Effects of Mechanical Fasteners and Gaps between Insulation Boards on Thermal Performance of Low-Slope Roofs

Abstract

This paper presents the results of laboratory experiments for the thermal performance of metal-decked, low-slope roofs with polyisocyanurate insulation. We measured the effects on thermal performance of fasteners through the roof insulation and gaps between insulation boards. Steady-state temperatures were held above and below the test assemblies in order to determine their overall thermal resistances as a function of the mean temperature of the insulation. The polyisocyanurate insulation used in the tests was not fully aged so its thermal conductivity increased during the tests. All data were corrected for this increase. Fasteners reduced the thermal resistances of the assemblies with two layers of insulation, each 5.1 cm (2 in.) thick, by 9% at winter conditions, by 6% at summer conditions, and by 7% at average conditions. The 2.5 cm (1 in.) wide gaps which we made in the configurations with two layers of insulation decreased their thermal resistances by 15% at winter conditions and 10% at summer conditions. With one layer of insulation the effect was larger: 17% reduction at winter conditions and 14% reduction at summer conditions. Modeling at steady-state was done to discern the effect of different fasteners. The 7% reduction at average conditions became 12% for a conventional fastener with a steel plate and 3% for a plastic/steel fastener. The results were extended by modeling that used weather data for typical meteorological years in six separate U.S. locations which included a wide range of climates. The effects of fasteners and gaps were to increase simulated annual heating and cooling loads by percentages about equal to the percentage reductions in thermal resistances at winter and summer conditions, respectively, in the tests.