Sponsored by the Ministry of Trade and Industry, a consulting firm performed an investigation of the possible energy savings in industrial halls outside working hours. The ventilation should be closed during shut-down periods, and air change through natural draught cut down to a minimum. The ventilation by leakage of an industrial building may be estimated at 0.1-0.5 air changes per hour, therefore the tightness of the constructions is imperative. The decrease of the temperature in industrial halls during non-working hours is imperative. It was found that at a shipyard in Rauma, about 50% savings in the heating costs were achieved when the night temperature was decreased, the building tightened and the ventilation completely closed at night.