Problems with high humidity levels and mould growth in cold attics has been increasing over the last few years. A recent Swedish study showed that as many as up to 60 – 80 % of the single-family houses in Västra Götaland (largely, the Gothenburg region) are showing significant mould growth and thereby risk developing serious moisture problems. The high humidity levels are to a large extent a consequence of the increasing demand on energy efficiency. Houses are frequently retrofitted with additional roof insulation, which leads to a colder attic space and hence a higher humidity. Furthermore, furnace heating is often replaced in favour of heat pumps or district heat. This may alter the air-pressure balance of the house, resulting in an increased thermal pressure on the ceiling with subsequent air-leakage up to the attic. Current theories and models for the growth rate of mould fungi at different climate conditions indicate a potential for significantly reducing the risk for mould problems by lowering the relative humidity by moderate amounts during critical seasons. For cold attics, the humidity reduction necessary is not greater than what should be possible to obtain with an optimised ventilation. There is currently a slight confusion on how to properly design the natural attic ventilation. The optimal air-exchange rate varies with the outdoor climate, and a static ventilation through open eaves and/or gable and ridge vents are not always the best choice. An alternative could be adaptive ventilation. A basic system would comprise mechanical fans and dampers controlled by attic and outdoor climate sensors installed in a sealed attic without vents. The ventilation system runs only when the outdoor air has a potential to dry out the attic. Advanced numerical simulations have shown that the risk for mould growth can be substantially reduced, and even eliminated, if the cold attic is sealed and fitted with adaptive ventilation. Ideally, the construction is well built and air-tight, but leaks can to some extent be compensated with an increased ventilation rate. Field tests and extensive measurements over complete annual cycles confirm the simulations and also show that a sufficient air-tightness can be achieved in practice with fairly small effort. The energy consumption for an adaptive ventilation system is in the order of magnitude of 100 kWh per year for a normal-sized one-family house. Hence, it is a very energy efficient way of moisture control, compared to, e.g., traditional dehumidifiers.