Mechanical supply and exhaust ventilation with heat recovery is usually slightly under-balanced. That means that the supply air flow rate is lower than the exhaust air flow rate. That is made in cold climates to prevent moist air from infiltrating the building construction parts. Usually, the exhaust air is more polluted so the exhaust filter pressure rises quicker than the supply filter pressure. The under-balanced ventilation therefore gives a margin to avoid over-pressure with time. If the ratio between supply and exhaust airflow rate is zero, the ventilation system can be understood as an exhaust only system. The under-balanced supply air flow affects the pressure difference between the inside and the outside of the house. This pressure difference in turn affects the sensitivity for infiltration due to buoyancy and wind. A higher under-pressure inside due to under-balance prevents air that comes in with the wind to pass through the building which means that the unintentional air leakage decreases. On the other hand, if the supply air flow is under-balanced, the available air flow for heat exchange in the heat recovery unit is decreased which means that the recovered heat decreases. This effect is dampened by the fact that the temperature efficiency related to the supply air of the heat recovery unit increases if the supply air flow is lowered. Still, the decreasing amount of recovered heat increases the need for heating the ventilation air. This study analyzed the effect of under-balanced supply air on the energy use. Theories were compared to simulations with actual wind data for Swedish climate. It was found that there is a minimum energy use for an optimal under balance ratio in some cases depending on the air tightness and nominal ventilation air flow rate.