The aim of this research is to quantify the influence of a louvered blind in a double-glazed unit during night-time conditions. First, an analytical study of free convection was conducted to obtain a set of correlations for the Nusselt numbers of the cavity. Second, a parametric study was performed to calculate the total heat transfer (convective and long-wave radiative) during night-time conditions. The analysis accounted for aspect ratio, blind thermal conductivity, surface emissivity and slat angle. Using these data, a biquadratic equation was developed to calculate the U-factor of a double-glazed unit with an internal louvered blind in terms of the U-factor of the unit without the blind, slat surface emissivity and slat angle. (C) 2011 Elsevier Ltd. All rights reserved.