Dampness promotes mould and dust mites in our homes which can cause respiratory problems. While dehumidifiers and ventilation systems help reduce the symptoms of the problem, they 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, mirrors and furniture.
  • Mould, stains or watermarks - on ceilings or walls.
  • Rotting wood - in the structure of your house.
  • Musty smells or mould under the house.

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

Sources of moisture

The average NZ family produces up to 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

The key strategy for avoiding dampness problems is:

  • Eliminate avoidable moisture sources - like not drying your clothes indoors
  • Extract moisture at the source in the kitchen, bathrooms and laundry
  • ‘Air out’ general moisture from the home regularly - through open doors and windows, or with a ventilation system
  • Keeping the home warm - with insulation and heating improves ventilation effectiveness and reduces the risk of mould growth on cold surfaces

Follow the steps below to tackle dampness issues at source:

Extractor fans

Use extractor fans that vent to the outside in your bathroom, kitchen and laundry. Fans should not vent into your ceiling space. Make sure extractor fans are:

  • Properly sized and located for the type of room - undersized fans or ducting will be noisy but ineffective. Ask your supplier for advice
  • Turned on before having a shower or bath - and shut the bathroom door. Leaving the bathroom window open slightly allows air flow into the bathroom and will improve the extractor fan's effectiveness
  • Left running for a few minutes after a shower or bath - until most of the moisture has cleared, with the bathroom door shut and the bathroom window open
  • Cleaned regularly - to maintain their performance

Bathroom, kitchen and laundry

  • Dry your towel outside - after having a bath or shower whenever possible.
  • Avoid drying your clothes inside. Dry them outside in the sun and wind, or under a covered. verandah, garage or carport. When the weather doesn't allow this, use a clothes dryer that is ducted to outside.
  • Use lids on pots when cooking - to reduce moisture release and to conserve energy.
  • Put a dome on your shower cubicle - to stop steam escaping into your bathroom.

Under floors

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

  • Look for leaks from showers or pipes under the house - and fix any issues.
  • Check that downpipes and gutters are not blocked or leaking - and that downpipes connect to stormwater drains. This is best checked during heavy rain.
  • Look for surface water flowing under your house during heavy rain - reshape the outside levels or install drainage channels or subsoil drains as needed – ask a licensed drainlayer for advice. Check there are vents on all sides of the house in the subfloor walls. Install vents (where they are missing) – ask a qualified builder for help on sizing. Inadequate ventilation is the most common cause of subfloor dampness problems.
  • Uncover any blocked vents - from plants, soil or pest barriers, and clear the subfloor area of any obstructions.
  • Put a vapour barrier (thick polythene sheeting) on the ground under your house - 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.

Damp concrete floors and walls

  • Check for damp patches and white mineral deposits on concrete floors and walls - which indicate that moisture is coming through. Lift flooring to check the concrete floor surface.
  • Waterproofing sealants / moisture barriers can be applied to seal the concrete or masonry - seek advice from a qualified builder about the best product for your situation, and also discuss any improvements to drainage that may be necessary.


  • Look for leaks in wall and roof cladding 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 of your home's materials is often the easiest way of finding hidden leaks - talk to a registered or accredited building surveyor who is experienced in non-invasive moisture measurement techniques.

Registered building surveyors - New Zealand Institute of Building Surveyors website

Accredited building surveyors - Building Officials of New Zealand 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 (with pipes fixed to the walls or portable) 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. To prevent mould growth, relative humidity should ideally be below 60% most of the time. Take readings over a few days or weeks in different rooms of your house.


  • 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:

  • applying methylated spirits or isopropyl alcohol (available from supermarkets and hardware stores), using a brush or spray bottle
  • let set for 30 minutes
  • re-apply methylated spirits or isopropyl alcohol - and let set again for 30 minutes
  • wipe with a clean cloth and methylated spirits or isopropyl alcohol
  • throw away the cloth when finished - to avoid spreading mould spores.

Keep children, allergy sufferers and people with a weakened immune system out of the room during mould treatment. Air the room well until all fumes have evaporated. Keep all chemicals well away from children, and open flames or fire. Do not smoke or use candles whilst handling methylated spirits or isopropyl alcohol. Note that treatment may discolour some materials and as such this may not be appropriate for all surfaces. Refer to manufacturer for further advice.

Vinegar is another alternative. Follow the same procedure above and rinse well with water at the end. However, vinegar does not kill all types of mould and any residue left behind may encourage more mould growth in the future.
Bleach and commercial mould removers can also be effective but release harmful fumes into your home over a long period of time.

Recurring mould

  • If mould keeps coming back after removing it, this is likely because humidity in your home is still too high. Follow the tips on this page to reduce dampness sources, and increase ventilation and heating to dry out your home.
  • Mould on walls or ceilings that reappears within a couple of weeks may be caused by dampness inside the wall or ceiling cavity, with the mould growing from the inside out. Consult a registered or accredited building surveyor for advice.


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.