I have some questions on a roof stackup. This is new construction in Zone 5 with 120 MPH winds. Roof is 2x10 rafters at 16oc with 5/8 T&G zip system sheathing. I want to use cellulose in the rafter cavities and two layers of 2" XPS on top of the sheathing then furring with standing seam roofing. Question #1 is there an easy way to space the strapping to allow eave to ridge venting under the metal (maybe diagonal? or alternating 4' lengths horizontally?) Question 2 Anyone have suggestions for the weather barrier material and placement? I assumed it would go on the foam with the strapping over and was thinking of a material like VaproShield although it's very pricey. I intend to clamp S-5's to the seams to hold solar modules - doe the roofing need to be solidly backed to do this or is strapping at 16" or even 24" OK? I'm imagining the roofing denting as solar installers walk on it (roof is a 45 degree (12/12) pitch. By the way, I know iso would be a better application but for various reasons XPS is what I have on site. Many thanks in advance.

Posted By Birdman on 25 May 2013 10:56 PM
I have some questions on a roof stackup. This is new construction in Zone 5 with 120 MPH winds. Roof is 2x10 rafters at 16oc with 5/8 T&G zip system sheathing. I want to use cellulose in the rafter cavities and two layers of 2" XPS on top of the sheathing then furring with standing seam roofing. Question #1 is there an easy way to space the strapping to allow eave to ridge venting under the metal (maybe diagonal? or alternating 4' lengths horizontally?) Question 2 Anyone have suggestions for the weather barrier material and placement? I assumed it would go on the foam with the strapping over and was thinking of a material like VaproShield although it's very pricey. I intend to clamp S-5's to the seams to hold solar modules - doe the roofing need to be solidly backed to do this or is strapping at 16" or even 24" OK? I'm imagining the roofing denting as solar installers walk on it (roof is a 45 degree (12/12) pitch. By the way, I know iso would be a better application but for various reasons XPS is what I have on site. Many thanks in advance.

First, change the 4" XPS to 4" of polyiso, which will give you a bit more dew point margin. Having only R20 would be pushing your luck on the cold edge of zone 5 with a ~R55 total R.) and reduce the (quite significant) global warming hit from the blowing agents of the foam by more than 99%.  The IRC prescriptive values for the exterior-R is a minimum value, with a presumptive R49 center-cavity total R. With 9.5" of cellulose and 4" of XPS you would be more than 10% above that for a center-cavity R, and will need to adjust the foam-R up accordingly. If XPS is what you already have on site and there's no going back, either add an inch of polyiso on top of that, or apply a flash-inch of closed cell foam on the underside of the roof deck before adding the cellulose. (It's probably cheaper to add the inch of iso, which would also be higher-R overall.)

Mounting the roofing on 2x4 purlins that are segmented, with 6-12" gaps to allow soffit-to ridge air flow works.

If you feel the ZIP isn't sufficient (and I'm agnostic on that issue regarding ZIP as roof decking) the WRB should be a peel'n'stick membrane type (eg Grace Ice & Water Shield) between the roof deck and the foam.  With the metal roofing purlin/furring mounted any moisture blowing through the metal seam roof dries pretty quickly, and taped seams on the foam is sufficient- no WRB or other sheet goods requires. If the roofing is to be mounted tight to the foam it needs to have the proper slip-surface to manage both the expansion/contraction of the metal, but also needs to be sufficiently wicking and air-permeable to allow reasonable drying rates.

I prefer the Advantech for roofs over the ZIP, since the ZIP is made with ordinary OSB & the Advantech is treated with a water repellent & stands up better to moisture. You can tape the joints with SIGA Wigluv which sticks to practically anything.

There are several good permeable roof membranes on the market which would work well over the foam & under the metal. We like RoofTopGuard II. Theoretically the tape on the foam will handle condensation, but there are questions as to whether tape will last, so in this type of situation where you may have repetitive condensation on the underside of the metal this may be a better bet. this is not a replacement for the tape - you have to do that anyway.

Question 1: I would recommend checking the metal roof material manufactures installation instructions.

Question 2: Again, what does the manufacturer say? Strapping installed directly over insulation, even with a barrier may not be the best answer. All instances of insulation on the roof deck I have seen have been sandwiched between sheathing on both sides. Does the manufacturer of the XPS recommend this type of installation? Sorry for more questions than answers but we do not have enough specific information as to the manufacturers of the roofing or insulation material.

Dana - Unfortunately I'm "married" to the XPS and can't afford to peddle it at a loss in order to buy iso. I am technically in Zone 5 but realistically more like a Zone 4 (Block Island - off the east end of Long Island - and tempered by the ocean). Would a 3/4" furring be OK? 1 1/2" seems like overkill. I was thinking ripped ledgerboard might be a good compromise at 1" actual. Does anyone have an actual pattern for the furring that has worked well under metal? Dana, wouldn't Ice & Water be another vapor barrier inhibiting drying (there will be no barrier on the interior except latex paint) The Zip is the primary air barrier and one of the reasons I chose it was for its permeability. I prefer not to rely entirely on the zip and tape for the weather barrier so I'd like to get recommendations for actual products. Bob I'll look into the RoofTopGuard II. Strong, I haven't settled on a metal yet though I'm leaning toward a Kynar finished aluminum. I have a meeting with the roofer tomorrow so I hope many questions will get answered.

Ice & Water is VERY vapor retardent (so it's important to have sufficient R outside the roof deck & membrane when you go that route) but it can also be useful and reliable as the primary air-barrier.

Since you're on the transitional edge of zones 4 & 5 you'll be fine with R20 foam, and R30-something in the rafter bays. ( It's unlikely you'll be over R35 in the rafter bays at any cellulose density.) In a solidly zone-4A location the IRC prescriptive value drops to R15:

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_par100.htm

Any air gap is better than no-air gap, and even some underlayments for metal roofing are fairly good about wicking & dispursing bulk moisture that finds it's way behind the metal. On Block Island snowpacks that stay around for weeks on end are all but unheard of, and on a metal roof of any significant pitch it'll slide off shortly after outdoor temps break freezing, so the prospect of a steady drip-drip-drip saturating the foam is pretty remote, unless you have very low roof angles. XPS won't take on much moisture anyway (iso would, over extended soakings), so as long as you have a very good WRB at the roof deck, it doesn't really need much in the way of drying capacity.

You may find Hobbit's Deep Energy Retrofit details on the roofing membrane, foam-over, and slip-surface under metal roofing issues useful.  They managed to screw it up in a few places, to be corrected after the fact, and the installers were apparently clueless about what the correct slip-surface material was.  (The whole thing is a fairly entertaining read, with lots of useful detail pictures and discussion.  I met the author/owner back in April when he attended an open house on a deep energy retrofit I had been involved with last year. He's definitely not your average homeowner- he knows how to read the materials specs with the best of 'em!)