This study presents a new approach to compute the heat transfer of ground-coupled envelope quickly and correctly which is suitable for implementation into building thermal simulation software. The heat transfer process is decomposed into three processes which are controlled by ground-coupled envelope surface temperature, outdoor ground surface temperature and the temperature difference of ground-coupled envelope surfaces. The three processes are computed by computing one-dimensional Equivalent Slab, simplifying outdoor temperature as harmonic and computing one-dimensional Extra Partition Wall. Validation indicates that the heat flux of ground-coupled envelope surface computed by the new approach is very close to that computed by Finite Difference Method (FDM) under unsteady boundary conditions. The computing time is much less than that with FDM. (c) 2007 Elsevier B.V. All rights reserved.