Overheating / Cooling
The challenges:
There is increasing scientific evidence in New Zealand and internationally that our planet is warming:
“Climate change is already happening in New Zealand and around the world. In future climate change is likely to have an even more significant impact on our environment, our way of life and how we make a living. More hot days – days when the maximum daily temperatures are over 25°C – are projected for most of New Zealand. For example, the Far North District is projected to experience between 22 and 110 more hot days on average by 2090.”
Ministry for the Environment:
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“Wellington is spiralling towards 30-plus degree heatwaves every year, and the most sustainable housing we have risks becoming unliveable in a warmer world… A townhouse owner in Auckland told Radio NZ that her bedroom was an insufferable 30 degrees at nine o’clock at night. On a 24-degree day, (the) temperature inside was between 28 to 32 degrees.”
Kasey McDonnell, 2024:
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This is already contributing to difficult levels of heat in New Zealand homes, with particularly severe overheating problems in apartments and townhouses.
BRANZ research during summer 2023/24 reported that indoor temperatures have risen by between 6% and 10% - or between 1.3°C and 2.1°C – compared with 20 years ago. Further monitoring data found:
Average evening interior temperatures exceeding 24oC in both living rooms and bedrooms
36% of the bedrooms monitored in the study recorded night-time temperatures over 26oC
BRANZ report:
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Overheating causes significant health risks. The World Health Organisation WHO reports that:
“Extended periods of high day and nighttime temperature conditions create cumulative stress on the human body, increasing the risk of illness and death from heat exposure. Heat is an important environmental and occupational health hazard. Heat stress is the leading cause of weather-related deaths and can exacerbate underlying illnesses including cardiovascular disease, diabetes, mental health, asthma, and can increase the risk of accidents and transmission of some infectious diseases. Heatstroke is a medical emergency with a high-case fatality rate. Heat-related mortality for people over 65 years of age increased by approximately 85% between 2000-2004 and 2017-2021.”
WHO 2024:
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Higher temperatures also impact the structure and materials of the building, causing roofs and cladding to warp, blister or crack and, when combined with moisture and humidity, causing mould growth.
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The current Building Code has no provisions for protection against overheating in buildings or high indoor temperatures. There are no requirements setting maximum temperature levels.
The WHO 2018 Housing and Health Guidelines recommends a minimum temperature of 18oC, and recommends strategies to protect populations from excess indoor heat, but does not establish a maximum level.
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What can be done to reduce overheating:
Overheating is a consequence of many factors and there are a range of measures to help prevent it, both in the design of new homes and renovations of existing homes.
Orientation
The design of the home should take into consideration the prevailing weather, winds, and solar exposure on the site, across the whole year with seasonal variabilities.
Size and location of glazing
Large windows on the north and west sides of the home will increase the heat gain in the home in summer without adequate eaves or shading.
Shading
Eaves and roof overhangs on the north and west side of the house to block the sun in summer while letting in winter sun.
Screens, shades, sails, pergolas, louvres and awnings for windows and decks exposed to the sun.
Plant deciduous trees on the north and west to provide shade in summer.
Insulation
Insulation helps keep steady, consistent interior temperatures – preventing overheating in summer, as well as helping keep the home warm in winter.
Insulation in the roof is particularly important to minimise overheating.
Ventilation
Install a heat exchange ventilation system that exchanges the stale, heated air with fresh cool air from outside.
Natural air flow
High level clerestory windows to support “stack ventilation” where the hot air rises and creates a flow through the home, bringing in cooler air through lower level windows.
Windows
High performance double or triple glazed windows, with PVC, timber or thermally broken aluminium frames.
Glazing with low-E glass and other high-tech coatings to protect from the sun and reduce solar heat.
Cooling systems
Heat pumps can be used on the cooling function to reduce indoor temperatures.
Ventilation systems can be connected to deep underground coils of piping that cool the air before circulating it through the home.
“A key principle of a Superhome is modelling and managing hot and cold, the comfort level of the home is the crucial factor. It's easy to heat a home up by whatever means, or to retain the heat – it’s another thing to control it and get rid of the heat when it’s over-hot. So a Superhome is designed around natural shading, how the sun impacts on the rooms, what areas of the home are shaded or likely to overheat – designed ahead of time as part of the Suprhome design.”
Guy Gillespie: Character Builders
“Mould is a really under-rated as an issue in NZ homes. There's BRANZ data, and data from the Asthma Foundation information on moisture in homes, how it causes mould, what it does to our health... It’s something that can be solved.”
Andrew Krupa: Koffman Windows
“Cooling is a new priority whereas, historically the focus has been on warmth. Design need to consider, shading, orientation, pergolas, cost efficient things – ventilation, insulation, rationalise glazing to avoid overheating, heat pumps, AC – a holistic approach needed for cooling.”
Marcus South: Warmth.nz
“Warm Roof system or Warm seal through SealCo works in both ways, it helps reduce your heat loss from inside your building, and helps reduce your heat build up from outside, for example extreme summer time weathers.”
Jeff Fitnss: StealCo
“Cooling the house – during the foundation stage of the excavation, installed some coils underground, about 60 metres, buried about 800 mm underground – during summer, open the manifold into the inlet of the ventilation unit and that pulls the air from outside underground to cool it and come back into the house during summer – it works, a little bit surprised, I was like it might shave off two or three degrees, and yeah the air is cool, we don’t overheat too much upstairs.”
Clayton Mitchell: Statement Homes
