Cold houses and impact on health

We spend more than 90% of our time indoors, and for that reason, the indoor environment and its effect on our health are more important than is often assumed.

Many New Zealand homes share a number of features that can lead to health issues. One of the biggest problems is that our houses are simply too cold. This can lead to dampness, mould and respiratory illness. This is a particular issue with older homes, but not all new ones are exempt.

Recent research has shown that insulation and effective heating can make a significant difference.

How does the temperature of our houses compare with world standards?

The World Health Organization recommends a minimum indoor temperature of 18°C, and ideally 21°C if babies or elderly people live in the house.  The average daily indoor temperature in the winter for most New Zealand houses is just 16°C.

How do cold houses affect health?

If house temperatures fall below 16°C, the risk of respiratory illness increases. This is because cold houses are also usually damp, which can lead to respiratory symptoms.

House occupants produce a significant amount of moisture in their day-to-day activities, for example, cooking, showering and drying laundry. Even breathing has an effect – each person produces one litre of moisture per day this way. Moisture condenses on cold surfaces, such as uninsulated walls.

As well as dampness being a health risk in itself, it can lead to mould growth, which may also contribute to respiratory problems. Mould growth is worse when there is also poor ventilation, such as when a house has well-sealed aluminium windows that are kept shut, or doesn’t have an extractor fan in the bathroom.

Living in a cold environment is also physiologically stressful for people who are old, sick or very young.

How might mould contribute to illness?

More than one third of our houses contain mould. Most mould is not harmful for healthy people. However, some species release substances which are potentially toxic and may cause adverse reactions in some people, such as those with pre-existing respiratory conditions. Mould also produces spores when it reproduces. When these are inhaled or come into contact with skin they may cause allergies and skin irritation, and aggravate asthma.

Why are our houses so cold?

Our houses are cold mostly because they are difficult to heat. Two major reasons for this are poor insulation and inefficient heating systems.

Poor insulation is very common in older houses, which generally have no wall, floor or ceiling insulation or double glazing (unless added later). In 1978, the New Zealand Building Code introduced minimum insulation standards, but three quarters of current buildings were built before this, meaning that a large percentage of houses do not have insulation.

Many of our heaters are inefficient, and some produce indoor air pollutants. Unflued gas heaters, for example, produce nitrogen dioxide, carbon dioxide and carbon monoxide.

Our heating practices are also a problem. Most households do not heat the whole house during winter.

Do new houses have temperature problems too?

New houses can also be colder and damper than ideal. Some of the problem areas that remain are:

  • windows without double glazing (from mid-2008, double glazing will be a legal requirement for most new houses)
  • concrete slabs without insulation underneath recessed downlights that allow warm air to be ducted into the ceiling cavity poorly installed insulation (for example, even very small gaps can reduce its efficiency by up to 50%)
  • poor ventilation.

How can houses be made warmer?

Two major ways in which houses can be made warmer are by installing insulation and effective heating.

Insulation can be installed in walls and ceilings and under floors. Double glazing is also a type of insulation. In many older houses, insulation can be easily and relatively cheaply added to the ceiling cavity and under floors. Options for ceiling insulation include fibreglass or wool batts, or loose fill. Under-floor insulation includes perforated foil and polystyrene boards.

It is very important that insulation is installed correctly or it becomes much less efficient. For example, batts must not be compressed or have folds or tucks, and must fit tightly inside framing.

Efficient, less polluting heaters include heat pumps, wood pellet burners and flued gas heaters.

Heat pumps use the same technology as a refrigerator, but in reverse. They are very energy efficient, with some models producing up to 5 kilowatts of heat for every 1 kilowatt of electricity they use.

Wood pellet burners are like traditional fireplaces but burn pellets made from compressed wood shavings and sawdust. They are more controllable than traditional fires (and therefore more energy efficient), and they produce less emissions.

Research: does improving insulation and heating improve health?

Two research projects led by researchers at the University of Otago, Wellington have looked at the effects of improved insulation and better heating on house temperatures and health.

The Housing Insulation and Health Study involved a group of households, each containing at least one family member with a respiratory disease such as asthma, bronchitis or emphysema. Their homes were insulated with ceiling insulation, draught-stopping around windows and doors, and under-floor insulation. The aim was to find out whether this would increase the temperature, lower humidity and energy consumption, and improve the health of the occupants. The results show that insulation made houses drier and warmer, and the health of occupants improved.

The second project, the Housing, Heating and Health Study, involved a group of households with asthmatic children. Houses were insulated and their heating systems were upgraded to a heat pump, wood-pellet burner or flued gas heater. Results so far show that the average daily temperature increased between 1°C and 2°C, but temperatures were still below the WHO recommendations of 18-21°C. Condensation and mould were reduced.


Howden-Chapman, P. et al (in press) “Retrofitting houses with insulation to reduce health inequalities: Aims and methods of a clustered, randomised community-based trial”. Social Science & Medicine.
University of Otago (2007). “Healthy housing study reduces asthma severity”(press release).
Howden-Chapman P, et al (2007). “Warmer houses reduce children’s asthma”. Build August/September 2007 p 40-41.
Howden-Chapman, P. et al (2007).”Effect of insulating existing houses on health inequality: cluster randomised study in the community”. BMJ 27 Feb.

This Science Byte was reviewed by Dr Jeroen Douwes, Centre for Public Health Research, Massey University