Internal humidity can be controlled by:
• passive ventilation by opening windows for cross ventilation
• removing moisture at source, for example, using an extract fan in the bathroom, using a rangehood in the kitchen, venting a dryer to the outside and using only externally vented gas heaters
• raising indoor temperatures by heating or insulating, since warmer temperatures imply lower relative humidities, see above
• occupants not drying clothes on racks inside.