Many Kiwi bedrooms are too cold in winter, despite homes being warmer than they were 20 years ago, a new report says.
The Building Research Association of New Zealand analysed indoor temperature data from more than 250 NZ households and found bedrooms often fall below recommended healthy temperatures during winter.
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Vicki White, Senior Research Scientist at BRANZ and lead researcher on the report, comments:
“This is the most comprehensive picture we’ve had of how New Zealand homes actually perform in winter in more than 20 years, and it allows us to compare like-for-like with the original Household Energy End-use Project (HEEP) study for the first time.
“Overall, the story is one of progress. Across the more than 260 households we studied, average indoor winter temperatures have increased by around 2.6°C to 3.5°C since the early 2000s, and living rooms are now typically reaching 20.4°C in the evening – comfortably above the World Health Organization’s recommended minimum of 18°C.
“That reflects improvements in insulation, heating and building performance, as well as shifts in how we use and heat our homes. But that progress isn’t consistent, and that’s where the concern lies.
“What this study makes very clear is that while our living areas are generally warmer, bedrooms remain a weak point. On average, bedroom temperatures drop to around 16.5°C overnight and fall further by morning. At around 6am – often the coldest point of the day – one in four bedrooms is below 14°C.
“That matters, because 18°C is the recommended healthy minimum indoor temperature for a healthy adult. Some people, like young children, the elderly or those with an illness will need it warmer than that. This study shows that many New Zealanders are regularly exposed to conditions below that threshold, especially while they sleep.
“Another area of concern highlighted by the data is that in the warmer months we are seeing homes reach temperatures that could be uncomfortable for occupants, including in bedrooms during sleeping hours. This issue of summer overheating has emerged as a growing concern over the twenty years since the first HEEP study.
“This dataset confirms that we’ve made meaningful gains as a country, but it also highlights that too many Kiwis are still living, and particularly sleeping, in homes that are not consistently at a healthy and comfortable temperature.”
Conflict of interest statement: Vicki White is lead researcher on this report.
Helen Viggers, Senior Research Fellow in He Kāinga Oranga The Housing / Health Research Programme in the Department of Public Health, University of Otago, comments:
“It’s exciting to see the first of the final results from HEEP2 being released. This study explores how middle New Zealand is doing – almost all of the homes studied were owner occupied. We know that rental housing tends to be in poorer condition and colder than owner-occupied, and this study doesn’t contain enough renters to show the conditions in the most problematic New Zealand housing.
“The authors suggest there may have been an approximate 3°C (varying by time of day) improvement in the winter warmth of owner-occupied New Zealand homes since the initial HEEP study in the early 2000s. This is an impressive improvement. However, the average changes masked a wide variation in the conditions experienced in the individual homes – the maximum temperature in the coldest homes seemed colder than the minimum temperature in the warmest.
“Despite the 3°C improvement and the relatively affluent households sampled, over 40% of the overnight winter bedroom temperatures were below 16°C. The World Health Organisation recommends 18°C as a “safe and well- balanced” temperature so there is still a pressing need to make sure that all households are living in dwellings that they are able to affordably heat (or cool) to ensure healthy temperatures.”
“The building code corresponds to the poorest quality housing it is legal to build; and the sub-study of houses in Christchurch shows the value of improvements to the code. Winter night-time bedroom temperatures were warmer in buildings built above-code than those built just-to-code; and the buildings built to current code performed better than those built to previous versions. The above-code dwellings also appeared better at protecting the residents from the extremes of summer heat than dwellings built to prior codes.
“If we as a country continue to improve housing quality then one day every New Zealander may be able to be warm in winter.”
Conflict of interest statement: “I have received funding from BRANZ, and interact with several of the study authors in professional groups.”
Nevil Pierse, Research Professor Co-director of He Kainga Oranga / Housing and Health Research Programme, Department of Public Health University of Otago, comments:
“The new BRANZ report is a high-quality, nationally representative study—and its findings are stark. Three quarters of New Zealand households report condensation in winter, one third report damp, and almost half report mould in their homes.
“At the same time, many homes remain too cold for healthy living. Bedrooms are almost entirely fall below the WHO recommended minimum of 20°C, with large proportions of time spent under 16°C—conditions linked to increased respiratory and cardiovascular risk.
“This has major public health implications. Cold, damp, mouldy housing is a key driver of New Zealand’s high burden of respiratory disease and contributes significantly to persistent health inequities, particularly affecting renters and low-income households, who are more likely to live in poor quality housing.
Improving housing and energy efficiency has consistently proven to be cost beneficial from the health effects.
“This is not just a current housing problem, current standards ensure it is a future one too. Even homes built to minimum Building Code standards can fail to maintain healthy indoor temperatures. However, the report shows homes built to higher performance standards can consistently achieve safe, stable conditions.
“Current building standards are still too low. New Zealand standards remain 40 years behind international standards for energy efficiency and healthy housing. We also rapidly need to raise the standards for rental housing.”
Conflict of interest statement: “No conflicts of interest.”
Dr Nigel Isaacs, Reader in Building Science, School of Architecture, Victoria University of Wellington, and leader of the previous report in 2010, comments:
“Everyone lives somewhere, yet we know surprisingly little about the conditions inside our houses. Cold and damp NZ houses did not just happen. They come from design, construction, local climate, changes in technology and of course how people live. To understand, we need data from real houses.
“The first Household Energy End-use Project (HEEP), finished in 2010, provided some critical insights. But since then changes, including technology, have had a dramatic impact. Some examples:
- high efficiency LED lamps have replaced inefficient incandescent, halogen and fluorescent lights; They save money but reduce the winter heating benefit (yes, that was a real issue identified in the original HEEP);
- increased levels of thermal insulation. This is not only due to the NZ Building Code for new houses, but also as more existing houses have extra insulation added (retrofitted);
- heat pumps and the drive to reduce open fire air pollution have revolutionised space heating; and
- energy for cars now comes from the petrol station and the electric wall plug.
“This report explores summer and winter temperatures (the ‘cold’). A later report will deal with humidity (the ‘damp’).
“HEEP2 found a range of indoor temperatures. It found not all homes are healthy and comfortable. A positive finding is mostly higher wintertime living room temperatures. This is possibly due to improved space heating (heat pumps) and thermal insulation. However, overnight bedroom temperatures are still too often below healthy levels.
“Not so positive is the increasing summer overheating. This occurs both during the day and night. It is probably due to aspects of design and increased glazing, made worse by poor understanding of whole building performance. This is highlighted by the excellent monitored performance of the well-designed, high-performance Christchurch houses.
“This report, and its supporting data, provides many opportunities to better understand how we live in our homes. HEEP2, thanks to BRANZ, gives the temperature performance of NZ houses a voice. Will this form the basis for policy and action?”
Conflict of interest statement: “I led the original HEEP research and have provided comments on the development and reporting of HEEP2.”
Professor Robyn Phipps, Dean, Faculty of Architecture and Design Innovation, Victoria University of Wellington comments:
“Both HEEP studies are critical research that will inform construction codes, energy usage and public health. HEEP2 showed that improved insulation levels and heating systems have reduced very cold and unhealthy low temperatures, but many homesare not consistently in a healthy temperature range.
“Homes with a household income above $200,000 were warmer than lower-income households, with households with incomes below $200,000 reporting being colder than they would like and having mould. Energy hardship impacts homes, even with high household incomes. Most bedrooms were below healthy temperatures, even those occupied by children. The condition of the house’s envelope had a significant impact on indoor temperatures, and homes with draughts were the coldest, as heat escapes.
“Bedrooms were uncomfortably warm in some homes that were built to the building code. Opening windows was the main method used to reduce heat, but adding external shade and correct window design would be more effective. While some might jump to the conclusion that adding insulation is the cause of higher temperatures, this is a simplistic and incorrect observation.
“A pertinent result is that homes that were built to an “above the building code” standard were consistently warmer in winter and cooler in summer. These homes were consistently within a healthy and comfortable range and buffered the outdoor temperatures. A feature of the “above code homes” is that they are designed and built as a whole of home system with insulation, window sizing quality and placement, exterior shading and heating designed for all year-round comfort.
“The Building Code is the lowest legally acceptable solution. HEEP2 shows that some improvements have been made to the comfort of New Zealand homes, and that greater improvements would be achieved with further improvements to the code.
Conflict of interest statement: “I am a past Director of NZ Green Building Council, which advocates for and certifies above-code homes. This was a voluntary role, and I received no financial benefit from this role.”
Dr Mikael Boulic, Senior Lecturer, School of Built Environment, Massey University, comments:
“This report confirmed what we expected, which is good. Temperatures in NZ houses have increased in the last 15 years (based on results from HEEP1).
“The living room is the preferred location for heating use. Bedrooms are still cold, which is a problem mainly for young people, elderly people, and people with respiratory issues.
“Vulnerable populations (low income) still have issues warming up their indoor environment.
“The Warm Up New Zealand program is still needed because of fuel poverty experienced by low income populations. The last program was Warmer Kiwi Homes, which provided clean heating devices in living areas for eligible households (that do not already have suitable heating), and the installation of retrofitted insulation for houses without (or with insufficient) insulation.
“New houses are generally warmer than old draughty ones, showing that improvement in material (insulation, double glazing, heating performance) was positive. We need to keep increasing the energy efficiency in NZ houses.
“A new issue is “overheating”. Lack of shading (design with limited eaves or overhang) and a lot of glazing facing North turn housing into a “hot box”. Fuel poverty in summer (affecting cooling) will probably come in NZ.”
Conflict of interest statement: “I have no conflict of interest. I participated in the MOTU-led evaluation of the Warmer Kiwi Homes program (as an advisor).“
Priscila Besen, Senior Lecturer in Sustainable Architecture, Auckland University of Technology, comments:
“The BRANZ Household Energy End-use Project (HEEP2) provides comprehensive national evidence on how homes are performing, drawing on detailed monitoring from over 250 dwellings. It is encouraging that winter indoor temperatures have improved – now around 2.6–3.5°C warmer than two decades ago – with evening living room temperatures typically exceeding 18°C.
“However, there are still significant challenges in achieving thermal comfort. Almost half of households (48%) report their home is colder than they would like in winter, 21% experience conditions cold enough to shiver, and 48% report mould. Thermal comfort is not just about air temperature but also humidity, ventilation, surface temperatures and other environmental and personal factors.
“Bedrooms remain a critical concern. They are consistently colder than living areas, with substantial time spent below 16°C and even 14°C overnight.
“This reflects both building performance and heating practices: while living rooms are usually heated, 41% of households never heat bedrooms, and 31% report never heating bedrooms used by children under 5 – an especially concerning finding for their health.
“The report also highlights inequities. Higher-income households tend to achieve warmer conditions and heat bedrooms more frequently, pointing to affordability and energy hardship issues. The monitored sample is weighted towards owner-occupiers, so questions remain about the performance of rental housing.
“At the same time, the focus on “warm and dry” homes in recent years has meant overheating has been relatively overlooked in Aotearoa NZ. HEEP2 shows this is now an emerging issue: indoor temperatures in summer frequently exceed comfort thresholds and remain elevated into the evening and overnight, including in bedrooms. Addressing comfort must therefore be year-round, not winter-only.
“Beyond the building-related factors explored in the report, urban form, densification and loss of green space may also be contributing to rising indoor temperatures.
“Encouragingly, the report shows we already know how to deliver better outcomes. The sample of high-performance homes maintained stable, healthy temperatures across seasons. Beyond just insulation, other factors such as solar orientation, airtightness, external shading and ventilation play a critical role in creating healthy homes.
“Certifications such as Passive House, EnerPHit and the latest version of Homestar can support better design and retrofit pathways, and these approaches can help ensure that all rooms are comfortable and healthy, beyond the current practice of only heating selected rooms.”