In the example building located in San Francisco, there was a decrease in the total amount of water within the roof system over time, and the relative humidity was never high enough to cause condensation within the roof system. Taking that same building to a colder climate, such as Anchorage, Alaska, different results are obtained. The WUFI analysis for Anchorage shows that the total water content in the roof system increases each year, which is an unacceptable condition. Figure 1 shows the estimated content in the gypsum roof board. Figure 2 shows that in the gypsum roof board the relative humidity approaches 100 percent, and after the second year it remains at nearly 100 percent throughout the year. The top layer of polyisocyanurate insulation experiences similar conditions. Clearly, this roof assembly would experience premature roof failure because of excessive moisture accumulation. By adding a vapor retarder, the revised assembly would be suitable.
This Web exclusive information is a supplement to Calculating heat and moisture.