ZIP system R-sheathing on a Roof
I’m looking for a poor-man’s alternative to thick polyiso on a roof and was wondering why ZIP system R-sheathing is not “allowed” to be on a roof? From Huber’s web site:
“Zip System Râ€Sheathing is currently limited to woodâ€framed wall applications only. DO NOT use roofs or light gauge metal framing. For systems requiring multiple layers of waterâ€resistive barriers, ZIP System Râ€Sheathing is intended to replace only the first.”
My roof is extremely unique and you will all probably roll your eyes, but here we go:
1. My Roof is vented with baffles
2. The attic conditioned space is: 1/2″ OSB baffle, 3″ Closed-Cell, BATT
3. There is a 2″ air vent above the OSB baffle and attic conditioned space
4. The “roof layer” consists of 1/2″ CDX with ZIP-system R-sheathing above it (the 1″ version of 7/16″ OSB with 1/2″ polyiso adhered to it)
Question: is this configuration, 1/2 polyiso effectively sandwiched between ZIP OSB and plywood CDX with a vent underneath and shingles on top “safe”? The plywood can dry down toward the vent, the ZIP can dry down through the 1/2″ poly and plywood to some degree.
Fundamentally I don’t see this being any different than 7/16″ regular ZIP as a roofing material with polyiso blown in underneath it directly. I’m questioning whether to put a waterproof membrane on top of the ZIP to keep it 100% dry…
Here is a PDF showing my proposed roof assembly:
Thoughts?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Christiaan,
First of all, it's unclear from your post whether this roof assembly is proposed but not yet built -- or already built.
Briefly, it makes no sense to install any insulation above a ventilation channel. The ventilation channel's purpose is to invite outdoor air (cold air in winter, hot air in summer) to flow above your insulation layer. Once you have invited that outdoor air into your roof assembly, any insulation installed on the exterior side of the ventilation channel is wasted. The Zip-R sheathing is outdoors. It isn't doing you any good.
Needless to say, it makes no sense to install building products in a way that violates the manufacturer's installation instructions. (Violating a manufacturer's installation instructions is a code violation.)
Martin,
Assumption: The OP is building an unvented Attic.
Q: Could the OP basically build a SIP using plywood as roof decking + ZIP-R nailed on top of it + roofing material ?
We don't really know why Huber doesn't recommend ZIP-R on the roof, but lets assume they gave the "ok" for this type of stack up.
Chris,
It's common to use nailbase (a panel consisting of rigid foam with OSB glued to one side) on roofs. If you Google "nailbase," you'll find lots of manufacturers. Nailbase is suitable for use on roofs.
I'm not sure why Huber's Zip-R isn't appropriate for roofs. But since so many manufacturers make nailbase, I see no reason to choose a wall product for this application.
Steve,
I doubt whether it makes sense to install Zip-R directly on rafters or trusses -- clearly, the framing members would dig into the soft foam under a snow load. It's possible that Zip-R works fine when installed on top of existing roof sheathing, however, which is the way that nailbase is usually installed. (But I'm speculating -- and in any case, it sounds like Huber doesn't want their product used on roofs.)
Of course, that's not how Zip-R is installed on walls. On walls, the product is fastened directly to the studs.
UPDATE: Huber just told me that they're currently testing ZIP-R using a similar assembly that I described. We'll see.
Huber has not had ZIP-R evaluated for roof applications, so it is not code compliant. I would speculate that they hadn't pursued this evaluation because there is no market opportunity.
Hey everyone, thank you for the responses. I agree with most of your points, but I think the reason I'm putting a foam layer on my ventilated roof decking is for thermal bridging purposes, not insulation purposes. What are you thoughts on that?
This assembly is not implemented, only the 1/2" plywood layer is up right now.
Well, your solution is akin to installing foam on your exterior walls while also decideding to leave your windows open.
I might be reading this wrong, but appears as if there is an insulated roof deck sandwiched between a vent channel and conditioned attic. I would propose the attached detail instead if a true conditioned attic is what you are after. Ignore the spray foam note, that will be DP Cellulose.
Christiaan,
Q. "I think the reason I'm putting a foam layer on my ventilated roof decking is for thermal bridging purposes, not insulation purposes. What are you thoughts on that?"
A. The way we address thermal bridging problems is with insulation -- so these two purposes ("thermal bridging purposes" and "insulation purposes") are really the same.
If you are worried about thermal bridging, it's fine and dandy to address the thermal bridging with a layer of continuous rigid foam insulation. But all of the insulation needs to be on the interior side of your ventilation gap. The ventilation gap is basically outdoor air.
Thanks Martin, I agree with you philosophically. I should have mentioned this in the beginning, but R-sheathing on the roof is a last-ditch attempt at avoiding the thermal bridging of the 120F-140F roof into the attic. I'm not as concerned about the 90F ambient air bridging in, which it will through the vent.
I have it on good authority that the only reason ZIP R-sheathing is not approved for roof applications is because Huber hasn't found a business case to go through the testing requirements.
Any update on the applicability of ZIp-R on roofs? I wonder if this is used as a secondary sheathing layer (i..e used as a nailblase over 2" of foam which in itself is over 5/8" sheathing) does it make a difference?
A mer 1/2" polyiso sandwiched between two layers of OSB or plywood isn't much thermal break. On a 2x12 raftered roof system it's not enough to move the needle much.
The 3" of closed cell foam bridged by rafters is pretty much wasted too, and not very green at all. That's more polymer than it takes a full cavity fill of half pound open cell foam, and packs a hefty greenhouse gas hit from the HFC 245fa blowing agent (compared to water, the blowing agent for open cell foam.) The difference in total cost between 3" ccSPF + fluff and 11.5" of half-pound foam would be enough to offset the cost of 2" nailbase panels to install above the structural roof deck for MUCH better thermal break.
With 11.5" of open cell foam it's air tight, but to limit summertime moisture cycling using vapor barrier latex or a smart vapor retarder on the interior side might be in order.
Dana,
I did not lay out the plan clearly, The idea is to use the Zip-R (2.5") over 2" of polyiso sheets that would be on top of std sheathing. Below that (in the bays) woudl be densepack.
The idea is to reduce the labor cost by eliminating one of the three layers (combining last foam layer with the osb overcoat)
That make sense?
Edward,
The product you are looking for exists. It is called nailbase. Nailbase manufacturers recommend their products for use on roofs, so they will back up their product with support from the manufacturer.
Zip-R is not designed for roofs, so the manufacturer (Huber) will not back up the product if you use it that way.