How sustainable is your solar passive house?

By Alex Bruce
Sustainable Energy Engineer
Posted on 17 June 2011
Filed under Assisting & Encouraging, Carbon Reduction

So you've worked hard with solar passive design concepts to achieve an 8 or 9 star rated house and you feel comfortable you won't be needing any air-conditioning. You've got layers of insulation, double glazed windows, they are in the right spots to keep the sun out in summer and let it in during winter, you can make use of the lovely cooling breeze, and it's so air tight you could take it to Mars and be comfortable. You've also dropped a massive polished concrete slab on the ground for thermal mass, keeping things nice and warm in winter. You've then complemented the lovely house with a lovely solar hot water system (perhaps Australian made) and maybe even some solar photovoltaic power panels.

Pretty happy you've ticked the box for reducing carbon emissions, and comfortable in the knowledge that while living in it you wont be responsible for any carbon pollution?

Well, what about all the energy and carbon that went into producing the materials for your house, transporting them to site, assembling them, and then maintaining them over its design life? This is generally referred to as "Embodied Energy" and in most cases is responsible for more carbon than the average house will emit through the use of air conditioners over its entire life.

Building the average 4x2 Australian home and then maintaining it (material repairs and replacement) for 40 years results in about 110 tonne of CO2e being produced (Embodied Energy).

To keep this building comfortable with an air conditioner over that same 40 year period, assuming it has now been built to a “Six Star” rating, will produce about 78 tonnes of CO2e. As you can see, the importance considering “Embodied Energy” in the built form sits alongside that of Solar Passive design and Thermal Performance.

The true carbon footprint of a building is determined through quantifying this "Embodied Energy" adding it to the "Operational Energy" (the air conditioner, hot water system, fridges etc) and dividing it by the design life of the building. This process is often referred to as "Life Cycle Assessment" (LCA) and it allows us to truly identify how "sustainable" a product or process is by quantifying its impacts past, present and future.

LCA is an accounting method that assesses each and every impact associated with all stages of a product or process over its life span. It is not a new approach to determining a product's environmental impacts, but one that has been gaining a lot of momentum recently as people start to ask the tougher questions on the true sustainability of the products they are consuming.

The approach is sometimes referred to as a “Cradle to Cradle” assessment if it accounts for full recycling at the end of the design life of the product, or just “Cradle to Grave” if it takes the product through to disposal only.

As mentioned before, in regards to the built form, LCA requires quantifying the “Embodied Energy”, “Operational Energy” and the “Design Life” or expected lifespan of the building.

Embodied Energy in the built form can be broken down into the following components:

  • Materials – Energy and Carbon used to extract the raw material and process them to a useable building product at the gate of the factory (Cradle to Gate). 
  • Transport – Energy and Carbon used to transport the building material from the factory gate to the building site
  • Assembly – Energy and Carbon used to construct and create the building
  • Recurring – Energy and Carbon used to maintain and replace certain building elements (such as paint) over the entire life span over the building
  • Demolition and Recycling - Energy and Carbon used to demolish and recycle the building and feed these materials back into useable elements

Fortunately for us we have a large data base of materials and their associated "Cradle to Gate" carbon co-efficients given in either kgCO2e/m3 or kgCO2e/kg. To calculate the total "Cradle to Grave" Embodied Energy of your design we need to know the type and volume of materials used, where from and how they were transported from factory to building site, the assembly energy and then how often various components will need to be patched up or replaced over the design life of the building. 

Operational Energy is something that everyone is already pretty familiar with and is dealt with in solar passive design, solar hot water, high-efficiency appliances, and that can be offset using distributed renewable energy (such as a solar PV system).

Design life is ultimately required to amortize the embodied and operational energy over the expected life span of the building. For instance, a house made of recycled cardboard might have a really low initial embodied energy but if it needs to be replaced every two years, then over its life span it is going to look pretty bad.

It is interesting to note that buildings in Australia very rarely get to the end of their design life due to the materials' durability limit. In other words, Australian homes do not fall over but they get knocked over for redevelopment. According to a 2009 study conducted by Forest & Wood Products Australia, 9 out of 10 buildings in Australia will see this fate.

The average Australian house is lucky to make it past its 40th birthday.

Design life is therefore dictated by design quality, correct density (building high density in high density suburbs) and things such as having adjoining walls so a developer will need to purchase multiple dwellings before they can knock down a set of town houses. If we can design buildings that double in life span (only need to get them to age 80) then we can halve the embodied energy impact.

The key is being able to tie all of the components of “Embodied Energy”, “Operational Energy” and “Design Life” together to ensure you are not compromising one aspect while trying to improve another.

If carbon reduction is your aim you need to complement your energy-efficient solar passive design with a LCA or you might be compromising your environmental objectives.

Fortunately, conducting LCA of the built form using suitable software is now pretty easy. Indeed, many designers are already producing designs that not only remove any carbon from the operational energy but also offset emissions associated with the embodied energy. Better yet, designers are finding that these houses are not only cheaper to run but are often also cheaper to build.

When considering both the embodied energy and the operational energy, the built form is responsible for about 35% of Australia's carbon footprint. It follows that intelligent design using LCA philosophy can have a substantial impact on reducing Australia's contribution to climate change.

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