Passive Design

Designing the building and the spaces within it to benefit from natural light, ventilation and even temperatures.

Controlling Humidity

Humidity affects both thermal comfort and indoor air quality. Although people cannot easily detect the level of humidity:

  • high relative humidity (very damp air) can make people feel chilled in cold weather and hot and sticky in warm weather, and can also damage building materials and cause odours in poorly ventilated spaces
  • low relative humidity (very dry air) can cause dryness and discomfort in the nose and make skin feel dry and itchy.

In addition to its direct effect on people’s comfort, damp air:

  • promotes the growth of fungi (mould) and bacteria, which can cause respiratory problems and/or allergic reactions – moulds such as stachybotrys which can grow on materials containing cellulose (timber, plasterboard (the paper lining) and fibre-cement) can be toxic
  • provides ideal conditions for dust mite populations, which can make asthma worse
  • can cause condensation (liquid water droplets) to form on windows and on walls/ceilings that are colder than the air temperature.

Appropriate humidity range

The amount of moisture in the air is expressed as relative humidity (RH). RH is the ratio of water vapour present in the air compared to the amount of water vapour the air is capable of holding if it is totally saturated (for example, relative humidity of 30% means that the air contains 30% of the moisture it can possibly hold).

As air temperature increases, so does its capacity to hold moisture. So if air temperature rises and its moisture content remains the same, the RH decreases. Designing interiors to be warm in winter generally reduces the RH (but if owners to not maintain the levels of ventilation this reduction will not occur and the RH will remain higher).

In general, most people will be comfortable in a humidity range of 30-80% so long as the air temperature remains in a range of 18ºC and 24ºC. New Zealand has year round high natural relative humidity levels of 70% to 80% in coastal areas, and about 10% lower inland. On clear nights, RH levels reach 90% to 100%.

Condensation

In houses where the relative humidity is high and temperatures are low, condensation occurs on cold surfaces. This effect is apparent on cold, impervious surfaces such as glass. It is not always so noticeable on plasterboard walls, although the result may be mould growth on walls and ceilings. One sign of this is when ‘pattern’ staining can be seen on the walls showing where the hidden framing timbers are behind the wall lining.

Reasons for humidity getting too high inside homes

Sources of moisture that can raise relative humidity in homes include:

  • leaks through the exterior envelope or from plumbing
  • damp ground conditions under suspended timber floors – soil under the floor is continuously emitting water vapour which, unless it is removed by natural ventilation, can penetrate into the interior spaces
  • construction moisture (that is, moisture retained in materials after manufacture or installation such as timber framing, concrete floors and plaster) – as an example, a new 100 mm-thick concrete floor slab contains about 10 litres of water per square metre, and a concrete slab floor in an average house will hold about 700 litres which must dry out, a process that will take at least 4 months even under favourable drying conditions; some drying will occur during construction, particularly after the roof has been installed, but ventilation of spaces is required after the building has been closed in and completed, to remove moisture
  • clothes dryers that are not vented to the outside
  • occupants
  • cooking
  • baths and showers
  • unflued gas heaters and gas stoves and clothes dryers

Controlling humidity

Humidity inside the home can be reduced through:

  • effective passive ventilation (and, if needed, active ventilation) to remove moist air from the home and replace it with drier air
  • effective ventilation of the underfloor (see NZS 3604)
  • insulation, which keeps RH down by maintaining warmer air and surface temperatures
  • providing background heating to habitable spaces
  • eliminating or reducing sources of moisture – for example, by specifying that gas heaters, moisture-producing appliances such as clothes dryers and active extract systems (kitchen and bathroom) are vented outside
  • for existing houses and under E2/AS1 for new building work, where sufficient ventilation cannot be provided, specifying a ground cover such as polyethylene sheet with a vapour flow resistance of no less than 50 MN s/g will restrict the amount of moisture evaporating (subfloor ventilation openings of 700 mm2 per square metre of floor area must still be provided
  • specifying automatic control on mechanical ventilation equipment
  • specifying a concrete slab RH of 75% or less before tiled and vinyl finishes are applied and 70% for timber installed directly onto the concrete. This can be measured on-site with a flooring hygrometer.

For your clients

To help your clients understand moisture and air quality issues, refer them to www.smarterhomes.org.nz/design/moisture and www.smarterhomes.org.nz/design/unhealthy-air.