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Building a low carbon house – more stars or Life Cycle Design?

Ask the average sustainable/eco/green consultant what the principles to achieving a low carbon design are, and more than likely you will get the response:

  • Orientation
  • Glazing
  • Ventilation
  • Insulation
  • Thermal mass
  • Shading
  • And maybe a few more

These solar passive design principles are all very important elements but miss the core of the question – “how do I achieve a low carbon design outcome?” To answer this you have to be able to answer the question “what is a low carbon building?”, and prior to that “what is low carbon”?

The planet can deal with around 1,000kgCO2e per person per year without catastrophic failure. If your house was around 10% of your total carbon footprint (including everything from the organic food you eat to the bike you ride to work) it would need to be around 100kgCO2e/occupant/year to be “sustainable”. Currently the average Australian house’s impact is around 3,900kgCO2e/occupant/year. Although rather depressing, there are a growing number of houses in Australia that come in below 100kgCO2e/occupant/year.

So back to getting a low carbon design outcome – the first place to start is to ask where the carbon impacts are – you can’t improve what you can’t quantify. This graph illustrates the average Australian dwelling’s carbon footprint comprised of several large components such as Thermal Control (air-conditioning), Hot Water, Other Appliances, Embodied Carbon (the carbon that goes into building and maintaining it) and several smaller elements like Lighting and Cooking.

If the “low carbon design” achieves optimum orientation, glazing, ventilation, insulation, thermal mass and shading you could even score the highly sort after “10 Star” performance. “Star rating”, for Australian houses, is a program administered by NatHERS that determines the thermal performance of a dwelling. A “0 Star” rating is synonymous with a glass box in the desert and a “10 Star” rating would suggest no air-conditioning is required to keep it comfortable all year round. By obtaining a “10 Star” rating you would remove all of the “Thermal Control” impacts (about 15% of the “average dwelling”), but still a fair way from realising the “low carbon” design outcome we set out to achieve.

It should be noted that the graph is based around an average Australian climate zone, if you were in tropical Darwin or on the top of the Snowy Mountains the heating and cooling impact would be greater than 15%. Regardless, it’s pretty clear that “more stars” alone isn’t going to do much for your carbon footprint especially considering you have to be at the “6 Star” mark to build anyway. (Note 1) Star Rating v eTool Rating Life Cycle Design Small We could then include a solar hot water system, LED lights, super efficient appliances and finish it off with a lovely big solar PV system to produce more power than we use. Now the design would be ‘net zero’ and at a point where all of our “operational” carbon has been dealt with but there is still that big dark blue section associated with the “embodied carbon”. That element alone is enough to put you out of range for achieving a sustainable carbon footprint. So enter Life Cycle Design…. LCD gives you the ability to understand all of the carbon impacts associated with your design over its entire life span. This starts with the building materials, their transport, construction and assembly, operation, and maintenance through to end of life. LCD Circle Small

Further more LCD looks at the function of the building and aims to optimise the carbon footprint in regards to its occupancy and design life. This sits inline with the initial goal of achieving the 100kgCO2e/occupant/year outcome. Some of the easiest things to dramatically cut your design’s footprint include making it smaller, having more people living in it and ensuring it will last longer. These three points alone will achieve far more to cut your carbon footprint than all of the solar passive design principles put together. If you want to see some more good ideas for getting a lower carbon footprint click here.

LCD allows you to put all of the carbon impacts of design choices in front of you with real numbers, and presents quantifiable ways of making improvements. Rather than targeting small elements without quantifying them, LCD is totally technology agnostic and makes judgement on real performance outcomes only. When you also include Life Cycle Cost in the process, you end up with a pretty powerful design methodology for a cost effective and sustainable outcome.

Next time you’re advised on an element that will improve your design’s carbon footprint make sure to ask “how much?” and ask for an actual number on the kgCO2e/occupant/year. If the advisor can’t answer this then it’s likely they are just repeating someone else’s advice who is just repeating someone else’s advice who has probably never stopped to actually quantify it…. In short, put it to the LCD test and you’ll find out pretty fast whether the advice is sound or silly. While LCD can’t account for everything, such as the social benefits of a comfortable house, you’ll find it nearly impossible to get a low carbon design without it. When you do include LCD from the beginning, getting a good outcome is super easy and super satisfying.
Get in touch with us if you genuinely want to know how your design will perform or want to learn more about LCD…..

 

Notes:

  1. Any new code compliant house has to achieve a minimum of “6star” which is a pretty good level of performance.
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    7 replies
    1. Henrique Machado de Mendonça says:

      Hi Nick,
      The figure is for design life of 54 years which results in 118 t CO2e per dwelling for embodied carbon, including materials, assembly and recurring impacts.

      Reply
    2. Urban Digestor says:

      Thanks for the article, wondering where the 6kg or so CO2e/day of embodied energy for a typical Australian home is coming from? I had thought that typical embodied energy in Australian homes generally 30T to 60T?

      Cheers Nick

      Reply
    3. Urban Digestor says:

      Thanks for the article, wondering where the 6kg or so CO2e/day of embodied energy for a typical Australian home is coming from? I had thought that typical embodied energy in Australian homes generally 30T to 60T?

      Cheers Nick

      Reply
    4. Sid Thoo says:

      Great article Alex! The graph convincingly illustrates how the NatHERS star rating is just one of a number of considerations that need to be accounted for when trying to achieve a genuinely sustainable outcome.

      Reply

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