The assessment of U-values for insulated roofs

Abstract

This paper reports the results of a series of thermal conductivity and thermal transmittance tests which were designed to investigate the discrepancy between U-values calculated i sing CIBS guidelines and some results obtained by the Agrément Board in their “hot box” apparatus. The most significant factor was that for permeable insulation the measured conductance (from which the U-value is derived) was very dependent on the air speed at the upper surface of the insulation in Agrément's test rig. The air speeds applied during these tests were higher than those subsequently measured by BRE in three loft spaces. The applied speeds were also higher than those normally associated with the surface resistances measured by Agrément. Indeed in several tests the measured values of surface resistance were higher than theory would predict is possible even if still air conditions are assumed. Because of these questions about the air movement conditions used in the Agrément tests, the results do not justify changes to the presently accepted methods of assessing the U-values of insulated roofs.