Mould in the house

Mould is a type of fungi which belongs to a group of organisms including mushrooms and yeasts.

To allow mould spores to grow and reproduce, a food source (dust, dirt or organic matter) and moisture is required.

Mould can be black, grey, green or white. Mould often looks like a stain or smudge, and may smell musty.

Where can mould grow

Moulds are present virtually everywhere, indoors and outdoors and can grow in and on materials such as:

  • food
  • furniture
  • fabrics
  • carpets
  • walls
  • paper
  • timber 
  • plumbing. 

Mould can also grow in decaying leaves, stale damp soil and compost.

Controlling mould growth

Parts of the house that are prone to mould growth are those areas with:

  • condensation or high humidity such as kitchens, bathrooms and laundries
  • restricted ventilation - such as in corners and/or cupboards
  • reduced dry heat – such as when the inside temperature is lower than the outside temperature in winter
  • ineffective insulation in walls and ceilings.

When water is heated it changes into vapour. Condensation occurs when vapour cools and changes back into liquid.

When the air is already moist, condensation occurs with the slightest drop in temperature.

Avoid conditions encouraging mould growth by using heat, insulation and ventilation.


Moisture and humidity levels are required to support mould growth.

The cheapest and easiest way of reducing moisture and humidity levels is by ventilating a room by opening a window or door. All areas of the house should be continuously ventilated where possible.

The most effective method of reducing moisture is to use exhaust fans in areas where water vapour is created. There must be enough ventilation for an adequate intake of fresh air to replace the moist air.


Condensation and mould growth can be reduced by providing a continuous low level of dry heat. Continuous, even heating will allow warmth to penetrate the walls and ceilings. On cool days try to keep the inside temperature at least 5°C higher than the outside temperature.


Condensation and mould growth will be minimised if your house has good
insulation. Insulated walls and ceilings stay warmer, reducing condensation, and also keep drying heat in.

Can mould affect our health

The most common types of moulds are not hazardous to humans,
however some moulds which may be found inside your home may cause
health problems. This will depend on:

  • the type of mould
  • exposure time
  • your general health.

Some moulds may:

  • trigger allergic reactions (for instance asthma or hay fever) when spores are inhaled, (often caused through direct handling of mouldy materials) or through accidental ingestion.
  • cause symptoms from coughs, congestion, a runny nose, eye
    irritation, headache to skin irritation.
  • cause more serious health effects such as fevers and breathing
    problems in people who are immuno-compromised or suffer from respiratory diseases.

Ways to reduce mould growth

Keep windows and walls dry inside the home by:

  • ventilating rooms with open windows or doors or by using
    extractor fans
  • wiping away condensation
  • heating rooms with dry heat

Keep the roof, cladding and guttering in good repair.

Ensure that stormwater is discharged to an appropriate outlet.

Family/lounge room

Reduce air moisture from kerosene heaters or unflued gas heaters by:

  • opening curtains and blinds during the day
  • opening windows and doors when possible
  • switching to an electric or flued gas heater.


Reduce moisture/humidity levels by:

  • using an exhaust fan or opening a window when cooking
  • using lids on pots and saucepans
  • checking plumbing for leaks.


  • Open a window or door or use an exhaust fan when having a shower
    or bath to control air moisture
  • clean and dry surfaces that get wet regularly.


Reduce air moisture by:

  • hanging wet clothes outdoors
  • opening a window when using a clothes drier or venting the drier
  • opening a window or door when using hot water.

Cupboards and bedrooms

  • open blinds and curtains to warm rooms with sunlight
  • ensure clothes and shoes are dry before being put away
  • keep cupboards and bedrooms uncluttered and well ventilated.

Storage space

  • dispose of any wet, badly damaged or musty smelling items
  • store dry items in sealed plastic containers
  • maintain good air movement in storage areas.


  • divert water drainage away from house walls by landscaping and paving
  • clean gutters to prevent back flow of stormwater
  • change topsoil in pot plants if mould appears
  • manage composting correctly.

Removing mould

Mould should be removed as soon as it appears. Completely eliminating mould and its causes can take some persistence.

Small areas of mould can be cleaned by using a bleach mixture (1 part bleach to 3 parts water) or a suitable commercial product (follow the manufacturer’s instructions).

Wear rubber gloves, take care not to splash the cleaning solution and make sure the area is well ventilated.

Don’t dry-brush the mouldy area as a brush can flick mould spores into the air which may cause health problems.

If mould returns, there may be an underlying problem. If mould contamination is extensive then a professional cleaner should be consulted.


If a mould-affected room is to be painted:

  • clean the area with a commercially available mould remover and allow to dry thoroughly before repainting
  • use a low sheen, semi-gloss or gloss enamel rather than a flat acrylic paint. Mould resistant paint and additives are available from paint stores.

Further information

For further information on mould contact either: