Hot unvented cathedral roof in north Georgia, what kind of underlayment under standing seam?
I’m going to have a unvented hot roof (no attic) with closed cell on underside of deck. Do I use permeable or impermeable underlayment under the standing seam metal roof? It is humid and rains quite a bit here. The first roofing estimate (so far) I received states I should use an ice and water dam underlayment. What if I used a entangled mesh over the underlayment?
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Standing seam roofs are water tight. The only areas I would use any I&W is in places code requires or questionable transitions and intersection. For the rest, synthetic underlayment is all you need. Make sure any I&W you use is high temp rated and non granulated, stuff tends to be pricey.
My local roofing supply stock permeable synthetic underlayment and it is only a couple of dollars more. The striations and ribs of any standing seam allow for a bit of airflow, if the underlayment allows for a bit of drying, I think it makes for a better assembly.
A roof with only closed cell is expensive and less environmentally friendly than other alternatives. If you have sufficient depth in the roof framing, you could use the flash and batt approach. See https://www.greenbuildingadvisor.com/article/the-flash-and-batt-method for more info.
I will be, I'm more concerned on what kind of underlayment to use. Since there is no attic, I am concerned about undetected leaks. I am thinking of a redundant zip system followed by an underlayment at this point to lower risks.
I understand. I had similar concerns when working on a previous home. The reality is that leaks are hard to detect under the best of circumstances, which is why you want a good roof assembly and installation. Water also has a funny way of traveling some distance from the entry point to wherever it becomes visible.