Final Verdict on Unvented Attic and Standing Seam Roof
I’m in Climate Zone 3a with a 12:2 low slope standing seam metal roof and unvented attic (most of the house has no attic, so is cathedral ceiling). I’ll be doing an OSB roof deck with seams taped with ZIP. Directly underneath the deck will be closed cell spray foam. On top of the deck will be underlayment, then standing seam panels.
1. Should I provide an air gap between the underlayment and the standing seam panels (purlins or furring), along with a permeable synthetic underlayment, to allow drying to the exterior?
2. Should I use an impermeable synthetic underlayment instead, with the standing seam applied directly on top, and just assume that the OSB will stay dry?
Appreciate any help very much. I’ve read all the articles and comments, but have not been able to reach a conclusion.
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Replies
swooley,
This is a bit different situation in that the roofs in the other discussions didn't have the risk associated with being low-slope.
Metal roofing typically relies on laps to stop water intrusion through capillary action at places like the ridge, end-walls, skylights, chimneys, etc. With a 1/12 pitch these rely on sealants and gaskets, and more vulnerable to leaks, especially if debris accumulates.
So on low pitch roofs the underlayment plays a larger role in ensuring the roof stays watertight. With a 1/12 roof I w0uld consider using an impermeable Peel & stick membrane (like I&WS) over the whole surface. That would preclude drying to any vent space.
Hey Malcolm. Thank you so much for the help!
I realize based on your response that I mistyped and it’s a 2:12 slope.
So in the case, would you still suggest:
Standing seam
I&W shield
OSB w/ zip tape
Closed cell spray foam (all the way to r-38 or supplement with open cell)
Drywall
Interior of house
Having the impermeable sandwich around the OSB is ok here?
swooley,
That's good! 2/12 is a lot less risky than 1/12 - altogether still low enough our code precludes it without taking other measures (sealing, gaskets) to offset the low slope.
You are into a grey zone where although I have my own preferences, there are two legitimate views on this.
The situation is different than when we discuss vented roofs where I don't see any case for venting above the sheathing or using permeable underlays. In your situation a vent space above, coupled with a permeable underlay, would add some drying potential. However that vent space will also encourage condensation, and needs effective inlets and outlets to allow the air to move - something that I almost never see in over-vented metal roof assemblies.
I'm going to fall back on what RDH said then they discussed this in the context of the PNW climate: The benefits of permeable underlays were offset by the increased risk from moisture damage due to roof leaks.
I&WS will definitely yield a better roof. For me it really comes down to price as to whether it's worth it over a good impermeable underlay.
Thanks so much again, Malcolm. It sounds like the realistic option is either an Ice & Water Shield underlayment or something like a GAF Tiger Paw -- impermeable but much cheaper.
This is because putting the metal roof on purlins or furring of some kind, with a permeable underlayment below, would require some very intentional venting on a low slope roof, and even then we just don't have enough data to know that it would be more successful than simply sealing everything up and hoping for the best.
If you have any other suggestions or if there's anything else I should know, please do not hesitate to let me know. In my case, something like Tiger Paw would be $500, whereas Grace I&WS would be $2600. Would have to really, really be worth it.
About to just do the whole thing in GAF Liberty and call it a day.
swooley,
If it were me I'd stick with the metal, use something like the Tiger's Paw, and wouldn't worry about that roof at all.
I was advised by my roofer not to use "standard" ice and water shield under a metal roof. He stopped me before I applied the ice and water shield and traded the rolls out for a product they use under metal.
Apologies I don't recall the product name.
Love it Malcolm. I think that's what I'll do. Your comments have been really helpful - thank you.
Edit: Oh, one more thing if you care to comment. Is the ZIP tape on the OSB seams worth doing?
Swooley -
If you use I&W make sure to get the High Temp I&W shield. This is probably what D_Hallowell was referring to. The High Temp version comes with a fabric type covering on the metal facing side that keeps the metal roof panels from sticking to the I&W shield when it’s hot outside.
If you do use I&W, would there be any benefit to using Zip on the roof? The I&W should provide the continuous barrier right?
Yeah, that’s what I’m thinking too. No need to seal the seams if going with an impermeable underlayment. Unless the point is to have a redundant system in case the I&W fails or is ripped off in a storm.
But if standing seam and I&W are both removed in a storm, I’m not sure a taped seam OSB is going to prevent further damage.
We’re in Oklahoma so tornadoes do happen, but the ZIP seam sealing is $450 in tape, so that’s a good aspect of this to question. What do you think?
If your ice and water comes off, your osb is also coming off. Gracie makes ultra for metal roofs. Gracie has one of the best adhesions of any impermeable peel and stick. There is a big difference in adhesion between brands.
swooley,
I don't know that much about construction in high wind areas, but a blog here (sometime in the last six months) detailing the appropriate measures to take did recommend either a self-adhered underlayment, or taping the sheathing.
The standing seam clips should be screwed through the ice and water and into the sheathing. It has self healing properties so not a big deal from that aspect. If the roof and ice and water come off, the sheathing is gone as well.