This article presents an experimental and numerical analysis of cooling buildings using night-time cold accumulation in phase change material (PCM), otherwise known as the 'free-cooling principle'. Experimental and numerical studies of the ceiling and floor free-cooling principle, as well as passive cooling, are presented. The free-cooling principle is explained and some of the types of PCMs suitable for summer cooling are listed. An experiment was conducted using paraffin with a melting point of 22 degrees C as the PCM to store cold during the night-time and to cool hot air during the daytime in summer. Air temperatures and heat fluxes as a function of time and dimensionless cold discharging values are presented for different air velocities. Experimental analysis of one week of measuring under real conditions is presented in order to show how cold storage functions. Copyright (C) 2007 John Wiley & Sons, Ltd.