Development of Psi Factors for Thermal Bypass Due to Insulation Gaps in Low-Slope Roofing Assemblies

Sudhakar Molleti,David Van Reenen

doi:10.3390/buildings12010068

Abstract

In commercial roofs, the presence or formation of gaps could be due to improper installation, thermal expansion, and dimensional changes in the insulation boards. The heat loss from these gaps could lead to higher thermal transmittance in the roof assembly. The current research study conducted around 70 experiments to investigate the effect of gap height, gap width and gap offset on the thermal transmittance of the roofing assembly. The measured data showed that in a staggered insulation layout with a joint offset of 610 mm (24 in), formation of 6.4 mm (1/4 in) to 12.7 mm (1/2 in) gaps at the insulation joints could contribute to an average decrease of 2% to 9% in the effective R-value of the roof assembly. As the insulation thermal resistance increases or becomes thicker, the thermal losses in the roof assembly increase. Generalized gap impact curves were developed to provide the relation between gap parameters (i.e., gap widths and height) and the thermal performance of the roof assembly. The experimental data were further analyzed using the psi factor approach of linear thermal bridging generating thermal transmittance data to support the calculation of thermal bypass from gaps in the thermal roof design.

Highlights

It should be emphasized that the results presented here are for a standard industry practice of staggered gap arrangement of 610 mm (24 in) between the insulation boards
In assessing the impact of thermal bypass, more than 70 experiments were conducted highlighting the effect of staggered gaps, the gap width, through gaps, cross joints, and the impact of fasteners and gaps
Thermal bypass increases with the gap width between insulation boards

Summary

Introduction

Roofs occupy a large portion of the surface area of the envelope. The typical components of a conventional roof assembly include waterproofing membrane, cover board, thermal insulation, vapor barrier, and roof deck. The thermal resistance of these roofing components and exterior and interior air films control the heat flow through the roof assembly. The thermal insulation is the primary component to provide resistance to heat flow within the roof assembly, i.e., the highest R-Value within the roof assembly. The efficient use of thermal insulation materials in the building envelop thermal design remains one of the most effective ways of improving a building’s energy performance

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Conclusion