Dampness is a common issue in older New Zealand homes, especially during winter.

Houses with excessive dampness are harder to heat, more unhealthy and can 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.

Principles of moisture removal

  • Eliminate
  • Extract
  • Air Flow
  • Warmth

A client recently requested help to upgrade their subfloor ventilation and get rid of the musty smell and dampness in a recently purchased house.

Signs of dampness


  • Rotting timber
  • Musty smells or mould under the house


  • Musty smells
  • Cold moist air
  • Damp or mouldy clothes or shoes in wardrobes
  • 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.

Where does excess moisture come from?


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

Poor subfloor ventilation restricts airflow underneath a raised house. Airflow is a key principle in drying moisture and preventing excessive dampness. Most homes with concrete ring foundations and suspended timber floors have existing vents but often they are too small or partially blocked.

Country and Coast Construction Wellington - Home dampness
Original vents are often too small or partially blocked


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.

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.

How to combat sources of home 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.
  • 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.
  • 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.
  • 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.

Calculating your subfloor ventilation requirements

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. You can talk to a registered or accredited building surveyor who is experienced moisture measurement techniques.

Upgrading your windows

Single glazed windows are a big factor in excess condensation being produced in your home during the year.

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 or 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

New or recently renovated homes often have 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.

How to tackle sources of home dampness inside

  • 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 during the day 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.

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

Living areas and bedrooms

Avoid unflued gas heaters

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 100mm or more to avoid mould growing behind furniture in winter.
  • Lift 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. 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.

For landlords

The healthy home standards set out requirements to landlords in New Zealand to provide warm and dry rental properties.

From July 1st 2019

  • There must be fixed heating devices, capable of achieving a minimum temperature of at least 18°C in the living room.
  • The minimum level of ceiling and underfloor insulation must either meet the 2008 Building Code, or (for existing ceiling insulation) have a minimum thickness of 120mm.
  • Ventilation must include openable windows in the living room, dining room, kitchen and bedrooms. Also an appropriately sized extractor fan(s) in rooms with a bath or shower or indoor cooktop.

From July 1st 2021

  • Landlords must ensure efficient drainage and guttering, downpipes and drains. If a rental property has an enclosed subfloor, it must have a ground moisture barrier if it’s possible to install one.

Healthy Homes Standards

For more information on energy saving issues https://www.energywise.govt.nz is a valuable resource.

Credit https://www.energywise.govt.nz/at-home/dampness/