Controlling moisture and damp

Last updated: 9 August 2023

controlling moisture

Ideas for reducing moisture in your home.

Dealing with moisture

Reducing moisture levels in your home involves:

  • venting moisture at the source to outside
  • eliminating or reducing sources of moisture
  • ensuring that the home is well ventilated, so that moisture is removed.

If you eliminate moisture sources, heat to healthy temperatures (18-22oC)  and improve ventilation, particularly in wet areas, you should not need a dehumidifier. If you do use a dehumidifier, ensure the water collected is emptied frequently and close your windows and doors when running it.

Building and maintaining a weathertight home

If you are having a new home or addition designed, your designer will need to be a licensed building practitioner (unless you have an owner builder exemption issued for the work by your Building Consent Authority) and the design will need to comply with the New Zealand Building Code. In choosing your design you will need to weigh up the appearance and complexity of the design against the difficulty of building and maintaining your home.

Choose the right people for your type of building work

Get information about choosing the right people, including when to use a licensed building practitioner (LBP).

Get the right people for your type of building work

If you are looking at purchasing an existing home, it pays to have a pre-purchase inspection carried out. Houses built between the mid-1990s to 2004 may be associated with the leaky building problem. For these houses a report from a specialist weathertight building assessor is recommended.

Locate an assessor near you using the NZ Institute of Building Surveyors website.

Weathertight Services has a guide to identifying leaky homes, and how to address weathertightness issues.

Maintenance is critical to keeping your home dry. Check your roof, guttering, windows and flashings, balconies and decks for leaks, and get any leaks repaired quickly.

Ventilate and heat your home properly to avoid damage from too much internal moisture.

Keeping moisture out

A surprising amount of moisture – up to 40L per day – often gets in from under the floor, even if the soil under the house appears dry. You can prevent this by:

  • covering the ground under the house with a moisture or vapour barrier. This is a sheet of thick polythene, laid out on the ground reaching several centimetres up the walls and taped at all joints and piles
  • using a damp proof membrane and polythene under a new concrete slab foundation
  • ventilating enclosed sub-floor spaces
  • checking for, and repairing, any plumbing leaks under the house
  • dealing with surface runoff or underground water – drain it away from the house
  • careful placement of garden irrigation systems.

Also, it's not a good idea to use air from a roof space for ventilation. Research has found that, in winter, this air is likely to be higher in moisture than the air in your heated home – so you may be bringing more moisture into the house.

Additionally, the air in your roof space is likely to be polluted with dust, mould and other contaminants. Its quality is dependent on the efficiency of the filter and how clean it is kept. The supply air for home ventilation systems should be sourced from the outside, not from the roof space.

Reduce excess moisture in your home  on the Gen Less website has more information.

Avoid unflued gas

Don't install or use gas heaters unless they're vented outside. Unflued gas heaters release a litre of moisture inside your home for every hour that you use them. Not only that, they also emit other pollutants that can be unsafe or even deadly for you and your family.

If you have to use an unflued gas heater, make sure there is enough ventilation when it's being used. These appliances need a lot of ventilation to make sure the appliances burn cleanly and dilute and remove toxic gases from combustion such as carbon monoxide. You will need to keep a window open during use - even if it's cold outside. Do not use them in bedrooms, small spaces, or areas without good ventilation. As with other heaters, furniture and other flammable items must be kept at least a metre away.

To reduce the risks posed by portable LPG cabinet heaters, they should be checked and serviced each year before winter by a qualified service agent. For more information, see WorkSafe's Energy Safety website, pages on Cabinet heater safety and Gas heating on the WorkSafe website have more information.

LPG cabinet heaters are also very expensive to run – about twice as expensive as electrical plug-in heaters for the same amount of heat produced.

Heating your home has more information.

Extract moist air

The most significant sources of moisture in your home are the kitchen, bathroom and laundries with dryers without vents to the outside. These rooms should have range hoods or extractor fans to remove moisture. The moist air needs to be vented to the outside (not into the roof space), to avoid moisture accumulating and damaging your insulation and internal roof structures.

Clothes dryers also produce a lot of moisture. Vent your dryer outside, too, or use a condensing or heat pump dryer.


Effective ventilation will help to remove moist air from your home and bring in cleaner, drier air that's healthier to breathe.

Ventilation has more detail.

Insulation and insulated glazing

Insulation and insulated glazing such as double glazing with insulating frames help keep the warmth inside your home. It also keeps surfaces warmer so moisture is less likely to form on them when it gets cold outside. A well ventilated, well-insulated room is much less likely to have problems with condensation and mould on walls and windows.

There are legal minimum requirements for insulation in new homes and renovations - it's worth exceeding these requirements to get a drier, more comfortable home.

A warm home is usually a drier home so using passive heating or an effective heating system to keep your home warm will also help to reduce moisture.

Insulating your home has more information.

Five simple things to do to keep your home dry

You can take simple and inexpensive steps to reduce the moisture level in your home:

  1. Don't dry your clothes inside, otherwise the moisture will evaporate inside your home. Use an outdoor clothesline, ideally out of the rain. If you need to dry clothes during wet weather, use a clother dryer (that is vented to the outside, or use a condenser/heat pump type clothes dryer).
  2. Cover pots when they're boiling on the stove – and use your range hood extractor fan.
  3. Ventilate regularly – for example, by opening windows several times each day for 15 minutes.
  4. Fix leaks in the roof, and around windows and doors, so the weather stays outside the house.
  5. Avoid over-watering indoor plants.

This information is published by the Ministry of Business, Innovation and Employment’s Chief Executive. It is a general guide only and, if used, does not relieve any person of the obligation to consider any matter to which the information relates according to the circumstances of the particular case. Expert advice may be required in specific circumstances. Where this information relates to assisting people: