Rooms left unheated in winter are unlikely to consistently achieve the World Health Organisation's indoor temperature recommendation of 18 degrees, according to a BRANZ report.
The latest Building Research Association of New Zealand (BRANZ) House Condition Survey found almost half of all New Zealand households did not heat bedrooms in winter, including those occupied by children under 18 years old.
This was despite the World Health Organisation (WHO) recommending indoor temperatures of at least 18 degrees in occupied areas of the house or higher for vulnerable groups like young children, elderly or those living with illness.
On top of this the survey found half of New Zealand houses lacked adequate insulation and were under-heated during the colder months.
The condition of owner-occupied houses remained well above those being rented, with rentals having double the rate of poorly maintained features.
University of Otago professor of health Philippa Howden-Chapman said renters had "few rights" when it came to the quality of housing and often lived in homes without inbuilt heating or insulation.
"There are many people who just have to take what they get in rental properties because the rental properties are likely to be older and built in the previous era when building standards were even lower."
She said the latest census showed 5 per cent of people didn't use any heating in their homes.
"We're really concerned that as winter comes on, even though we've had this glorious summer, there will be people still not using heating and that means air gets damp, mould grows and that's when we start to have people getting respiratory diseases."
Younger children and older people were vulnerable groups and most likely to be living in under-heated homes, she said.
"Infants and young children's thermoregulatory systems, the way they keep themselves warm, and their lung capacities aren't nearly as great as those in people who are youths and adults, and older people tend to have a reduced lung capacity."
Improved housing was the best way to keep people out of hospital during winter and the professor said she was delighted to hear Master Plumbers' call to ban unflued heaters last week.
"Our studies have come out and the number of people who now use them have now dropped from 26 per cent of households to 13 per cent. That's very pleasing. We think that they should only be used in an emergency."
The professor said while New Zealand had lower housing standards compared to other parts of the world, the Government now recognised the problem and was working to deal with it.