New Metal Deck and Metal Roof / Condensation

Nate Kerstetter
12/16/2015
I am working for a General Contractor on a project that is replacing a metal roof. The new design determined that the existing roof purlins were spaced too far apart, therefore, a metal deck was added to the roof construction. Design then calls for z-strips to be installed over the metal deck and fasten the roof panel to the z-strips. The z-strips create an 8" cavity between the roof panel and roof deck. 8" batt insulation and vapor barrier will be installed in this cavity. Inspector is questioning the design, stating that condensation will build up within the flutes of the metal decking, or the underside of the metal roof panel. Is this a valid concern? Thanks, Nate
Todd Miller
Isaiah Industries, Inc.
12/17/2015
What sort of use will this building have? Is there any ventilation at all, or other insulation? Where exactly will the vapor barrier be located? Are there other vapor barriers? I feel like you probably are okay as described but would love to have this additional information.
Nate Kerstetter
12/17/2015
Todd, The building is an Army Vehicle Maintenance Shop. The vapor barrier will be laid on the metal deck. So, the system will be from top down, Metal roof panel, insulation, vapor barrier, metal deck. That is the only vapor barrier to be installed. No other ventilation is to be provided. Thanks, Nate
Todd Miller
Isaiah Industries, Inc.
12/17/2015
The risk of condensation is actually when you have warm moist air inside the building and the roof gets cold due to an outside temperature drop. I think that the vapor barrier, provided it is installed correctly, will keep you safe with this installation.
Nate Kerstetter
12/17/2015
Todd, Thanks for your help. It always helps to hear from someone else. Nate

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