I'm currently in the midst of building my own house. 3/4 of it is catherdral ceiling and I'd like to insulate the best I can. I had a plan but after reading to many articles on here and other places I'm just not sure about it now. I'm in zone 5, Western pa What I have now is an unvented roof, 2x12 rafters, 5/8 osb, 1" blueboard, 7/16 osb over top that then titanium wrap underlayment. I'll be putting standing seam roofing down on the 7/16 per the manufactures recommendations. The underside will be 5/8 T&G. The plan was for sprayed open cell foam on the bottom then Certainteed MemBrain then the T&G. Now I'm thinking maybe I should be going closed cell, blown cellulose, Certainteed MemBrain. My concern is the 5/8 osb rotting, something to worry about with the way I was going to do it or am I just worrying to much? Open to any suggestion also

The 1" of XPS isn't buying you much in either thermal performance or dew point control. I wouldn't count on MemBrain to save the day with the foam that thin.

The "right" thing to do in zone 5 would be to put 40% or more of the total R above the structural roof deck, and mount a vented nailer deck or purlins for the standing seam above that. If you used reclaimed roofing polyiso the exterior insulation can be pretty cheap (and pretty green, since it's not using new material). I don't know how far these guys are from you:

https://pittsburgh.craigslist.org/mat/6041848139.html

With 2x12 rafters you can get about R40-R42 cellulose in the rafter bays, so to get above 40% you'll need to add another R25 or so on top of your 1" / R4.2 polystyrene. (I know it's labeled R5, but as it loses it's HFC blowing agents over a few decades it'll drop to R4.2.) That means adding 4.5-5" of used polyiso on top of the 1" blue-board. It'll then take 8" timber screws to mount a 3/4" OSB or plywood nailer deck, onto which you can mount the purlins for the standing seam. You might be able to get way with half-inch- run the fastener schedule for both the nailer deck and purlins or clips by an engineer (or the standing seam roofing manufacturer.)

If you go with closed cell polyurethane on the interior you'll need at least 3", leaving 8.25" for ~ R30 of cellulose. With the R4.2 up top and ~R18 of closed cell (don't believe any R7/inch claims here- R6/inch might even be optimisic), you'd have~ R22 of total foam and R1 wood for a total of R23 exterior to the cellulose, which is about 43%. At 3" most 2lb foam comes in between 0.3-0.5 perms, which gives the roof deck at least some drying capacity toward the interior.

MostHFC blown closed cell foam can't be installed in lifts over 2", but some of the HFO-blown stuff can be installed in lifts of 4" or more. Lapolla has a 2lb foam that can be installed at that depth in one pass.

Don't even think about a cut'n'cobbled polyiso solution under the roof deck. The air sealing of the rigid foam would have to be perfect, both on day-1 and day 20,000. In construction there is good, better, and best, but "perfect" just doesn't happen, particularly for the long haul.

ok, few more questions though to help me understand whats going on. I get that you are trying to control dew point within the roof, the extra 5" polyiso on top of the 5/8" osb keeps it out of that but how does spraying closed cell on the bottom help regulate the dew point of the 5/8 with out anything else being on top? with the 1" on top doesn't that trap any moisture from the outside that might accumulate between it and the closed cell? I also guess I don't understand what the difference in spraying all open cell (R 38ish) underneath with 1" on top and spraying closed cell on the bottom, cellulose and 1" on top if the roof going to dry to the inside since the open cell would be air barrier in itself at that point but still allow moisture to dry to the inside.

With closed cell foam on the under side of the roof deck the condensing surface is at the foam/fiber insulation boundary. With open cell foam the much higher vapor permeance allows wintertime moisture to collect in the roof deck, but at even 2" the closed cell foam is a Class-II vapor retarder, and the amount of moisture getting to the roof deck over a full winter isn't enough to cause a problem. But you still need enough total R exterior to the foam/fiber boundary to keep that condensing surface warm enough that the fiber insulation doesn't retain too much moisture. It may only take 2" to be sufficiently vapor retardent to protect the roof deck, but you'll need another inch to have reasonable safety margin on keeping the fiber insulation dry.

Even with 3-4" of closed cell foam on the under side there is still enough vapor permeance that the roof deck still has enough drying capacity toward the interior that it will dry just fine over a reasonable drying season. In a zone 5 PA location the drying season would typically be 8-9 months long. With R38 of open cell foam it would dry quicker, but the peak moisture content of the roof would be high enough to put it at risk.

Or maybe I'm not being clear enough in the stackup. From the top down it's:

--outdoor air

--metal roofing

--titanium underlayment (hopefully air tight)

--Foam: either just the 1" XPS, or the 1" XPS under 4.5-5" of polyiso

--t & g roof deck

--3" of closed cell foam (if no polyiso up top)

--cavity filled completely with cellulose or open cell foam (cellulose preferred)

--ceiling gypsum

--standard latex paint

--conditioned room air

Ok thanks for clearing everything up, I understood how the stack up went just wasn't sure why the open cell wouldn't work and how the open cell would allow the roof to dry to inside. I'll more then likely be doing the closed cell stack up. Wish I would have found this sote a little sooner. Thanks again

If closed cell foam, take the time to find an installer that uses a product using an HFO blowing agent. This is for two reasons:

HFC blown foam can only be blown in 2" lifts, or both fire safety at the time of installation and the quality of the installation are compromized. Most HFO blown foams can be safely blown in lifts of up to 4".

The HFC245fa used to blow 2lb polyurethane has a heavy environmental impact, not the least of which is it's greenhouse gas potential (~1000 x CO2), whereas HFO1234ze and other variants have a very greenhouse gas potential, less than 5x CO2.

A few brand names of HFO blown foam currently on the market can be found in this recent bit o' bloggery:

http://www.greenbuildingadvisor.com/blogs/dept/green-building-news/next-generation-spray-foams-trickle-market

The installed cost is really no different- it all depends on how hungry the installers are in your area. In my area the going rate has been about a buck a board foot, so 3" foam would run about $3 per square foot, but I've seen both higher and lower quotes than that in the past 5 years as the market forces shifted.