Ventilation is important to remove this excess moisture and pollutants such as carbon dioxide from the air inside a house and replace it with dry, fresh air. Removal of moist air can reduce problems of damp and mould.
Opening windows is an effective form of ventilation if windows are opened often enough, far enough and for long enough - see these ventilation tips from Energywise.
Previous BRANZ research that monitored the airtightness of 60 houses throughout New Zealand showed that householders were providing insufficient ventilation in about one-third of cases, especially in newer, more airtight homes - see the Build articles Changing the air indoors and Ventilation options.
Around half of bathrooms and kitchens in the 2015 House Condition Survey (49%) did not have mechanical extract ventilation venting to the outside. In addition, 49% of households had a clothes dryer, but only 23% were vented to the outside. Clothes dryers can release up to 5 kg of moisture per load.
Heating is also important for managing and controlling moisture generated in the home. Around half of households (48%) did not have heating in the main bathroom. Owner-occupiers were much more likely to have heating in the main bathroom (60% versus 37%).
Combining the data on mechanical extract ventilation and heating in the main bathroom shows that around two-fifths (42%) of main bathrooms in rental houses had neither heating nor mechanical ventilation. This compares to less than one-quarter (24%) of owner-occupied houses having neither.
This suggests a high proportion of householders in rental houses have limited means for controlling moisture levels in the bathroom, leaving it more susceptible to damp and mould.
There was also a difference between owner-occupied and rental houses in the use of whole-house air treatment systems - 21% of owner-occupied houses surveyed had these systems compared to just 2% of rentals.