Building and Environment 41 (2006) 1767–1778 A perspective on the effect of climate and local environmental variables on the performance of attic radiant barriers in the United States Mario A. Medina à , Bryan Young Civil Environmental and Architectural Engineering Department, The University of Kansas, Lawrence, KS 66045, USA Received 29 October 2004; received in revised form 30 November 2004; accepted 15 July 2005 Abstract This paper offers a perspective on how climate and local environmental variables affect the performance of attic radiant barriers across the United States. Transient heat and mass transfer simulations were performed on a vented triangular attic with insulation level of 3:5m 2 K=W (R-19) and the results were based on integrated hourly ceiling heat fluxes over 3-month periods during the cooling season. The ceiling heat transfer percent reductions ranged from 36.8% in the Tropical Savanna climate to 2.3% in the Mediterranean climate. Peak-hour percent reductions in ceiling heat flux ranged from almost 100% in the Marine West Coast climate to 23% in the Desert climate. The results suggested that local ambient air temperature, humidity, cloud cover index, and altitude had first-order effects. The amount of local solar radiation had no effect on the performance of the systems. r 2005 Elsevier Ltd. All rights reserved. Keywords: Radiant barriers; Building energy conservation; Building heat transfer; Climate and environment 1. Introduction The value of radiant barriers, thin aluminum sheets installed in attic spaces, in relation to reducing the ceiling heat transfer to and from conditioned spaces has been documented for years [1–18]. Several conclusions were drawn from the referenced literature. These are summarized below as follows: (a) Radiant barriers do contribute to a reduction of heat transfer rate in attics when compared to attics without radiant barriers. The percent reductions varied from approximately 40–45% if insulation with a resistance value of 1:95 m 2 K=W (R-11) was used to approximately 15–20% in the case when the attics had insulation with a resistance value of 5:28 m 2 K=W (R-30). (b) Radiant barriers installed over the attic ceiling frame (horizontal radiant barrier, HRB) consistently re- duced the heat transfer rate by approximately 5% more than a radiant barrier installed against the rafters that support the roof (truss radiant barrier, TRB), even in cases when the end-gables were covered. The HRB also used less material per attic structure, but its use has been discouraged because of dust accumulation. (c) Radiant barrier effectiveness was not increased by it having two low-emissivity surfaces instead of one. (d) The TRB reduced the heat transfer rate as effectively whether the low-emissivity surface was facing down or facing up. (e) Ceiling heat flux reductions as a result of using radiant barriers were larger in non- vented attics when compared to vented attics. However, once a small threshold ventilation rate of 1:27 ðl=sÞ=m 2 ð0:25 CFM=ft 2 Þ was achieved, radiant barrier effectiveness was independent of airflow. ARTICLE IN PRESS www.elsevier.com/locate/buildenv 0360-1323/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2005.07.018 à Corresponding author. Tel.: +1 785 864 3604; fax: +1 785 864 5631. E-mail address: mmedina@ku.edu (M.A. Medina).