How to tackle dampness

Damp homes promote mould and dust mites which can cause respiratory problems. While dehumidifiers and ventilation systems help reduce the symptoms of the problem, it’s important to track down the underlying cause of dampness in your home. The problem may be relatively cheap and easy to fix.

Signs of dampness


  • Musty smells
  • Damp or mouldy clothes or shoes in wardrobes
  • Mould forming behind pictures, mirrors and furniture
  • Mould, stains or watermarks on ceilings or walls

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


  • Rotting wood in the structure of your house
  • Musty smells or mould under the house

Where does excess moisture come from?


The average NZ family produces up to 8 litres of moisture in the home each day from activities like cooking and showering. This is normal and can be managed by insulating, heating and ventilating.

Find out if your house is damp


To prevent mould growth, the amount of moisture in your home (relative humidity) should ideally be below 65% most of the time, and rooms should be heated to at least 18 degrees.

To assess the temperature and relative humidity in your house, try using a simple, low-cost hygrometer. Take readings over a few days or weeks in different rooms of your house, especially in winter, to find out where you might need to address dampness issues.

?Learn more about hygrometers


Sources of moisture, such as leaking pipes or damp rising from underneath your house, are often hidden and can go undetected for a long time, damaging to your home.

How to tackle sources of dampness inside

Top tips

  • Eliminate avoidable moisture - dry washing outdoors rather than indoors.
  • Extract moisture by using extraction fans (vented externally) in the kitchen, bathrooms and laundry.
  • Air out the home regularly - open doors and windows to create a cross draft, or use a ventilation system.
  • Keep the home warm - insulation and heating improve ventilation effectiveness and reduce the risk of mould growth on cold surfaces.

Watch: How to fix dampness in your home | 2:55 min


Bathroom, kitchen and laundry

  • Avoid drying washing inside - dry it outside or under a covered verandah, garage or carport. If it’s raining, use a clothes dryer (ducted to the outside).
  • Use lids on pots when cooking to reduce moisture and save energy.
  • Install a shower dome to stop steam escaping into your bathroom.
  • Use rangehoods and extractor fans - read our tips below.

For kitchens, a rangehood above the cooktop captures steam and cooking odours more effectively than an extractor fan installed through an external wall, window or ceiling. However, a well-sized and installed extractor fan will still work well and may be the only option in some situations. Also, ducted extractor fans (with the fan unit located in the roof space) are less noisy than most rangehoods.

For bathrooms, extractor fans can be installed through an external wall, window or ducted through the ceiling. Ducted extractor fans (with the fan unit located in the roof space) are usually less noisy than fans mounted directly on a wall, window or ceiling. Many combined bathroom heat/fan/light units have undersized fans and ducting and are therefore less effective. To control the extractor fan, use a run-on timer switch (which keeps the fan running for a period of time after you leave the room) or a humidity sensor (which automatically turns the fan on whenever steam is detected in the bathroom air).

Ensure your rangehood and extractor fans are:

  • vented to the outside - they should not vent into your ceiling space, and rangehoods should not recirculate air back into the kitchen
  • properly sized and located for the type of room - undersized fans or ducting will be noisy, but ineffective. Position extractor fans close to the moisture source (cooktop, shower or bath). Extractor fans and all ducting should have a minimum size of 150mm diameter (smaller fans are acceptable in toilets). To enable good fan performance, ducting should be as short and straight as possible, without unnecessary bends and any excess length.
  • turned on before having a shower or bath - shut the bathroom door and leave the bathroom window open slightly to improve the fan's effectiveness
  • left running for a few minutes after a shower or bath with the bathroom door shut and window open until most of the moisture has cleared
  • cleaned regularly to maintain their performance - also check any ducting for damage (holes, tears or parts becoming disconnected).

Extractor fan running costs are very low

The electricity used by extractor fans typically cost less than 1 cent per hour. If you have two extractor fans in your home and run each of them for 90 minutes per day, they will add less than $1 to your monthly power bill.

Living areas and bedrooms

Avoid unflued gas heaters

Unflued gas heaters can be portable or fixed to the walls. They release large amounts of moisture and toxic gases into your house, and 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 in bedrooms.


  • Keep away from external walls (especially uninsulated walls) - leave a gap of 10cm or more to avoid mould growing behind furniture in winter.
  • Keep mattresses off cold floors - put them on a bed base to let air circulate underneath.
  • Leave wardrobes slightly open for ventilation.


Dehumidifiers are useful when it’s raining, but are unlikely to stop mould growth unless you tackle the sources of damp, 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 - a warm room makes it easier for a dehumidifier to extract moisture.

How to tackle sources of dampness outside

Under floors

  • Look for leaks from showers or pipes under the house and fix any issues.
  • Check for blocked or leaking downpipes and gutters, and ensure downpipes connect to storm water 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 drain layer for advice.
  • Check there are vents on all sides of the house in the subfloor walls - inadequate ventilation is the most common cause of subfloor dampness. Install vents where they are missing - ask a qualified builder for help on sizing.
  • Uncover vents blocked by plants, soil or pest barriers, and clear the subfloor area of obstructions.
  • Install a ground moisture 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.

?How to install a ground moisture barrier - NZ Standard for installing insulation

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

Floors, walls and roofs

  • Look for leaks in wall and roof cladding and flashings.
  • Check plumbing pipes and services for leaks and 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

Concrete floors and walls

  • Check for damp patches and white mineral deposits which indicate 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.

Construction moisture

If your house is new or recently 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.

Find out if your home is warm, dry and healthy

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