COP v3.0:internal-moisture; moisture-sources

10.8 Moisture Sources 

Everyday household activities, heating, indoor plants, pets and construction activities all contribute to indoor moisture.


10.8.1 Occupant Behaviour 

The occupants of the building create a significant amount of water vapour. Therefore, the air inside in a building typically has a higher moisture content than the external atmosphere.

10.8.1B Approximate Amount of Water Vapour from Occupant Behaviour

Occupant behaviourEstimated Amount of Water Released (per 2.5 inhabitants)
Cooking (unventilated)3.0 L / day.
Baths / Showers1.5 L/day. 
Clothes Washing0.5 L/ day.
Clothes drying (unvented)5.0 L/ load.
Dishwashing1.0 L / day.
Portable gas heaterup to 1.6 litres per 1kg of gas burned.
Breathing  (average)3 litres per day.
Breathing asleep (per hour) (average)50 ml.
Perspiration 0.5 litre per day.
Pot PlantsThe same amount as the input


Bathing and showering, cooking, heating, indoor laundries, and unvented clothes drying are the most obvious sources of water; respiration, perspiration, indoor plants, and pets also produce moisture.

Areas for moisture-generating activities should be well ventilated and the entire building should be mechanically ventilated to outside the structure. Proposed changes to NZBC G4/AS1 will require venting to the outside of appliances such as showers, baths, and cooktops.


10.8.2 Heating 

Some other sources of moisture are best avoided altogether, particularly unvented gas heating and kerosene heaters. Burning 1 kg of gas can release 1.6 litres of moisture into the atmosphere.

10.8.3 Mechanical Venting 

Supply-driven and exhaust-driven mechanical ventilation systems can pressurise or depressurise internal atmospheres in different areas of the building. Supply-driven systems can be problematic as increased internal pressures can drive moist air into the attic through openings in the ceiling.

Exhaust-driven systems can de-pressurise internal areas and increase the intake of moist external air, e.g. ground moisture via a vented cavity. Unbalanced mechanical ventilation can also encourage moisture migration by creating negative pressures in the ceiling cavity. These systems need to be well designed and maintained to avoid the risk of affecting internal moisture. A balanced mechanical ventilation system, where both intake and exhaust are connected to the outside, is the preferred system and will be most effective when the thermal envelope of the building is airtight.


10.8.4 Construction Moisture 

During construction, timber can become wet and take some time to dry out. Some activities, such as plastering and painting, also release water vapour.

Concrete floors are particularly prolific sources of moisture. During curing, a 100 mm thick concrete slab releases approximately 10 litres of water vapour per square metre of surface area. The curing period depends on various factors, but as a rule of thumb, a concrete floor cures at a depth-rate of 25 mm per month. Therefore, a concrete slab can affect internal moisture levels for a considerable period.

All new buildings, particularly those with concrete floors, must be kept well ventilated  (at much higher levels than required during normal use) until moisture levels of construction materials have stabilised.

10.8.4A Mould Damage

This building suffered mould damage to underlay and roof truss even before occupants moved in.


10.8.5 Ground Moisture 

Ground moisture can infiltrate living spaces by way of the floor or directly to the ceiling space by way of vented cavities. The cavity should be constructed to prevent the migration of water vapour into the ceiling space. Wet subfloors can be isolated by laying polythene tightly over the surface and taping all joints.  More information can be obtained from the Good Repair Guide: Damp Subfloors (BRANZ).

Concrete floors must be installed over a damp-proof membrane (DPM) to ensure that moisture from within the ground does not penetrate the slab. This membrane can be formed by a polyethene sheet that is taped at the laps and laid over compacted hard fill topped by a sand blinding layer. The DPM must be installed under the full extent of the slab, under any internal or perimeter foundations, extend up to the external edge of the floor slab, and lapped and sealed under the wall damp-proofing system.