Closed-cell polyethylene insulation has been installed in inverted roof systems for more than 40 years. In some of these applicationsthe insulation is exposed to humidity and gradually absorbs moisture while in service. This paper presents data aboutwet and dry densities and aged U-factors of insulation samples retrieved from roofs in service in Western Canada for between11 and 35 years. The data indicate that closed-cell polyethylene insulation exposed to wet service conditions absorbs sufficientamounts of moisture to significantly degrade the expected insulation U-factor and also increase roof loading. A wider study isrequired to isolate field variables such as assembly type, exposure history, insulation type and frost action to generalize findings.