Describes experiments carried out in 4 low energy electrically heated houses incorporating extra thermal insulation and heat recovery mechanical ventilation systems. Airtightness was made an objective so that the ventilation system would provide as much of the required fresh air as possible. Instrumentation was installed during construction to monitor the performance of the houses, with the cooperation of eventual purchasers. The houses were leak tested and sealed where necessary. Ventilation rate calibrations were carried out to provide data for estimating the ventilation rate when the houses were occupied. The mechanical ventilation systems were found to provide good internal conditions without opening windows, and freedom from condensation. Total ventilation heat loss was less than one quarter of the total heat loss from the house.