Key design features of high performance houses are investigated for improving energy efficiency in cold climates. Reference dwellings with typical constructions and system designs are compared with high performance houses using the best technology available. The dwellings used for reference are a multi-family apartment building and a single-family detached house, designed according to a mix of Nordic building codes of 2001. The high performance houses designed fulfilled the target requirements of IEA Task 28, Sustainable Solar Housing. Simulations of the buildings are performed using the computer programme DEROB-LTH and results from simulations give the hourly space-heating demand and peak load of the buildings. A comparison of reference houses to high performance solutions shows that the space-heating demand can be reduced by up to 83% for single-family houses and by up to 85% for apartment buildings. The climate data used for all simulations is Stockholm, Sweden. The environmental effects in terms of CO, equivalent emissions and use of non-renewable primary energy are quantified for each building type and construction. The energy saving potential of high performance houses in cold climates is demonstrated. (C) 2006 Elsevier B.V. All rights reserved.