This paper stresses the importance of a proper estimate of ground reflectivity in building energy simulation, particularly in the presence of snow. Several factors influencing ground reflectivity are reviewed. Two models for estimating ground reflectivity in the presence of snow are developed. The first one is based on the number of days with snow depth greater than 5 cm, and is appropriate for use with 'typical' meteorological years. The second makes use of actual records of snow depth, and takes into account snow accumulation, ageing and melting to calculate reflectivity values. The algorithms were implemented in ESP-r and tested by simulating a passive solar house for six Canadian locations. The sensible heating load calculated by the simulations is reduced by up to 10.9% on a yearly basis, and 23.3% on a monthly basis, when the ground reflectivity takes into account the presence of snow. (c) 2005 Elsevier B.V. All rights reserved.