Intuitively, the problem seems obvious. Boost the minimum insulation requirements for roofs, walls, windows and floors in new homes – as happened when clause H1 of the Building Code was reviewed in 2021 – and occupants will stay cosier in winter but swelter their way through summer.
‘That is a very common misconception,’ says BRANZ Senior Building Physicist Steve McNeil.
‘Insulation works by reducing the transfer of heat, helping to keep the indoor temperature more stable and comfortable. The usual focus is on retaining heat in winter, but in summer, insulation reduces heat gain – particularly from the roof space – and helps to keep the home cooler. The increased insulation requirements under H1 have not, on their own, caused an overheating problem. Design plays a crucial role – particularly when it comes to managing solar gain.’
Flexibility in compliance methods
Furthermore, Steve points out that two of the three available methods for showing compliance with the insulation requirements of H1 allow some flexibility in the quantity and placement of insulation.
‘In the past, the majority of designers have chosen the schedule method because it’s clear cut and easy to follow. But it’s a bit of a blunt instrument and with some roof styles can be challenging to achieve. This is one of the reasons we’ve seen decreased use of the schedule method since the recent H1 revisions,’ he says.
‘The calculation and modelling methods give a bit more freedom, allowing thermal resistance to be shifted around the building. The calculation method allows insulation to be reduced in some places and increased in others as long as the overall thermal resistance doesn’t drop below that of a reference building. The modelling method is more sophisticated and applies climate, occupant loads and other factors to assess energy use against a similar reference building. It has the potential to provide rich information about how a building is likely to perform.
Using these methods, designers and their clients can therefore engage a bit more with the design – thinking about the home’s physical characteristics, the climate and how occupant comfort might be optimised.’
The calculation and modelling methods won’t single-handedly prevent potential overheating problems in new homes, Steve adds. In fact, the calculation method focuses on heat loss, not heat gain. But they do enable designers to think about occupant comfort at the same time as achieving compliance.
Model designs
Longer term, BRANZ sees computer modelling underpinned by accurate and consistent data as a pathway to compliant designs that also ensure good thermal performance throughout the year in all new homes.
Insulation would be one of many considerations factored into such simulations. BRANZ research has shown that, to achieve year-round occupant comfort, homes need to be looked at as a system.
‘Factors such as the orientation of the building site, the number and position of windows, and shading and ventilation are critical to understanding outcomes,’ Steve says.
‘Add in good data on home occupancy and occupant behaviour – which we’re now getting from our HEEP2 project – and simulations become more robust and trustworthy assessments of building performance. We’ve also worked collaboratively with NIWA, MBIE and Kāinga Ora on the climate data used for simulation. It includes future climate scenarios, which will help us create buildings resilient to future conditions.’
Regardless of the compliance route chosen, Steve says that, the earlier in the design process that likely performance is assessed, the more cost-effective design choices can become – reducing the risk that expensive, poor-performing choices will be locked in.
Sector-wide support needed
Moving to modelled designs that comply with the requirements of H1 while placing occupant comfort at their centre will require buy-in, commitment and support from across the sector, Steve says.
‘We recognise that designers and builders would need to be trained and motivated to choose such a compliance pathway. Models will need to be consistent, easily accessible, easy to use and accurate. For example, they’ll need to be supported by accurate data for all climate zones.
‘In addition, overheating is best assessed using hourly computer models. The good thing is, computing power is increasingly cheap, so barriers to hourly simulations are reducing all the time.’
Steve adds that things are evolving similarly internationally, citing the ISO 52016-1:2017 Energy performance of buildings as an example.
‘The committee has gone to significant efforts to create a standard that uses the same inputs for both hourly and monthly methods, overcoming one of the frequent criticisms of hourly models. This gives much greater transparency and the ability for designers to understand daily trends – particularly with overheating.’
More broadly, Steve says support will be needed from the Building Code. While achieving national home energy efficiency aspirations that can consistently be applied, he says the Building Code should support a degree of design flexibility so occupant comfort can be achieved in homes in any part of the country.
There is also the vexed question of how the upfront cost of potential solutions compares with the long-term household energy costs that result. BRANZ provided analysis to support MBIE’s recent H1 consultation, comparing the upfront and ongoing costs of using the calculation and modelling methods for compliance – should the schedule method be discontinued. BRANZ is conducting further analysis to help the sector better understand the short-term and long-term financial implications of designing, building and living in better-performing homes.
Groundwork through BRANZ research
In addition, BRANZ is leading research that will lay the groundwork for a model-led approach to designing for thermal efficiency, indoor air quality and occupant comfort.
One new project aims to develop industry-wide capability in building simulation, particularly energy modelling, to improve knowledge and practices. The research will identify and address skill gaps, standardise practices and provide industry support.
Another project will develop a framework for reducing the impacts of our future climate on building performance. Recognising the high likelihood that climate change will exacerbate existing problems with building performance – including overheating in summer – the research will explore solutions that consider homes as a system. It will consider the cost and carbon impacts of ventilation, airtightness, indoor environmental quality and interstitial moisture together, and examine how effective retrofits might be achieved.
H1 compliance methods
MBIE describes three methods for showing compliance with the H1 Energy efficiency insulation provisions for housing and small buildings.
Schedule method – prescribes tabulated minimum construction R-values for the roof, walls, windows, doors, skylights and floors of a building based on its location in the country. To comply using this method, the minimum R value for each of these elements must be achieved.
Calculation method – based on simple equations and allowing a designer to customise the insulation levels between different building elements to give the same relative heat loss as a building that complies with the schedule method.
Modelling method – uses computer modelling to demonstrate that the proposed building does not require more heating and cooling energy than a reference building that complies with the schedule method. It provides the greatest flexibility to customise insulation levels.