Todd Miller - Roofing Expert

Metal Roofing and Condensation

Bob wrote and asked me the following:

Hello. I was reading about what you said concerning metal roofs. I have one and it sweats when the sun hits it in the morning. Can you help ? I understand the need to reduce moisture from the inside, so I took the boiling pan of water off of the wood stove. Is this a start ? It is an open beam ceiling. From the inside out are 4×8 beams, then 1×12 shelving board, then felt paper then 2x4s laid horizontally as lathes with 2 1/2″ styrofoam insulation, then the metal roof. I was told to take the metal off again and put this moisture barrier sheeting down then put the metal back on. What do you think? I have someone else who wants to take the metal off and just out some sheeting down, then the metal back on. Will this work? Please help.

My answer:
I assume that the decking they want to put down would be plywood or OSB, and then cover it with another layer of underlayment before the metal is put back down.

These situations are always a bit of a guessing game. First of all, eliminating any sources of excess moisture inside the house is important … make sure bathroom and laundry roof vents go outside. Ventless gas stoves also create a lot of moisture.

Would the decking alone be enough to tip the scale and end this problem? I really cannot predict. It would be a good step, undoubtedly. But my preference would be to add the vapor barrier behind the 1 x 12 boards as well as add a layer of decking and underlayment beneath the roofing. Of course, I’d also love to se soffit to ridge ventilation occur beneath that decking as well.

In addition to minimizing excess moisture inside the house, these three things (vapor barrier, Ventilation, and decking) I feel certain would end the problem. Is there a chance that one or two of them alone would fix the problem? Yes, but no guarantees.