Energy modelling software can create its share of frustration, none more so than that used for assessing dwellings against Section J of the National Construction Code – the modelling analysis also known as NatHERS.
There are a lot of fundamental issues here - the black box nature of the analysis (leaving energy modellers in the dark about the calculation), the absence of accounting for thermal bridging and air permeability, and the lack of education around the WERS database and more. But perhaps most frustrating of all is the rhetoric that is often purported that the star rating is an assessment of comfort.
An example of this is on the NatHERS website, where it states that “NatHERS helps to make Australian homes more comfortable for their inhabitants.”
In the current version of Green Star, NatHERS is used as a benchmark for proving compliance with the Thermal Comfort credit in the Indoor Environment Quality category (an average star rating of 7 stars is stated to being equivalent to a ‘high degree of thermal comfort.’)
Assessing the likely thermal comfort of a space before the building has been constructed of course has its complexities. In addition to this, we’re all wonderfully unique beings, and trying to quantify the extremely subjective nature of physical reaction to your local environment is incredibly hard due to both psychological and physiological idiosyncrasies. This is outlined by the accepted metric of Percentage of People Dissatisfied (PPD), where 20 per cent of people being dissatisfied represents a high level of thermal comfort!
Something that should be understood though is that glazing in facades can have a massive effect on the level of comfort felt by building occupants. I’m sure we’ve all experienced the impact from sitting near a poorly insulated window in the winter months. This discomfort is due to the radiant heat exchange with the window surfaces.
Additionally, cold surfaces induce drafts and the temperature asymmetry between the room and the window will also affect how a person feels. Although the air temperature in the room may be 20 degrees Celsius, the individual may feel like it’s 10 degrees Celsius. In the summer, clear glazing with no solar control allows a large solar load to penetrate, which can be very uncomfortable if directly falling on occupants.
So how do we alleviate this impact from windows? The use of double glazing, low-e coats and solar control can all be beneficial and the final choice in product will depend on the individual project’s needs, location and orientation.
Referring back to the metric for a ‘high degree of thermal comfort’ - the NatHERS standard - a number of building specifications were developed that represent a 7 star building, accessible from the NatHERS website. So what windows are recommended for a 7 star house? Low-e coats? Double glazing? Thermally broken frames?
The answer is: none of the above.
Shockingly, what represents a 7 star house for all the cities assessed - Perth, Darwin, Brisbane, Adelaide, Cairns, Canberra, Sydney, Melbourne and Hobart - incorporates clear single glazing, with aluminium frames across most of the facade.
It's baffling that the same window specification is used across these vastly different climates, with clear single glazing actively encouraged. This would cause discomfort during both the winter and the summer months, and definitely does not represent a ‘high degree of thermal comfort.’
Not only does single glazing degrade thermal comfort, it can result in condensation formation, potentially leading to mould growth. Couple that with a leaky facade, and you’ve got a recipe for discomfort and negative impacts on human health. Yet we’re still putting single glazing in residential buildings (and even aged care facilities).
Until a minimum standard for window thermal performance is introduced, we need to refrain from referring to NatHERS as a comfort assessment. And ideally stop using single glazing altogether.