White roofs and moisture in the US desert Southwest

Roof assemblies in the US desert Southwest have been instrumented for temperature and humidity. The low-slope roofs are composed of R-38 insulation installed directly beneath wood product sheathing boards, and are covered with a built-up roof and topped with a very reflective white coating. The air temperature at the underside of the sheathing remained very cold from December through March, rarely rising above the outdoor air temperature even in the middle of the afternoon. Observations confirm the presence of water in the insulation of some of the roof assemblies. Measured values showed high humidity (average RH>80% for several weeks during the winter) at the interface between the insulation and sheathing in some of the roof assemblies. This report presents theory as well as measured and modeled values indicating the“white roof problem”. With highly emissive upward-facing surfaces in clear-sky areas, the infrared loss from a surface to the sky is great; indeed, with high solar reflectivity, the solar gain cannot compensate for the sky loss. Since infrared radiation balance measurements are not available for this site, local airport cloud-cover measurements are used as a surrogate. Measured values of temperature and humidity from winters 2004-2005 and 2005-2006 are presented. Modeled values that account for sky radiation, solar radiation, conduction and surface convection are used to explain the measured values.