Moisture condensation under metal roofs is a problem encountered all architectural sheet metal contractors. While architectural contractors may furnish the substrate to which the metal roof attaches, they typically do not design the balance of the roofing system’s insulation and vapor barrier. Unfortunately, excess moisture accumulation under a metal roof can cause and give the impression that the metal roof is failing, when, in fact, a failure to properly consider the dew-point temperature is a flaw in the design. It is the failure—by the designer—to consider the dew-point temperature and the location of the vapor barrier in an insulated roofing system.
The following article recommended by SMACNA’s Architectural Sheet Metal Steering Council is an excellent roof-specific explanation of how designers might approach dew point calculations for insulated roofing support systems. While the illustrations and examples are specific to low-slope roofs, these methods can also be applied to any insulated roofing assembly.
To view the article “Calculating Temperature Gradients,” from Professional Roofing magazine, visit the Professional Roofing magazine Web site.