Two roofing systems are examined. The systems are identical, except for the insulation material. Both are analyzed for the climate in Chicago with an indoor relative humidity of 50% and indoor temperature of 68°F. Both have a black outer membrane and a tight metal deck. Fiberboard and foam systems are analyzed. Insulation thickness is 3 inches for both.
These results are common for both systems. Then, using Equation (4), for fiberboard:
The failure threshold for fiberboard is 0.20 lb/ft2. Since 0.046<0.20 lb/ft2, this system passes. Using Equation (4) again, this time for foam:
The failure threshold for foam systems is 0.012 lb/ft2. Since 0.027>0.012 lb/ft2, this system fails.
For the fiberboard system, the condition for failure is not met, so this system passes.
For the foam system, the vapor resistance ratio is calculated:
Ri/Rd = 1.39/1.56 = 0.9.Since 0.9 < 1.5, we fail.
Using Eqns. 7 and 8 to determine the relative time to dry for the fiberboard and foam systems:
t = 3 months (for fiberboard)The fiberboard system dries more quickly than the foam system, but both dry in less than the maximum of twelve months.
The fiberboard system passes all four requirements and therefore represents an acceptable design for moisture control for this roofing application. The foam system would likely see condensation at the roof membrane in the winter and represents a poor design for moisture control in this case.
Next Section - Comparison with Existing Methods
André O. Desjarlais