RDH Building Science

Wind washing is wind-driven air movement through or behind thermal insulation within enclosures. This bulk movement of air increases heat loss, resulting in increased energy consumption, risk of condensation on cooled surfaces, and increased space heating loads.

This paper presents a methodology for predicting wind washing impacts and laboratory measurement of heat loss impacts for insulation products exposed to cavity airflows. It is recommended that average winter wind speeds (or ASHRAE mean coincident wind speed) be used for wind washing analysis. A method is presented for adjusting wind speeds for sheltering and height impacts based on approaches used in structural wind load calculations. Hourly average air speeds as high as 0.1 and 0.7 m/s (0.4 and 2.4 ft/s) were predicted using simple airflow network analysis for well-ventilated brick cladding and slot panels, respectively. Analysis of full-scale wind tunnel pressure tap data for a small house was used to predict vinyl siding cavity air speeds as high as 0.5 m/s (1.5 ft/s). Heat flow measurements were taken for a number of insulation products in a purpose-built apparatus. Of the mineral wool board samples tested, only the 25 and 50 mm (1 and 2 in.) thick 70 kg/m3 (4.4 pcf) samples showed measurable wind washing impact. This impact was as much as 0.03 RSI (R-0.2) reduction of thermal performance. Hence, wind washing impacts are expected to be small for well-installed mineral wool board continuous insulation.

Note: This article was published in Proceedings of Buildings XIII, 2016. Copyright 2016 ASHRAE. Reprinted by permission at rdh.com. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. For more information about the Buildings XIII Conference Proceedings, visit http://web.ornl.gov/sci/buildings/2016/proceedings/.

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