Convective Heat Loss: A Critical Analysis of Conventional Rainscreen Design

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New energy mandates prompt enclosure designs that integrate continuous insulation in ventilated rainscreen facades. Classical rainscreens have the singular purpose of improving moisture transport by means of convective forces. These same forces may have detrimental consequences to heat transfer, especially in assemblies that rely solely on exterior insulation. This presentation examines the “Rainscreen Paradox:”“ the apparent conflict between improved moisture transport of rainscreen designs and the coexisting effects of convective heat loss due to wind washing. Simulations of modern ventilated rainscreens employed Computational Fluid Dynamics (CFD) to demonstrate previously unseen flow characteristics as air encounters inlets, cladding attachment systems, and insulation gaps. Findings demonstrate that convective heat loss in ventilated rainscreens is largely a function of wind speed, rainscreen geometries, and air-permeability of insulating materials. Reductions in effective R-values are similar in magnitude to those resulting from thermal bridging in steel stud construction. Prevailing assumptions regarding moisture performance are challenged with particular emphasis on the combined effects of thermal bridging and wind-washing. Strategies for risk avoidance are discussed in context with material selection, design, and construction quality control.