As warm temperatures continue, BRANZ’s landmark study into home energy use and indoor conditions indicates that indoor summer temperatures have risen by 6-10% over the past 20 years, and people are feeling the heat.

This rise in summer indoor temperatures occurs against a backdrop of outdoor temperatures increasing by 4-8% over the same period, according to NIWA data.

In-home monitoring data collected from 151 homes during summer 2023/4 suggests average temperatures have increased 6-10% or 1.3-2.1°C, depending on room and time of day, compared with 20 years ago when the original HEEP study was done. BRANZ’s new data shows that average evening temperatures in both living rooms and bedrooms exceeded 24°C.

More than one-third (36%) of the 310 bedrooms monitored were classed as ‘overheating’, based on an industry method for assessing overheating risk in homes*.

Approximately 70% of 425 households surveyed as part of the same study reported their home felt warmer than they would like at least some of the time in summer (48% ‘sometimes’ and 22% ‘always or often’). This means more people felt warmer than they would like in summer than colder than they would like in winter (34% said ‘sometimes’ and 14% said ‘always or often’).

BRANZ Senior Research Scientist Vicki White cautions on the issue of overheating: “Overheating homes are uncomfortable, can exacerbate health issues and impact sleep, and the issue is becoming more prevalent with climate change”.

BRANZ advocates that to achieve year-round comfort and energy efficiency, including in warmer weather, we need to consider home design holistically.

Factors such as the orientation of the building site, the number and position of windows, shading, insulation, heating and cooling systems, and ventilation work together to determine the level of year-round comfort.

Considering the entire house in the design phase is a fundamental key principle of BRANZ’s response to MBIE’s H1 Energy Efficiency consultation.

BRANZ General Manager Research Dr Chris Litten explains: “BRANZ supports MBIE’s proposal to move the industry towards using more sophisticated computer modelling to demonstrate energy efficient design as this will allow more flexibility and help tackle the issue of overheating”.

BRANZ acknowledges the positive intent of the current review but believes a more strategic, planned review of several other related aspects of the Building Code, such as interior environment, internal moisture and ventilation, would make the biggest difference to year-round indoor comfort.

“We need to think holistically about home design to improve comfort and more generally to achieve affordable, healthy and sustainable homes,” says Dr Litten.

BRANZ continues to develop new evidence-based research and practical solutions to help improve the quality of New Zealand homes.

For instance, alongside research and industry collaborators, BRANZ is exploring how a construction technique, known in the UK and parts of Europe as Warm Roofs, can reduce the large temperature changes characteristic of conventional roof designs.

BRANZ’s early tests have shown that Warm Roofs can help to maintain stable roof and living space temperatures, reducing the need for additional heating and cooling. This could dramatically reduce the risk of moisture problems in the roof structure and improve the durability of the roof.

Indicative data shows a 70% efficiency of heat recovery ventilation systems in warm roof construction, compared with 40% efficiency in typical cold roof designs. Further research findings will be released later this year.

*CIBSE 1b: over 26°C for more than 1% of night-time hours (22:00 – 07:00) in a year.

Notes:

More about HEEP2: branz.co.nz/heep2
BRANZ’s submission to MBIE’s consultation on house and building insulation requirements: branz.co.nz/sector-leadership/submission-h1-requirements-in-housing-and-other-buildings
More about Warm Roofs and other initiatives underway, see BRANZ’s latest video on improving quality in New Zealand buildings.

About the HEEP2 study 

The Household Energy End-use Project 2 (HEEP2) is a comprehensive national study of energy use and conditions in New Zealand homes designed to replicate the earlier HEEP1 study undertaken in 1999–2005. These are initial findings from HEEP2, which involves a national sample of over 750 households with data collected in various ways, including self-completion and on-site surveys, in-home monitoring and accessing metered energy data from retailers. Households are monitored for at least 12 months and data collection is due to complete in April 2025.

BRANZ partnered with Stats NZ Tatauranga Aotearoa to recruit households via its Household Economic Survey. The survey data presented in this second HEEP2 report is derived from the in-home interview and building and appliance survey completed for 425 households throughout the country. The detailed temperature analysis is based on a subset of 151 of these households monitored over summer 2023/24. As this represents less than half the HEEP2 monitored sample, these results are preliminary only.

This research is funded by the Building Research Levy, with co-funding from the Ministry of Business, Innovation and Employment Hīkina Whakatutuki. New Zealand Green Building Council provided additional funding for a related Canterbury-based high-performance and code-minimum study, which will be reported on next year.

Further analysis of HEEP2 data, to be released later this year, will explore why some of the participating households experienced higher temperatures than others and add to our understanding around the risk factors for overheating.

About the Building Research Association of New Zealand (BRANZ):

BRANZ is a trusted, independent expert in building construction. We provide practical research, testing, quality assurance and expertise to support better buildings.

Our aspiration: Affordable, resilient, sustainable and quality buildings for Aotearoa New Zealand. Find out more: branz.co.nz