Building Performance in Practice

Small House, Heavenly Court

The “Small House” was one of the first projects ever certified by eTool. Key features of the original 3 bedroom design included a lightweight timber frame, small 2 storey footprint and 6 star NatHERS thermal performance rating. John and Betty Saunders moved into the house in December 2012 and we thought it was time to pay them a little visit and see how the house is actually performing. There is a large gap perceived in the building industry between performance ‘As Designed’ and ‘As Built’. eTool has committed to auditing 1% of all projects. Through checking ‘As Built’ drawings, energy bills and site visits we hope to gain a better understanding of where anomalies lie and hopefully help bridge the performance gap.

Initial Reactions

Approaching the dwelling from the main road it clearly stands out amongst its surroundings. The double storey building sits on a plot much smaller than its neighbours whilst still boasting a large garden space. The use of space is far more economical than neighbouring buildings which mostly stretch right to the boundary of their plots – there is even room for a number of fruit trees and native plants which are scattered around the edges of the lawn.

Stepping into the house it is obvious that the design has been carefully thought through. The large north facing windows and open plan layout allow for plenty of natural daylight and the temperature on this fairly chilly July morning is very comfortable. John informs me that they have not required heating all winter – quite impressive for a building that only passes the current regulatory thermal performance requirement of 6 stars. Walking around the house it is barely noticeable that there are almost no windows on the south facing wall as the main rooms are located towards the north. The master bedroom does have a westerly facing window which will let in a lot of heat during summer months, however it also provides good cross-flow ventilation from the afternoon sea breeze and there are plans to install a pergola over the window. The Saunders only require the reverse cycle heat pump during the very warm 32 degree plus days when there is little breeze.


Real data has been obtained in the form of energy bills which combined with an audit of all appliances provides a reasonable picture of what the main uses of electricity are. The last electricity bills show a consumption averaging 5kWh per day which is 75% less than the average of 15 units for 2 person households in the area. Including the generation from PV which is likely to offset most of the daytime consumption, the total electricity consumption is estimated at around 7.3 units which is pretty close to the 8.4 units predicted by eTool LCA during the design stages.


Small House LCD 2


As well as a reduction in the energy required for thermal comfort, the house has lower carbon impacts associated with cooking. This is because an electric cooker and stove was assumed in design stages whereas a gas stove and electric oven have actually been installed. (For further info on gas versus electricity see here). There are also a number of ‘smart’ appliances including:-

Refrigerator – as well as being in a well ventilated space (for more on fridge ventilation see here) the fridge has a “holiday mode” as well as sounding an alarm when the door is left open.

Kettle – the kettle has the ability to set the temperature required. Any tea or coffee aficionados amongst you will know that the perfect coffee temperature is around 90 degrees and the perfect tea around 80. This simple function has relatively large savings due to the fact that it takes much more energy to convert water to steam (at boiling point) than it does to heat water by a single degree.

Drying line – The garage was not in the original design and adds slightly to the embodied energy of the building, however the side doors are located such that a good through draft occurs in the afternoon which is perfect for drying clothes without worrying about the rain.

The increase in the “other” category is largely down to the sophistication of the calculation during the design stage. This appliances template has since been updated a number of times and now splits the other category further into dishwasher, clothes dryers, workshops and all other appliances.

The embodied impacts are close to those predicted during design stages. There is an increase in the fittings category due to embodied impacts of electrical and light fixtures not being accounted for originally.  The increase in internal finish is because the buildings’ ground floor is finished with ceramic tiles which were not assumed in the design stage.

Silver medal house, gold medal people

The building as designed represents a 52% CO2e saving over the benchmark which is an eTool Silver Medal rating.  As built, the building does much better, achieving a 116% saving which is an eTool Gold Medal rating

The performance of the Small House is down to a lot more than good design. The Saunders are clearly conscious of their own environmental footprint and feel strongly that “if today’s humans don’t live more sustainably, then our children and grandchildren will suffer the consequences”. They have a sustainable lifestyle that aligns closely with their sustainable home – their electric scooters, for example, (powered by batteries charged from the PV panels) get them to Bunbury and back on a single charge and they even purchase large bags of fruit on offer and use a de-hydrator so that they keep for longer. According to NatHERS a 6 star house of this size should require around 3000MJ per year to maintain thermal comfort, however John and Betty require only around half of this. Although there are obvious deficiencies in the NatHERS methodology[1] the key difference is down to occupant behaviour more than anything else. The eTool Medal rating has risen to Gold because the occupants are making the best with what they have. Likewise it is all too easy for a house to be designed to Gold standards but fall far below that in practise because the occupants have all the gear and no idea.

This raises a very important point – as engineers we focus primarily on good, low carbon design and innovation, however occupant behaviour has an equally important role to play in reducing our environmental impacts. Persuading people to put an extra jumper on in the winter perhaps presents a far greater challenge than some of the more tangible aspects of delivering a low carbon society.

Read the original case study of Heavenly Court here >>


[1] Namely a lack of monitored data to support the calculations, questionable accounting for thermal mass, lack of air-tightness or thermal bridging calculations, no account of equipment efficiency or fuel source.

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