Metal roof over Zip-R vs. rigid
I am building a small cabin which will be *lightly* insulated, for 3-season use, but may be upgraded in the future. For aesthetic reasons, I spec’d T&G pine as sheathing, with exposed rough-sawn rafters – the inside will be unfinished. In other words the T&G is used as the structural roof deck as well as the finished ceiling. The plan is to add 2″ rigid foam on top of this deck and then exposed-fastener metal roof panels. So my questions are:
1. Air barrier – I’m thinking I need a peel n stick on top of the roof deck, but don’t want to use any rubberized products since I’m afraid the smell of asphalt will permeate through the cracks in the T&G. Should this be vapor permeable? Is there such a product – a synthetic peel n stick that is not butyl-based?
2. I am open to the idea of using Zip-R (even though it is not warranted for roof use, at least I don’t think yet) to provide insulation and a solid deck to attach the metal roof panels to. But I worry that I should have a rainscreen since exposed fastener roofs eventually leak – so wood purlins? Metal z-girts? Should they be installed in both directions?
3. Does the metal roof need to be installed over purlins anyway since it is black and will get very hot? Will the foam be affected? Should I use polyiso instead of the cheaper XPS?
Thanks for your help.
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European suppliers have zero-VOC membranes perfect for your situation. I would check out what's available from Pro Clima (foursevenfive.com), Siga (sigatapes.com) or Rothoblaas (rothoblaas.com). If the foam layer is impermeable to moisture, or if it is permeable but the metal roofing is not vented, there is no benefit to the membrane being vapor permeable but it wouldn't hurt anything if it was permeable.
Zip-R is not warranted for use on roofs but I believe they are working on it. Hunter has similar nailbase panels with integral venting, but I don't see a lot of benefit to having the venting in this situation. I would probably go with a regular Hunter panel. Actually I'd prefer to use materials with less embodied carbon than foam panels but I have used foam on roofs before and will probably do it again.
You don't need ventilation under a metal roof; in fact venting it causes problems with condensation that are easy enough to handle but require extra steps. If the roof assembly does not require ventilation, and yours does not, I (and metal roofing installers) prefer to install directly over the sheathing. If you're in a place that gets snow, at the eaves and other vulnerable spots you should use an ice-and-water-shield type product meant for use under metal roofing, designated by "hi-temp" or something like that. Trust me, you don't want to use the regular stuff and see it dripping down the fascia on a hot, sunny day. Over the rest of the roof you can use conventional roofing underlayment.