I was actually in this industry for quite a few years before I really understood condensation in a building and how a metal roof might impact it. In fact, I always sort of thought that metal roofs would regularly get condensation on the back side of the panels – between the metal roof panels and the roof deck. Turns out that is rarely if ever the case.
Condensation occurs when the dewpoint is reached as warm moist area comes in contact with a cooler surface. That really doesn’t happen on the back side of metal roof panels because both are out in ambient conditions without any significant temperature or humidity differential. When it comes to condensation, the discussion is no different with metal roofing than it is with standard roofing. The place where condensation might occur is in the building’s attic.
Fact is, we do all kinds of things to make our homes and other buildings tighter for energy efficiency purposes. We put in better windows and doors. We put house wraps or insulation on the outside of the home. The result is that the moisture we generate inside our homes from bathing, cooking, and etc. including even house plants and ventless gas stoves, now gets trapped inside the structure. That moisture migrates upward to the attic and, if it is not vented outward, it condenses on cool surfaces – usually the bottom side of the wood deck on the roof’s surface when it starts to cool down in the evenings. This really occurs no matter what sort of material is on the roof itself.
The only exception is with a vertical seam roof that has direct contact with the roof deck. This could drop the roof deck temperature a couple of degrees from what it would be with, say, asphalt shingles. And that couple of degrees could be enough to be the straw that breaks the camel’s back and creates condensation that never occurred previous to the metal roof. A metal roof such as a metal shingle, though, that has an air gap between the metal and the roof deck, does not pose this risk. In addition to ventilation, vapor barriers and things like closed cell urethane foam can be very helpful at avoiding attic condensation.