Unvented roof problem
I built a great room addition but didn’t do my homework first. The outside of the roof is finished.
Due to weird angles, dormers, hips, etc., it is virtually impossible to vent this thing. Also due to all the weird features, the roof deck was completely covered with Ice & Water Shield before the metal roof was installed. So now I can’t add foam to the outside, and the Ice & Water Shield will prevent any outward drying.
What do put in the 2×10 rafter bays that will work in zone 6? I was trying to avoid spray foam due to the cost, but is this my only option?
My original plan was to flash it with 1″ of open cell, add 9″ of Roxul, add 1″ of rigid under the rafters, sealing the rigid tightly and lastly airtight drywall. Would that work?
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Replies
Nope. You can't even install less than about 2" of open cell with any consistency, and it's way too vapor open to protect the roof deck.
Try 2" of closed cell directly on the roof deck, the rest Roxul, tightly fitted, air-tight drywall on the interior. The 2" of foam puts a ~0.6 perm vapor retarder (a class-II vapor retarder) between the interior an the wooden roof deck, and enough R value to avoid copious condensation at foam condensing surface where the foam/fiber meet.
At ~R42 center-cavity (R30 Roxul + R12 closed cell foam) the total R will be a bit shy if IRC 2012 code-min- if you wanted to come closer to code you could add 1" of unfaced Type-II EPS between the rafters and gypsum to take you the rest of the way there, but it really isn't necessary. R42 in an air-tight rafter bay outperforms typical R49 low-density fiberglass with no top-side air barriers. The primary benefit would the thermal break you get on the rafters. Avoid any foam with facers- you don't want to create a moisture trap.
I was under the impression that I could not use closed cell because of the complete coverage of ice shield on the exterior. Wouldn't this be sealing off the deck on both sides?
Darren,
Ideally, you don't want to encapsulate your roof sheathing with vapor-impermeable layers on both sides. In this case, however, it looks like you will have to.
Obviously, this insulated roof assembly will be unvented. Since you didn't install any rigid foam above your roof sheathing, you'll have to install spray foam on the underside of your roof sheathing. To learn more about your options, see How to Build an Insulated Cathedral Ceiling.
You are in Zone 6, where the code (2009 IRC) requires a minimum of R-49 ceiling insulation. If you fill your 9.25-inch-deep rafter bays with open-cell spray foam, you'll end up with R-34. To get R-49, you'll need 13.25 inches of open-cell spray foam. That will require to you scab on some framing to your rafters to provide another 4 inches of depth.
There are two caveats to this approach: (1) Make sure that you can find a spray-foam contractor who is willing to install open-cell foam at this thickness; and (2) The open-cell foam will need to be protected on the interior with a layer of gypsum wallboard painted with vapor-retarder paint.
If you want to install a combination of closed-cell spray foam and Roxul, the building code requires you to install at least R-25 of spray foam before the Roxul. That means that you'll need to install 4 inches of closed-cell foam, followed by about 6 or 6.5 inches of Roxul. The code requirements for this type of flash-and-batt job are spelled out in my article, How to Build an Insulated Cathedral Ceiling.
Closed cell foam is not a true vapor-barrier until it is many inches thick. At 2" it it is a class-II vapor retarder, about as vapor retardent as "vapor barrier latex". But unlike the vapor-barrier latex, it puts the condensing surface at an appropriate place in the stackup to be able to use cheaper fiber insulation to fill it all out.
While it's true that code specifies R25 in closed-cell foam before the Roxul, the code is simply wrong on that point, using the same R value for interior-side closed cell as what would be necessary to be protective with the insulation on the exterior of the roof deck. What the code-prescription misses completely is the vapor retardency of the closed cell foam, an important distinction between exterior vs. interior placement of the foam, since with the R on the exterior of the roof deck code allows a 5 perm (cheap latex paint) vapor retarder on the interior side to protect the roof deck. Two inches of closed cell is an order of magnitude lower vapor permeance, which makes the same prescriptive R-value extreme overkill.
A couple years ago John Straube of the Building Science Corp did a series of WUFI simulations over a wide range of climate zones that proved 2" really IS enough for zone-6. The detailed report (RR-1001) had been up on the Building Science website, but it currently isn't- you may be able to find web-archive copy somewhere.
the rafters are rough cut so they are more like 2.5 x10.5 so could i in theory stuff 3 rows of 3.5 roxul which would be r45 add 1" of unfaced foam for r5 and the thermal break with a carefully installed airtight drywall???
Darren,
The problem is that your roof sheathing has no insulation above it, so it will be very cold in winter. Cold sheathing is at risk for moisture accumulation and rot.
There are several ways to address this problem:
1. You can install rigid insulation above the sheathing to keep it warm. It's too late for that.
2. You can install a vent channel above the roof sheathing. It's too late for that.
3. You can install a vent channel under the roof sheathing. Your complicated roof geometry doesn't allow that.
4. You can install insulation that is air-impermeable (that is, spray foam) directly against the underside of the roof sheathing. This method requires you to make the foam thick enough, so that it meets minimum R-value requirements -- in your case, R-25.
If you install air-permeable insulation (like Roxul) against the roof sheathing, the sheathing will be at risk for moisture accumulation and rot.
Under no circumstances should you put fiber insulation in direct contact with the underside of the roof deck, even if you use a class-II vapor redarder on the interior. If you did that, once the roof deck becam satuated the outer inch or so of the fiber will develop frost over the course of the winter, and you will have reinvented the fine art of mold-farming.
As stated previously, cheating the code-R minimums Straube- style, with only half the code-mandated foam-R is safe, even if it's a code-violation, due to the low vapor permeance of the foam. With a bit of poking around, it looks like they re-named the report BA-1001 (was RR-1001):
http://www.buildingscience.com/documents/bareports/ba-1001-moisture-safe-unvented-wood-roof-systems
See Table 3, specifically the columns for 2"ccSPF + spray fiberglass or 4"ccSPF + spray fiberglass, for the row marked 6A Minneapolis, and 7A Int. Falls. The only time you have any real risk is with a light-colored "cool roof" finish to a metal roof, and run your conditioned space at the high-humidity profile all winter. (Read the whole thing if you want to really understand it fully.) But to quote the document directly:
"The hybrid 2” ccSPF and spray fiberglass or batt insulation performed well in all locations
except Zones 6 and 7 under light colored metal roofs. Again, the high interior moisture load
is somewhat unrepresentative as it is an extreme test "
To meet code you could go with 4" of closed cell foam (sprayed in two lifts of 2" per pass) with the rest as fiber. That isn't appreciably safer, but at about a buck a square foot per inch of depth it adds up. At 4" the drying rates for the roof deck are pretty slow, but still more than an order of magnitude faster than through 3-4-mil polyethylene- it's fast enough. Going with 2" is both cheaper, and offers 2x faster drying.
If you want further sanity checking for a foam-against-the-roof deck with fiber below approach read this:
http://www.buildingscience.com/documents/bareports/ba-1312-application-of-spray-foam-insulation-under-plywood-and-osb-roof-sheathing