Outside surface resistance: Proposed new value for building design

Abstract

This paper presents a critique of the design guidance currently offered by the Chartered Institution of Building Services Engineers (CIBSE) and by the American Society of Heating, Refrigeration and Air-conditioning Engineers (ASHRAE) with respect to external convection heat transfer at building façades. Their guidance to date is based on wind tunnel studies. Values are deduced for the convection heat transfer coefficients that form the basis of current advice. Deductions are compared with the findings of outdoor full-scale experimental studies. It is shown that these experimentally measured values for the convection coefficient are significantly higher than the values assumed in current design guidance. Based on this evidence, an expression derived from four full-scale investigations is presented for use by designers and building thermal modellers for correlating the external convection coefficient with surface windspeed. Using the expression, a modified design value of 0.04m2K W -1 for the external surface resistance Rso is proposed for the condition of 'normal' exposure (based on a design surface windspeed of 2 m s-1 as defined by CIBSE) to replace the current value of 0.06 m2 K W-1. As a result, it is recommended that the corresponding U-value for single glazing be raised to 6.3 W m -2K-1 from its currently accepted value of 5.6W m-2 K-1. A summary of results is also presented, showing the effect of window framework on external convection heat transfer.