Thursday, 9 March 2017

iLife - an energy performance guarantee for your new home

If you buy an energy efficient new home you expect low energy bills - but in practice this does not always happen. This might be something to do with you setting the thermostat to 25°C and opening the windows - or it might be due to defects in the building fabric and heating services. So if your bills are higher than you were lead to believe, what can you do? Currently not a lot. However, there is hope for the future. At Ecobuild yesterday I heard about iLife - an ambitious scheme to deliver energy performance warranties for new homes and hence drive better quality in design and construction.

The BPE monitoring project found many homes using twice as much energy as expected.
How much of the performance gap is due to build defects? The iLife team have been using data from the Building Performance Evaluation (BPE) to find out [1]. The BPE involved monitoring the first few years of use in a range of new dwellings that were supposed to be exemplars for energy efficiency. Out of 68 domestic dwellings, measured energy use exceeded expectations (based on SAP calculations) in the vast majority and by a considerable margin; often by a factor of two or more. Nineteen of the dwellings were built to Passivhaus standard and there were generally better, although one was particularly bad, using five times the expected energy.

On air tightness Passivhaus homes were much closer to target than other homes
Passivhaus homes should be extremely energy efficient, achieved by high levels of insulation, careful design to avoid thermal bridges even in difficult corners, and very good air tightness. To get Passivhaus certification you have to get your design approved and then show that it was built as designed - with photographs of every stage of construction. The air tightness requirement is very tight and more than half the Passivhaus homes did not quite achieve it. But the margin was small, on average just 0.5 m³/h/m²@50Pa. The other homes had much easier targets for air tightness but still only half achieved them and the average was 1.9 m³/h/m²@50Pa over. It seems the extra work and cost for auditing Passivhaus builds for certification makes a big difference.

Passivhaus homes show that fabric quality has a significant impact on energy use
Also, in the Passivhaus homes there was a fair correlation (0.6) between actual energy use and air tightness which suggests that the Passivhaus design was successful in eliminating many other potential heat losses. It also shows that variation in energy use due to residents behaviour (such as thermostat settings and window opening) did not completely overwhelm the variation due to build quality - fabric matters a great deal. In the other dwellings there was no correlation between air tightness and energy use and there was a wide range of defects: the full BPE report lists numerous examples of poor workmanship or mismatches between the design details and what was actually built. There were cases of missing or poorly fitted insulation, thermal bridges at junctions between walls and roof and around windows, ventilation systems with the wrong sort of ducts and so on. There is a lot that can go wrong.

The BPE cases that were Passivhaus standard show that it is possible to build homes that perform reasonably close to expectations. The fact that the others are much worse shows that there is a great deal of room for improvement.

A warranty for performance would incentivise build quality in all types of homes
Hence the i-Life project. A warranty scheme for energy performance would provide a real incentive for industry professionals to up their game. Insurance premiums would be lower for builders and designers that produce good results. i-Life is a partnership from across the industry: BLP for insurance, builders Wilmott Dixon, and energy modelling, monitoring, and analysis by Virtechs, Encraft and Oxford Brookes University.

The challenge is to allocate blame for poor performance without expensive surveys
The big challenge is to find a way to disentangle the part of the bill due to occupant behaviour from that due to the building performance. (There are also some grey areas where occupants don't do what the designers expected them to, like remembering to change the ventilation system filters regularly. If this is because they were not told it was necessary then this is hardly their fault, though at least it is easily remedied.) Unfortunately most defects are hidden behind floor and wall coverings and difficult to find. It costs hundreds of pounds to have an air tightness smoke test and a thermal imaging survey, and if this was necessary for every claim the cost of insurance would be ruinous.

More detailed energy modelling and smart meter data can help
Fortunately, there are other tools to help. For example it is possible to adjust the SAP model to reflect actual heating regimes, real numbers of occupants and more accurate water use (based on numbers of baths and showers). Also, when smart meter data is available this gives energy use at 30 minute intervals that can be correlated with weather records. Rajat Gupta from Oxford Brookes University reckons a years worth of energy use data would be enough to show if the building was performing to specification in most cases.

Fixing the defects may cost more than paying the extra bills - but with added benefits not least mitigating climate change.
Insurance will inevitably add to the cost of a home and not all buyers will choose to pay for it. iLife expect that social landlords will be their first customers because high energy bills often lead to rent arrears and that is bad for their business. Some private owners may be up for it too but only if the cost is less than the energy savings. At the moment energy is very cheap. For example, poor air tightness might cost you an extra £30 on your bill per year. Is that worth an insurance claim? In fact the cost of fixing it would probably be more than the savings made and it would be cheaper for the insurers to compensate you for your bills than to rectify it. But money isn't the only thing that matters. If you care about your thermal comfort, peace of mind, and climate change, then you will want the problem fixed.

[1] Building Performance Evaluation Programme: Findings from domestic projects (Innovate UK) Jan 2016
[2] Performance Gap from the iLife workshops ( June 2016

1 comment:

  1. Thanks to share great informations about energy performance. Wilco Energy