Glenn tells Design Plus Drafting how he did it.
Describe the cottage
It’s a simple rectangle on an East-West axis. with two bedrooms and one bathroom. The exterior is weatherboard with a 2kW solar PV system on the roof.
Bundanoon is 660 metres above sea level so it is a cool climate.
The gardens are designed to handle the extremes of weather and provide shade in summer and produce food. Sustainable design goes well beyond the building envelope.
Why did you decide to go net-zero?
It was an ethical decision. We wanted to reduce our carbon footprint. After that, it became for comfort. Then it was for the economy.
What are the benefits of having a Carbon Net-Zero House?
It’s a very comfortable house. There are stable temperatures. It’s pleasant and very light and bright. There are very small bills. The last two have been negative.
We’re grid-connected, so we use our grid as out battery. Much of the year, we produce much more energy than we use. Over the year, it costs us less than $400 for all of our energy and that includes our barbecue, electric bike, and electric lawnmower.
[An image taken 3 years after the building was finished shows electricity IMPORT 40698 and electricity EXPORT and 62784]
What were the design principles you used?
The Design principles were really simple.
- Get maximum winter sun penetration for warmth.
- Exclude summer sun penetration to keep it cool.
- Insulate and draught seal appropriately, R5 roof, R2 walls, R1 slab edge and double glazed windows with thermally broken frames.
- Incorporate thermal mass (a concrete floor slab was enough).
- Face entrance away from prevailing winds.
Was it expensive?
There’s a misconception that a sustainable house is going to cost $4,000 for every square metre, which is double what the average house would cost.
We built our house for $1,500 per square metre, so it was a very cost-effective build.
Because we were able to divert funds from components that would not have added much to performance, we were able to put in a few high-end finishes like glass splashbacks and stone benchtops.
Loungeroom in Autumn with sun seeping through.
Was it difficult to build?
It’s really simple to build a net-zero house in Sydney. Basically you just have to orient the house properly.
The main factors are how the building faces and where you put windows and awnings and things. This is the biggest thing that makes a house energy efficient.
The site needs to have access to all-day sun. If there’s shading, that makes it more complicated. Ideally, you also need to be somewhere with established blocks on both sides around you so you can tell whether there will be shading later. It helps you be able to position your building so as not to have light affected.
You were an owner-builder. How did you manage the project?
There was an excellent spreadsheet from the Owner Builders course that we used to plan the project and cost the components.
The main tool was in finding the sweet spot, a cost-benefit analysis for each component. It showed whether an increase in price would result in an increase in performance for every element. We used it to see where we could get the most bang for our buck.
We mostly subcontracted the work: the slab, the framework, plumbing, and roofing.
Myself and a carpenter did the rest of the work. I was able to do the electrics.
How did you choose your materials?
In New Zealand they have a system called BRANZ, which is a quality assurance system that looks at the full life cycle of materials.
The same materials are available here in Australia so we used that system to help us choose the optimum of lowest carbon footprint with longest lifespan.
We wanted a durable build that would be readily modifiable in the future. For that reason, we chose standard materials on advice, so that they would be easy to source and replace in the future.
We also used standard 600mm modules to reduce waste in materials.
Did you design and draft the house yourself?
I found an American website that had a plan that I decided to pirate (with permission) and modify. For the council to approve it, the drafting needed to include things like where the washing line would be, and so on, so I took advice from a few people.
Floorplan of net-zero house
We’re all at home now because of Covid. How is life at your place now?
We’re coming into the middle of Autumn and as the days start to cool, we have more sun coming in. That’s the beauty of passive solar design. The colder it gets, the more light comes in.
After we finished the house, we also built a granny flat but with tiny changes to the plan. We wanted less space. The smaller the space, the less energy it uses.
I tested both with a thermal imaging camera and the granny flat works better. There, we used all-metal architraves. I also spaced the weatherboards off the wall so they were not in direct contact with the outside. We also stopped any heat leaking from the slab, which caused an increase in performance.
Some people think it’s really new and radical to live sustainably or have a carbon net-zero house. There’s nothing new about this. It’s simple, liveable design.
Socrates said, “admit the winter sun into buildings.”
A detailed report on the construction can be downloaded at bundanoonnetzerocottage.com