Two energy-efficient single-family houses (known as ESPI houses) with competitive overallcosts were set up during the study in Finland. The consumption of energy for room heating inESPI houses was reduced to a half at the construction stage, by employing simple solutionswhich can be used by every builder. The level of thermal insulation of the houses wasimproved remarkably. The houses were equipped with a controlled ventilation system and anefficient exhaust air heat recovery unit. One of the houses was oil heated and the other waselectrically heated. In ESPI houses the indoor air quality and thermal climate were good.Against common opinion there was no conflict between low energy consumption and goodindoor air quality. Most important thing was adequate and energy-efficient ventilation. Theconsumption of energy for room heating in ESPI houses was 60 kWh/m2 per year. The totalheating bill of the ESPI houses was only EUR 27-38 per month. Due to the reduced heatingdemand the heating system can be simplified, and as a result the building costs do not increasesignificantly.