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I am installing ribbed metal panels over a singlt slope roof on a new workshop. I was planning to install directly over 2x4 purlins, which are mostly already installed. This shop will be insulated but mostly unheated except when I'm working in there (say, 10 hours a week). I will have continuous soffit vents and will install a couple wall vents up high on the walls.
My question is - should I install a synthetic underlayment over the purlins? I'm in Blacksburg VA if that helps. Mean annual temperature is 51F and we do get snow/cold in winter (often cold, not very often snow).
Thanks. Yes, underlayment should be in place. I am unsure whether you have solid decking beneath the purlins. If you do, i suggest the underlayment on top of it. If not, then underlayment on top of the purlins it is. Your risk is when warm moist air from inside hits the cool back side of the roof system. It will want to condense. Insulation and ventilation are going to be keys for you. The underlayment doesn't really impact condensation in your case -- the condensation will just occur on it rather than on the metal itself. Or both places.
Thanks for your quick reply. There is no solid decking. Purlins are already attached with a hundred screws so they are not coming off. So the only question is should I put an underlayment over them. It will make installation harder in my view. If I don't put it, I can do a lot of work from a ladder rather than clambering over the purlins and trying not to rip the underlay.
In your view, is there any other option to keep condensation down? It sounds like I can expect condensation on the underside of the underlay which sort of seems like it wouldn't really serve much purpose. Could I fit XPS in the rafter gaps after the roof is up to create a thermal break? Or would insulation in the ceiling joists (not the rafters) serve the same purpose?
The purpose of the underlayment is largely that it is required by code. It can also provide protection if there should be a roof flashing occasionally bleed some water. That said, I am not keen on underlayments over battens as it tends to drape and droop. Avoiding condensation requires insulation, ventilation, and vapor barrier. You need at least 2 of the 3 and, with no decking, I'd say all three. Can you add a polyethylene vapor barrier (completely sealed to the bottom of your rafters, with insulation above that? Yes, that all could also be done done below the ceiling joists if this is "normal" construction with an attic. In that case, do you have drywall on the ceiling yet? If not, a vapor barrier behind it would be wise.
Great thanks. I am in the process of building. It will be 'normal' construction with a single pitch ("shed") roof with attic. I was planning to put insulation in those ceiling joists that form the attic and will include a vapor barrier per your advice.
Thanks again for your help!
We are getting ready to put a metal roof over shingles on the house and garage in Ohio. Does it matter if we place the wood slats 24 inches or 18 inches apart. Also, will we need anything else for a moisture barrier
Richard, the spacing of your battens / purlins needs to come from the panel manufacturer. It will be based upon the design and testing of their product. I do suggest underlayment over the existing shingles. Not all folks will agree with me on that.
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