This article presents an experimental analysis of cooling buildings using nighttime cold accumulation in a phase change material (PCM), otherwise known as the "free-cooling principle". 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 nighttime and to cool hot air during the daytime in summer. Air temperatures and heat fluxes as a function of time are presented for different air velocities and inlet temperatures. (C) 2008 Elsevier B.V. All rights reserved.