zone, anything over R25 on the exterior for the roof assembly should be enough of a thermal break for batts or cellulose underneath in rafter bays. Agree on the zip with ice and water shield on it.
A continuous R26 isn't even close to meeting code min for zone 6, let alone "...a high performance roof assembly...". Code min on an R-value basis is R49.
Hitting code min on a U-factor basis with just exterior insulation would require about R35-R36 for the insulation layer.
To be moisture safe at the roof deck in zone 6 using a combination of fiber between the rafters and foam board up top requires more than 50% of the total R be above the roof deck. With milled 2 x 8 rafters (7.25" deep) there is enough room for R25 low density fiber glass batts (R24 when compressed to 7.25") , or ~R27 of cellulose, either of which would at least meet code on an R-value basis, but if properly derating the polyiso for temperature 4" isn't quite enough for dew point control- it really needs to be 5" for R24-R27 insulation in the rafter bays. If going with R30 rock wool or 1.8lbs dense-packed fiberglass in the rafter bays the polyiso needs to be at least 6" (which would be enough to meet code-min on a U-factor basis with no rafter-fill).
To take the financial sting out of 5" - 6" foam, use reclaimed roofing foam or factory seconds foam. This outfit in Thorndike ME is always advertising factory-blem foam at about half price:
https://maine.craigslist.org/mad/d/thorndike-factory-seconds-of-foilback/6989620502.html
Running these searches every now and again will dig up other sources, some of which may be near enough to you:
https://maine.craigslist.org/search/sss?query=rigid+insulation
https://nh.craigslist.org/search/sss?query=rigid+insulation
https://boston.craigslist.org/search/sss?query=rigid+insulation
Thanks for the feedback, sorry I left out the underside, we discussed doing blown in DP cellulose or batts which would get us to R53 / R50 respectively. Based on my calculations in my zone, anything over R25 on the exterior for the roof assembly should be enough of a thermal break for batts or cellulose underneath in rafter bays. Thanks for the tips on rigid foam, I will definitely look into.
Dana,
I see your point about the downgraded polyiso for cold temperatures, so changing to two layers of 2.5" would get us to R33 on the exterior, keeping us above the R25 threshold. Couple that with an air tight membrane above the original deck boards and Rockwool R23 batts in the rafter bays would equal a total assembly of R56. Do you see any issue with this approach?
R23 Rockwool Batt (5.5") Orginal rafters are 2x7 rough cut (6.5" actual)
Original deck boards
Air tight membrane (Mento)
R33 Two layers of polyiso 2.5" staggered seems & tapped
Zip with flashed seems
Roofing (shingle)