Moisture in the home

If you aren’t already thinking about getting your home warm and healthy for winter, now’s the time to start.

21jun moisture in the home newhero

Every winter we hear and see stories of people living in extremely damp, mouldy conditions.

It’s a symptom of living in older wooden houses in a temperate climate with inadequate heating, ventilation and insulation. However, it isn’t just extreme cases that need to be fixed — most of us would be more comfortable if we reduced the dampness in our homes.

A damp home is an unhealthy home, there’s no argument there. But it isn’t just that — damp air takes more energy to heat than dry air, so it literally pays to remove moisture from your home.

Musty smells, mouldy walls and ceilings, weeping windows and damp clothes in wardrobes are all signs you need to reduce moisture and increase ventilation or heating.

Start by reducing dampness sources. Moisture gets into your home in many ways. One of the biggest sources is moisture evaporating up from the ground through your floors (as much as 40L per day per 100m2). Check for dampness under your house and fix any drainage, guttering, downpipe or plumbing problems — then consider installing a sealed moisture control sheet.

Other daily activities can add moisture to your home: ![Use pot lids when cooking to contain steam and a kitchen rangehood or fan that vents outside.][1]

  • Drying clothes inside can add 5L of moisture per load. Make use of good winter days to dry clothes outside. Using the fastest spin speed on your washing machine minimises the amount of drying needed. If you use a vented clothes dryer, ensure it vents outside.
  • Cooking can add up to 3L a day. Use pot lids when cooking to contain steam and a kitchen rangehood or fan that vents outside.
  • Showers and baths add up to 1.5L per day per person. Use an extractor fan when showering or taking a bath, or at least open a window. You can also fit a dome to your shower to contain moisture.
  • Don’t use unflued gas heaters to heat your home. Not only are they dangerous unless well vented, they add up to 1L of moisture to the air per hour.

Sources of moisture

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

Once the main sources of dampness are removed or reduced, you can think about ventilation. Just living and breathing adds moisture into the home, and we can’t stop doing that. However, we can open windows to let a breeze through and vent moisture-laden air. It seems counter-intuitive to open windows on a cold winter day, but removing moisture will be healthier and make your heating more efficient. Get in the habit of airing your home every day or leaving windows slightly open.

Many of these tips are free or low-cost, but they can be highly effective. Further options start to get pricier.

You could consider a dehumidifier to target parts of the home that don’t get enough airflow and remain damp. However, they aren’t a magic bullet — the best dehumidifiers in our tests remove up to 9L per day at 12°C, but only desiccant models perform when it’s colder. They are effective at drying smaller spaces, and act as a small heater too (they put out about 300 to 400W of heat).


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. However, 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, which can cause inflammation, allergies and infections.

New homes

In new buildings, some moisture is trapped during the construction process. Wet timber may also have been used. The sequencing and timing of the construction process is important to avoid this and be sure to avoid storing construction materials out in the open where possible.

Under the floor

To prevent damp air from building up under the floor:

  • Ensure there is sufficient ventilation. Vents are often obstructed by vegetation and should be cleared.
  • 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.
  • Cover the ground area beneath your home with heavy-grade polythene, taped at the joints and trimmed neatly against the foundation walls.


Let moist air out and dry air in by:

  • Opening your windows – especially in wet areas such as bathrooms and kitchens. Even in winter, it’s worth opening windows to let the house dry out.
  • Closing doors to contain steam/condensation in wet areas.
  • Using extractor fans in bathrooms and kitchens (but never vent them into the roof space 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 end up creating thousands of litres of damp air.
  • Installing vents into windows.
  • Using a forced ventilation system. That said, forced ventilation systems can be expensive to install. 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 it’s 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 or wall insulation.
  • Consider installing double or secondary glazing.
  • Check 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. 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.
  • Well-fitted heavy curtains, drawn on winter nights, will help retain heat gained from the sun during the day.


  • If the indoor temperature is at least 7ºC warmer than the outdoor temperature, it’ll prevent condensation forming on colder surfaces. The World Health Organisation recommends an indoor temperature of at least 16ºC in bedrooms and 18ºC in living areas.
  • Use low levels of heat all the time rather than high levels in short bursts. This reduces condensation.
  • Don't use unflued gas or kerosene heaters inside – they release up to 1L of water per hour.
  • In damp cupboards, consider installing a cupboard heater.

This page was put together with the help of BRANZ.

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