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Good Afternoon, I am about to install a metal roof on my 24'x27' garage. I laid out 15# roofing felt over the obs roof sheathing to get the space dried in. My metal will be here in a couple of days. In your opinion will that 15# felt be enough of a vapor barrier for my roof? This is on a 4:12 gable roof in SE Tennessee with little snow or freezing weather. Or do you think it would be smarter to rollout some synthetic underlayment over the 15# felt? Thank you for your time.
I would refer you back to the roofing manufacturer as far as the minimum underlayment they recommend. Most metal manufacturers recommend 30-pound felt or a synthetic underlayment. As far as a vapor barrier -- I guess I am curious what you want to accomplish. Will you have moisture inside the garage that you're trying to stop from driving outward? Tell me more about what you want to accomplish and I will weigh in. Thanks!
Well, I just thought it was always a good idea to have a vapor barrier under any roof especially metal. The only moisture source in the garage will come from the combustion of propane for heat and humans breathing. So I doubt there will be tons of moisture to worry about. That and the open trusses will be reasonably well vented. Also, I will have kraft faced fiberglass insulation installed in spaces between the trusses.
Thanks! So, roofing underlayment, while largely vapor impermeable, is not really a vapor barrier but a moisture barrier. I would refer you back again to the roofing manufacturer as far as their recommendation. Usually it is 30 pound felt or synthetic. My concern is always with warm moist air inside the structure trying to drive outward and then condensing when it hits a cold surface such as the underside of the roof deck or the metal roof itself. Metal roofs do tend to be colder so that's a consideration. We usually prevent that from happening with things like vapor barriers behind the ceilings, insulation on top of the ceilings, and ventilation in the attic cavity. In your case, it sounds like you will have ventilation and also no large sources of moisture so I believe you should be okay.
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