Home maintenance combating dampness herol

Combating dampness

A damp home is hard to heat and can make you and your children sick. Damp homes are associated with increased numbers of doctor’s visits for respiratory problems such as asthma.

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Dampness encourages mould and mildew, which can also harm your health. Damp homes also deteriorate more quickly and they’re uncomfortable to live in.

There are 3 ways to combat persistent damp in your home: insulate, ventilate and heat.

If you are certain that the dampness is not caused by leaks brought about by weathertightness problems (roof, claddings, around windows, doors or deck areas) then follow the measures described below.

How damp are our homes?

A 2005 BRANZ survey of the condition of New Zealand houses found:

  • Many are damp and poorly ventilated.
  • More than 20% had at least one dehumidifier, and without a dehumidifier, it was estimated that at least 30 percent of homes would be damp.
  • Most bathrooms relied only on windows for ventilation.
  • Only half of kitchens vented moist air to the outside.
  • 40% of timber-framed houses had poor or seriously inadequate under-the-floor ventilation.

For more information on this survey see the BRANZ survey.

How do I know if my home is damp?

Signs of high levels of internal moisture are:

  • Mildew
  • Condensation on hard surfaces
  • Water stains.
  • Swelling and rotting wood or wood-based materials.
  • Rotting carpet.
  • Bubbling wall linings and vinyl floor coverings.
  • Musty smells.

A damp home may have only some of these signs or many.


Mould is known to cause inflammation, allergies and infections.

It is a relatively straightforward process to remove mould from hard non-porous surfaces such as glass or ceramic tile. Removing mould from porous substances such as wallboard, wood and carpets is more difficult and more hazardous. This is because spores can be released when disturbing rotten material and these can cause inflammation, allergies and infections.

For advice about dealing with mould, see the Ministry of Business, Innovation and Employment’s Building Performance website.

New Homes

In new buildings, some moisture is trapped during the construction process. Wet timber may also have been used. This will dry out eventually over the first year of the building’s life, as long as the house is properly heated, ventilated and insulated.

Under the floor

To prevent damp air from building up under the floor:

  • Make sure there is sufficient ventilation. See "Bulletin 457" Ventilation of Enclosed Subfloor Spaces on www.branz.co.nz for advice on your type of floor and ways to improve ventilation under the floor. Note: you will need to pay for this information.
  • Make sure water isn’t draining from paths or gardens under the house. You may need to create channels or underground drains to divert surface run-off.

To prevent damp air from being drawn into your home through the floor:

  • Cover the ground area beneath your home with heavy-grade polythene, taped at the joins and trimmed neatly against the foundation walls.
  • Look at insulating beneath the floor. You will need reasonable access under the floor and, in windy sites, the insulation will need to be protected by a separate layer of lining material, for example plywood or fibre cement.

WARNING: If you have foil underfloor insulation be very careful. Get a competent person to test that it’s not electrically live. People have died when they touched live underfloor foil.


Showers, cooking, especially on the stovetop, flueless gas heaters, and indoor drying of clothes all create large amounts of water vapour leading to condensation.

The best remedies are to reduce the amount of moisture or extract moisture as close to its source as possible. Some good ways to reduce condensation include:

  • Not drying clothes indoors.
  • Putting lids on pans when cooking.
  • Using extractor fans when cooking or showering.
  • Keeping your showers short.
  • Cover fish tanks.
  • Choosing household plants that don’t require a lot of water – for example, yuccas, succulents and dracaenas.

Where can moisture come from?

Activity Litres
Cooking 3.0/day
Clothes washing 0.5/day
Showers and baths 1.5/day (per person)
Dishes 1.0/day
Clothes drying (unvented) 5.0/load
Gas heater (unflued) Up to 1.0/hr
Breathing, Active 0.2/hr per person
Breathing, Asleep 0.02/hr per person
Perspiration 0.03/hr per person
Pot plants As much as you give them


Let moist air out and dry air in by:

  • Opening your windows – especially in wet areas such as bathrooms and kitchens. Even in winter on dry days it’s worth opening windows to let the house dry out. You will have to heat the drier outdoor air but it will be easier to heat than the moist indoor air.
  • Closing doors to contain steam/condensation in wet areas.
  • Using extractor fans in bathrooms and kitchens (but never vent them into the roofspace above the ceiling or under the floor. Always vent them outside).
  • Venting clothes dryers to the outside. A typical load contains 5 litres of water. This is released as water vapour, which can create thousands of litres of damp air.
  • Installing vents into aluminium windows.
  • Using a dehumidifier or forced ventilation system – however, forced ventilation systems can be expensive to install and dehumidifiers can be costly to run. Try to address the cause of the moisture build-up instead.


  • Block off draughts. Use draught seal tape around windows and doors. Block off unused chimneys but ensure that it is not an airtight seal as air needs to circulate into the chimney to allow drying if rain gets in.
  • Insulate your ceiling. This helps keep your home warm and reduces condensation; 42% of heat loss is through the ceiling/roof. Older homes often don’t have any ceiling insulation.
  • Consider installing double or secondary glazing.
  • Check that existing insulation is in good condition. As a guide, insulation should fill up to the top of the roof/ceiling joists.
  • Insulate beneath the floor. The floor accounts for 10% of heat loss.
  • Well-fitted heavy curtains, drawn on winter nights, will help retain heat gained from the sun during the day.


  • Aim to keep the indoor temperature at least 7ºC warmer than the outdoor temperature to prevent condensation forming on colder surfaces. The World Health Organisation recommends an indoor temperature of at least 16 ºC.
  • Use low levels of heat all the time rather than high levels in short bursts. This reduces condensation.
  • Avoid flueless gas or kerosene heaters – they release up to one litre of water per hour.
  • In damp cupboards, consider installing a cupboard heater, especially if it has an external wall.

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