This paper presents data on the behavior of moisture and temperature in a roof assembly of a western frame constructed, three story high, twelve unit multifamily building in Bellevue, Washington, just outside of Seattle. The data covers a multiyear span and reveals the hygrothermal behavior of two different roof assemblies, first a standard western frame, platform construction vented flat roof assembly, and then a retrofitted unvented and energy efficient roof assembly. The retrofitted roof strategy is only allowed as an exception by the local code authorities, compelling evidence must be presented. This paper provides results from field monitored roof sections and from roof simulations. A state-of-the-art hygrothermal computer program was employed to simulate the complex moisture dynamics, dependent on the specific interior and exterior environments and with the impact of air leakages exchanges in the air cavity. The analysis and measurements were performed for both roof design strategies. Both experimental and advanced modeling provided invaluable insight on the complex hygrothermal transport present in the roof. Results on the moisture accumulation in the roof assemblies, temperatures and relative humidity within the roof envelope systems are presented to quantify the merit of the two different roof assemblies.

Presented at Thermal Performance of Exterior Envelopes of Whole Buildings X – December 2007

Units: SI