About 30% of homes are damp, and many houses have mould. Dehumidifiers and ventilation systems help fix the symptoms of the problem, but don’t always tackle the cause itself. In many cases, dampness is relatively cheap and easy to fix.

Symptoms of dampness

  • Musty smells in rooms that are closed for any period of time.
  • Damp or mouldy clothes or shoes in wardrobes.
  • Mould forming behind paintings and mirrors.
  • Mould, stains or watermarks on ceilings or walls.
  • Rotting wood in the structure of your house.
  • Damp or mould under the house.

Condensation on windows - especially in bedrooms - isn't necessarily a sign of dampness if it only happens occasionally during winter.

Sources of moisture

The average NZ family produces around 8 litres of moisture in the home each day from activities like cooking, showers and breathing. This is normal and can be managed with the right balance of insulation, heating and ventilation.

Dampness can also be caused by moisture getting into your house from outside, underneath or through leaky plumbing. These sources of moisture are often hidden and can go undetected for a long time, causing damage to your home.


Video - How to fix dampness in your home

Tackling the source

Unless you've looked for the source of the dampness, trying to solve it with dehumidifiers and ventilation systems may just mask the problem rather than fixing it. If you’re having issues with excessive moisture or dampness and poor indoor air quality, follow the steps below.

Bathroom, kitchen and laundry

Make sure your extractor fans are vented to outside your house.

  • Put a lid on your shower cubicle - to stop steam escaping into your bathroom.
  • Avoid drying your clothes inside. It’s better to dry them outside in the sun and wind, or when the weather doesn't allow this, use an externally vented clothes dryer.
  • Use lids on pots when cooking to reduce moisture release and to conserve energy.

Under floors

Check under your house - if you can get under your floor, check for signs of dampness like mould. Take some dirt and rub it firmly on your hand - if it stains like mud there is too much moisture present.

  • Check for water getting under the floor from drainage, guttering, downpipes or plumbing problems.
  • Install vents (if there aren't any).
  • Uncover any vents blocked by plants, soil or pest barriers and clear the subfloor area of any obstructions.
  • Put a vapour barrier (thick polythene sheeting) down on the ground. This keeps the moisture in the ground and stops the air under the floor from getting damp.
  •  New Zealand Standard NZS4246 has detailed instructions on how to do this

 New Zealand Standard for installing insulation

If you’re not sure about any of these actions, talk to a qualified builder.


  • Look for leaks in wall and roof claddings and flashings.
  • Check plumbing pipes and services for leaks, and for moisture getting into walls or floors near showers and baths. Leaks like these are often hidden and can go unnoticed.
  • Measuring the moisture content in your home is often the easiest way of finding hidden leaks - talk to a qualified building surveyor who is experienced in non-invasive moisture measurement techniques.

Find a building surveyor near you - New Zealand Institute of Building Surveyors website

Construction moisture

If your house has been recently built or renovated, there’s probably excess moisture in some of the construction materials that needs to dry out. This may take a few months, but you can speed up the process with heating and ventilation.

Unflued gas heaters

Avoid using unflued gas heaters as they release large amounts of moisture and toxic combustion gases into your house. They can also be a fire hazard. If you’re using a gas heater or LPG portable heater without a vent or flue, always keep at least one window open to allow fresh air to enter the room. Never use unflued gas heaters in bedrooms.

Humidity gauge

Use a humidity gauge (called a hygrometer) to keep track of humidity in your house. 40% to 60% relative humidity is ideal - make sure your house doesn’t exceed 70%.


Keep furniture away from external walls, especially if they are uninsulated. Leave a gap of 10 cm or more behind large objects like furniture to avoid mould growing behind them in winter. Keep mattresses off cold floors - put them on a bed base to let air circulate underneath.

Leave wardrobes slightly open for ventilation.

Mould removal

Look for mould regularly in hidden areas like wardrobes, under carpets, behind curtains and furniture. If you find any mould remove it by:

  • spraying it with white vinegar
  • letting the vinegar set
  • wiping with a clean cloth, soap and water
  • throwing away the cloth afterwards.

Learn more about mould in homes and how to remove it on the Housing New Zealand website.

Controlling mould in your house - HNZC website


Dehumidifiers are useful during periods of damp weather, but are unlikely to stop mould growth unless you also address the points above, and heat and ventilate your home.

At low indoor temperatures, desiccant dehumidifiers tend to be more effective than refrigerant compressor dehumidifiers. Although they use electricity, it’s eventually released as heat and helps warm up the room.

Whatever type of dehumidifier you use, run it together with a heater. Having a warm room makes it easier for a dehumidifier to extract moisture.