Humidity and Condensation in the Home

Humidity and condensation can compromise building occupants' health and comfort, damage interior finishes, and raise heating costs.

On this page:

  • relative humidity (RH)
  • reasons for high humidity
  • controlling humidity
  • maintaining a comfortable humidity range
  • condensation.

Relative humidity (RH)

Air usually contains water vapour, the amount depending primarily on the temperature of the air. Warm air can hold more moisture than cold air, so as the air temperature falls, the maximum amount of water the air can hold also falls.

The ratio of water vapour in the air to the maximum amount of water vapour the air can hold at a particular temperature is expressed as relative humidity (RH). For example, a RH of 30% means that the air contains 30% of the moisture it can possibly hold at that particular temperature.

When air can hold no more moisture at a given temperature (i.e. the RH is 100%), the air is said to be saturated.

As air temperature increases, its capacity to hold moisture also increases, so if air temperature rises and its moisture content remains the same, the RH decreases.

Humidity affects both thermal comfort and indoor air quality. For example:

  • high RH (very moist air) will make people feel chilled in cold weather and hot and sticky in warm weather
  • low RH (very dry air) can cause dryness and discomfort in the nose and make skin feel dry and itchy.

In addition to the direct effect on comfort, damp air:

  • facilitates the growth of fungi (mould) and bacteria that can cause respiratory problems and/or allergic reactions
  • provides the conditions for dust mite populations to grow, which can affect asthma sufferers
  • results in odours in poorly ventilated spaces because of fungal growth
  • will result in condensation forming on windows, walls and ceilings that are colder than the air temperature and potentially damaging building materials.

Reasons for high humidity
Household activities such as cooking, washing and using unflued gas heaters, as well as peoples’ breathing, provide the primary sources of moisture that cause humidity indoors. (A person exhales approximately 200 millilitres of water vapour per hour while awake and approximately 20 millilitres of water vapour per hour during sleep).

Other sources of moisture may also include:

  • water leakage through the building envelope
  • damp ground conditions under suspended timber floors
  • retained construction moisture, i.e. moisture retained in building materials such as timber framing, concrete floors and plaster, after installation
  • plumbing leaks.

Controlling humidity
Internal humidity can be controlled by:

  • passive ventilation by opening windows for cross ventilation
  • removing moisture at source, for example, using an extract fan in the bathroom, using a rangehood in the kitchen, venting a dryer to the outside and using only externally vented gas heaters
  • raising indoor temperatures by heating or insulating, since warmer temperatures imply lower relative humidities, see above

To prevent moisture from the space under a floor getting into the building and increasing the levels of internal moisture:

  • ensure there is good ventilation under suspended timber floors – clear openings of 3500 mm2 per square metre of floor area must be provided
  • cover the ground with a vapour barrier such as polyethylene sheet where there is high ground water content under the building or where sufficient underfloor ventilation cannot be provided. (Even with a vapour barrier, minimum subfloor ventilation openings of 700 mm2 per square metre of floor area must still be provided.)

The most effective passive ventilation to remove internal moisture is simply to open windows. These should preferably be on opposite sides of the building to maintain a good cross air flow.

Vents in window frames allow air movement while maintaining security when the house is closed up. The recommended minimum vent area is 4000 mm2 of air opening per room space for an average size room. This can be achieved by a 600 mm long vent in a window frame.

Maintaining a comfortable humidity range
New Zealand has a year round, outdoor RH of between 70–80% in coastal areas and about 10% lower inland. Indoor relative humidities are generally lower than outdoor relative humidities ranging, in New Zealand dwellings, from 30% to 65% during the day-time in a “dry” house, and 50% to 75% in a “damp” house. Cold bedrooms can have relative humidities of 80% - 90% at night-time. Generally, most people will be comfortable in a humidity range of 30–80% if the air temperature is in a range of 18–24ºC. By insulating to help retain heat in winter and providing adequate ventilation to remove indoor moisture generated by occupants, heating requirements (and costs) may be reduced without compromising occupant comfort.

Condensation

Condensation occurs when warm, moisture-laden air comes into contact with a colder surface such as glass. The air temperature in contact with the colder surface suddenly drops, reducing the amount of moisture it can hold. This results in moisture formation, or condensation, occurring on the cold surface.

Condensation is most obvious on uninsulated, heat conductive surfaces like glass, and is less noticeable on surfaces such as plasterboard. Nevertheless, it does occur on all surfaces that are cold enough and becomes apparent by mould growth on walls and ceilings. It can also be seen where ‘pattern’ staining on walls identifies the location of timber framing behind the wall lining.

Condensation causes damage to interior paintwork, the inside surface of wall linings, floor coverings, curtains, and furnishings. It results in increased heating costs (as additional energy is required to convert condensation back into vapour which is taken up by the air as the temperature rises), and presents a health hazard.

Condensation can be controlled in two ways: first, by reducing humidity (by reducing sources of humidity and through effective ventilation as explained above) so that air is less likely to become saturated; second, by reducing the likelihood of warm air coming into contact with cold surfaces. This can be achieved through insulation. Insulated glazing units significantly reduce window condensation.