Bragging rights are that guilty pleasure of pointing out that your [insert item here] is better than someone else’s [item] for some reason. When it comes to sustainable homes, the energy star rating is an important design factor that’s often discussed. Producing a NatHERS 10 Star design is the absolute pinnacle and would certainly give a home owner bragging rights.
The Nationwide House Energy Rating Scheme (NatHERS) predicts the amount of heating and cooling energy a home will consume annually to keep the occupants comfortable. All new Australian houses are supposed to achieve a minimum level of 6 Star. However, clever designers can push their homes to 10 Star. That’s impressive because it means a comfortable interior, year round, without the need for any heating and cooling devices.
After our last visit to The Cape, we knew that a 10 Star home existed but knew little about how the design worked. Should we aim for the best possible thermal rating? What are the advantages and disadvantages?
Luckily for us, Beyond Zero Emissions (BZE) recently arranged a presentation by Tim Adams who is an architect experienced in designing sustainable homes.
What does a 10 Star home look like?
Going for a 10 Star home does not limit a designers’ imagination as shown by the entries in the BDAV 10 Star Challenge.
Tim highlighted the variety in the designs submitted to the annual challenge. They cover the spectrum from “wouldn’t look out of place in any suburban street” to “wildly different”. There are many interesting ways to use passive design principles.
One that captured our attention was the Nissan Hut, designed by Tim’s firm F2 Design. Tim pointed out the name was a play on the name of the pre-fab military Nissen Huts used in WWI. In this case, the name was changed to the Nissan Hut. F2 Design wanted to appeal to environmentally conscious families who drove a Nissan LEAF electric vehicle. Mission accomplished Tim! As LEAF owners, we’d be happy to live in this house.
To experience what it’s like to walk into a 10 Star home, please visit the The Cape estate at Cape Paterson. Seeing a design on paper is all very well, but does it feel warm in winter and cool in summer? Go to The Cape to find out. Next time we’re in the area, we’ll visit the display home and check out the passive design features.
Should we aim for 10 Star?
Just because it’s possible to reach a 10 Star design, does this mean every new home should be designed that way?
The energy savings are attractive with a 6 Star home needing 114 MJ/m2 per year to heat and cool while a 10 Star home needs zero. The financial savings can be quite large given the steady increase in gas and electricity costs.
Tim explained that the BDAV Challenge was set up to encourage architects to explore the upper limits of house design. If nobody had a reason to go there, would anyone ever find out what a 10 Star home looked like? Although these homes are unlikely to become mainstream dwellings, the lessons learned in designing them are useful for making improvements to 6 Star designs. Knowing what’s possible makes it easier to improve from the minimum requirement to a 8 or 9 Star design.
So, building a 10 Star home isn’t for everyone.
Based on his experience, Tim believes there is a sweet spot between 8 and 9 Stars. Getting to this rating means the house requires between 25 and 54 MJ/m2 per year which can probably be met with the electricity produced from solar panels.
Our designs were rated at 8.4 Star so we’re in the sweet spot.
What can we learn from the 10 Star challenge?
What tips and tricks could we take from designers who explored the heady heights of 10 Star homes?
Know how you use electricity. Residents can reduce electricity consumption by up to 17% just by monitoring the when and where of power consumption. This information will be readily available from the systems we plan to install with the solar cells and batteries.
Vent warm air in summer. Sloped ceilings and high windows are ideal for venting warm air from the house at night in summer. We’ll explore Tim’s suggestion to use hopper windows as vents because they avoid any warm air being trapped below the ceiling.
Consider using adjustable eaves on the northern walls. Eaves are essential to prevent summer sunlight reaching the thermal mass inside the house. However, using a system that allows adjustment of the width of the eaves means getting more warmth into the house during spring and autumn. Adjustable sails on the eaves could do the trick. Something to think about.
Select windows with high solar heat gain (SHG) – In our Victorian climate, Tim recommends maximising the heat gain through the double glazed windows in order to get the benefit of winter warming. It’s important to allow the energy to pass through the window so it passively heats the thermal mass inside the house (eg the concrete floors in our design). Shading the windows with good eave design minimises the problem of heat coming in from the summer sun.
10 Star is inspirational.
The presentation confirmed we are on the right track with our 8.4 Star designs. We collected some ideas which could be helpful for tweaking the design to achieve a little more.
Speaking of inspirational, BZE will soon release a study they’ve done on how to produce zero carbon cement. That mightn’t sound like a big deal until you discover that making cement contributes between 5 and 10% of the world’s carbon dioxide emissions. It’s up there with road transport which produces 15% of total emissions. It’s good to know change is coming when it comes to using concrete for thermal mass.