Comparative Summer Thermal Performance of Finished and Unfinished Metal Roofing Products with Composition Shingles

Abstract

This paper presents an overview of results from experimental research conducted at FSEC's Flexible Roofing Facility in the summer of 2002. The Flexible Roof Facility (FRF) is a test facility in Cocoa, Florida designed to evaluate a combination of five roofing systems against a control roof using dark shingles. The intent of the testing is to evaluate how roofing systems impact residential cooling energy use. Recent testing emphasizes evaluation of how increasingly popular metal roofing systems, both finished and unfinished, might compare with other more traditional roofing types. All of the test cells had R-19 insulation installed on the attic floor except in the double roof configuration which had R-19 of open cell foam blown onto the underside of the roof decking. The test results were used to determine relative thermal performance of various roofing systems under typical Florida summer conditions. Measured impacts included changes to ceiling heat flux and attic air temperature which influences loads from unintended attic air leakage and duct heat gain. We also develop an analysis method to estimate total cooling energy benefits of different roofing systems considering the various impacts. The results show that all the options perform better than dark composition shingles. White metal performs best with an estimated cooling energy reduction of about 15%, but the spectrally selective metal shingles (12%) and unfinished Galvalume roofs (11%) do surprisingly well. Galvanized roofing did less well than Galvalume (7% reduction) and worse performance in the second year of exposure was observed due to corrosion of the zinc surface. The sealed attic with a double roof produced an estimated cooling energy reduction of only 2% -- largely due to increases in ceiling flux.