Helen Brown from Encraft explains the ‘performance gap' – what it is, why it happens and how we can avoid it on our projects.
Interview with Helen Brown
Helen has been working at Encraft for nearly 9 years and is currently head of their building physics department. When she started she was interested in renewable energy and micro generation, but she gradually became drawn to low energy construction in new build housing, with Passivhaus being a particular focus.
Encraft are renowned for their research projects
Previous research projects have included:
Warwick Wind Trials – Funded by Pilkington Energy Efficiency Trust and BRE Trust, this looked at the performance of building-mounted micro wind turbines.
Retrofit for the Future – The programme announced by Technology Strategy Board (now Innovate UK) aimed to achieve deep cuts in carbon emissions by retrofitting existing homes.
OWLs – A project using Beattie Passive TCosy system to refurbish a block of 6 flats.
I-LIFE – A current project funded by Innovate UK, to develop an insurance product that would guarantee a home's energy performance.
A possible future research project, likely to take several years in order to obtain a meaningful and large enough dataset, would be looking at the indoor air quality in homes by considering the effectiveness of different ventilation systems.
‘Performance gap' is the difference in actual energy consumption compared to the designed intent
The range can be from as little as zero gap, which means the house is performing exactly as intended, to as much as 200% where the house is actually consuming double the energy that was expected.
The main performance gap causes:
As identified by Zero Carbon Hub –
- Quality of workmanship on site
- Quality in the design stages
- Lack of skills, knowledge or understanding either in the design stages or during construction
- Using the wrong materials and products, not following the specification or not having a clear enough specification from the outset
Effective monitoring is required to assess how well a building is performing
Most new buildings aren't properly monitored to see how well they're performing, so clients aren't well informed about how the in-use performance relates back to the design stage targets.
If monitoring is to be performed effectively, it requires someone to review the data and feed the results back to the occupants, builders and designers.
In the case of Encraft's insurance product idea which sets out to guarantee a home's energy performance, monitoring would be required in order to verify that the performance targets are being met.
An airtightness test is an easy way to see if a building is performing according to its design intentions
All buildings will have an airtightness target, though not all homes are required to have an airtightness test. In large developments it may be only 10% of them that are tested.
Encraft have found that there is a correlation between airtightness and construction type. In situ concrete is an inherently airtight construction method which seems to consistently meet or exceed its airtightness targets, whereas masonry construction is much more likely to fail the airtightness targets that have been set.
In situ U-value measurements can be compared to performance targets
Testing whether insulation has been well installed can be achieved by measuring the temperature gradient across a wall and calculating its U-value.
Encraft have been looking at this data from the Building Performance Evaluation programme which shows a clear performance gap between the target U-value and the actual measured U-value in the houses that it studied. Their data included Passivhaus and non-Passivhaus developments and found that the performance gap within the Passivhaus subset was much smaller than the performance gap measured across the wider dataset.
‘People factor' affects the correlation between airtightness and total energy consumption
Encraft have found that when they've tried to map airtightness with total energy consumption they've found no correlation, which seems like it should be wrong because in terms of building physics there should be a direct correlation.
The reason why there is no correlation is purely down to the building occupants, which drives the variation from home to home in terms of energy use. Encraft would like to be able to somehow normalise the data to eliminate the affects of occupancy, because the people factor is the most complex and difficult to account for, while having the biggest influence on total energy consumption.
Find out more
Visit the website of Encraft
Follow Helen Brown on Twitter
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