The growing importance of energy efficiency has us all talking about ‘stars’. However, while we may all be familiar with the term we rarely consider what it is that stars are rating and how they are calculated.
Building energy stars and window energy stars - these are two entirely separate concepts used for different purposes.
A home with 6-star rated windows won’t necessarily have a 6-star rating itself, nor does a 6-star home necessarily have 6-star windows. Having said that, high performance windows are becoming an increasingly important factor in achieving a 6-star energy rating.
As applied to new homes, an ‘energy efficiency star’ refers to the Nationwide House Energy Rating Scheme (NatHERS). A NatHERS star rating is a ranking of a building’s ability to maintain (thermally) comfortable conditions in terms of the annual amount of energy needed for artificial heating and cooling.
The less energy used, the higher the star rating. A building that leaks more heat, in or out, will see its occupants using the air conditioner or heater more frequently (respectively) and therefore using more energy annually.
Worse still is that a poor star rating usually means that more peak cooling power is needed on a hot summer’s afternoon and more peak heating power is needed on a cold winter’s morning.
This is already straining Australia’s energy infrastructure – particularly the electricity grid. Remember, power is the rate at which energy is being consumed at any given moment.
This is limited by the number of power stations and by the capacity of the transmission system to deliver that power to millions of homes simultaneously.
The objective is to build homes that better retain a comfortable living temperature regardless of outside weather conditions and to do so with less annual energy and less power.
Taking all this into account, it would be understandable to conclude that it must be harder to achieve a higher star rating in some places than others.
For example, wouldn’t it be more difficult to reach 6 stars in regions with an extreme climate, such as Darwin or the Snowy Mountains, as opposed to areas with a more moderate temperature, like Coffs Harbour? This would be true if the stars implied the same amount of energy loss in each place, says building physicist and energy expert, Dr Peter Lyons.
“The modelling software used to calculate NatHERS ratings contains weather data for 69 climates (to be increased to 80 later this year), with a different annual energy assigned to each climate.
"A 10-star home requires no energy for heating and only a minimal amount for cooling (more specifically to remove humidity), while, conversely, a zero-star home retains little heat or coolness so the inside temperature basically mirrors outside conditions – little better than a tent.”
While NatHERS provides the framework for building energy star ratings, the actual calculation of ratings is undertaken using Second-Generation NatHERS modelling software such as AccuRate, developed by CSIRO. In addition to AccuRate, rating can also be performed using FirstRate 5 (from Sustainability Victoria) and BERS Pro (from Solar Logic in Queensland), both of which are derivatives of AccuRate.
As an alternative to software-based modelling, the deemed-to-satisfy (DTS) provisions of the Building Code of Australia (BCA) can be used to determine if a building meets a particular star rating. “The DTS provides a faster, prescriptive approach to detailed simulation,” explains Dr Lyons. “By applying specific, minimum specifications and requirements based on orientation, shading, roof, walls, windows and floor, DTS determines if the proposed design passes or fails.”
As the DTS ‘rules’ do not take into account the specific properties of a particular design and climate, using DTS to determine a home’s star rating means could mean missing out on some site specific benefits that may otherwise have provided for an increased star rating.
Experience shows that it can also result in more expensive designs than those derived (and optimised) by simulation on a caseby- case basis.
The windows used in Australian homes has changed very little over the past few decades, largely because they haven’t needed to.
"Until now, increased insulation in the walls and ceilings has in many cases been enough to meet modest 4-star and 5-star energy ratings for most house designs and climates,” says Lachlan Austin from the Sustainable Windows Alliance (SWA) steering committee, a committee which conducted a study into the impact of windows on residential energy efficiency.
“But with the implementation of 6-star ratings, builders and homeowners will have to start addressing the performance of their windows – in most cases ordinary windows will not be adequate for current home designs to meet increased building regulations.”
“The good news is there is no need for consumers to limit the size and scope of glazed areas – we just need to use better windows,” says Lachlan.
This is a view backed up by the SWA’s results, which found that a range of typical home designs around Australia can achieve an increase of up to 2 – 2.5 stars simply by going from ordinary aluminium clear single glazed window to the best-performing windows available for the climate.
”Now, more than ever, windows are critical to good house design and it is important that windows are considered early in the design process in order to achieve the best energy efficiency outcomes.”
Window star ratings
The star rating process for windows is independently operated by the Australian Window Association (AWA), through the Window Energy Rating Scheme (WERS).
Like star ratings for buildings, window star ratings are based on a 10-star scale. However, when it comes to windows there are actually two ratings: one for heating and one for cooling. Both star ratings take three factors into account: thermal transmittance (U-value), solar heat gain coefficient (SHGC) and air infiltration.
These ratings are based on the average performance of the window over a year. The idea is that over a whole year, the impact of a window on the home differs, depending on the time of day, season, heat loss, heat gain, etc. The star ratings then roll all of this together into two numbers that are easy for consumers to understand and which provide a means for comparing products.
On the 10-star scale, 10-stars is the ‘perfect’ window. In winter, the perfect window can be thought to be one that insulates perfectly against heat loss but simultaneously allows 100 percent of radiant solar energy to be transmitted into the home. In summer, the perfect cooling window is one that insulates fully against conducted heat gain into the home and also blocks 100 percent of radiant solar energy. In practical terms, we know the perfect window is very unlikely to ever exist.
However, many manufacturers offer products that achieve more than 5 and 6-stars for heating or cooling, and sometimes both. Unlike building star ratings, window star ratings are independent of climate; they are a ranking of how the product compares against the lowest performing in the market. In hot climates, the cooling star is what counts, and the heating star, while still valid, is not relevant or useful; vice versa for cold climates. In mixed climates, a judgment needs to be made based on the size and orientation of the windows.
The results of the SWA study provide information that can help guide these decisions.