The main aim of the paper is to investigate the possibility of substituting the thick and heavy thermal mass external walls used in high latitude countries by thin and light thermal mass ones whilst maintaining similar comfort levels throughout the year. To make this substitution the use of PCM (phase change materials) panels incorporated into external wall structure are considered. The analysis of wall structures is carried out by modeling the energy balance of a room concentrating on the heat gains and losses through the walls. The resulting dynamic building model has been used in numerical simulations. Simulation studies have been performed for different types of wall structure under changing weather conditions. A wall with external PCM composite panel and its dynamic thermal performance is simulated and results analyzed thermally. The results show that in a high latitude climate external light mass walls should contain insulation in an external layer if it is combined with a PCM panel Such lightweight external walls can substitute standard heavy mass walls comprised of insulation and brick (or concrete or other standard high thermal mass structure). (C) 2013 Elsevier Ltd. All rights reserved.