Cliff Elwell, CREDS buildings theme co-investigator and project lead, talks about his work at UCL on energy efficiency in buildings (interview by Mike Fell).
So can you say a bit about what you’ve been working on recently?
For the past few years I’ve been working on a few different topics, but with the common theme of the measurement of physical properties that we can use to characterise the thermal performance of buildings. By “I”, I of course mean my research group, the Physical Characterisation of Buildings group – we have projects on airtightness, ventilation, whole house heat loss, moisture transport through walls and whole house thermal characterisation.
What does that work involve?
We go out and measure a range of useful things, such as the internal temperatures in various rooms of a house, the external temperatures, energy use, solar irradiation, and heat flows. We then use physics and statistical techniques to work out parameters that are of use, such as the ventilation rate, or the heat transfer coefficient. We like to study real buildings and these are usually occupied, although this adds complexity, because it gives greater insight into how buildings really work than test cells or models.
OK, what sort of things have you found?
We’ve found some really interesting things, such as that by using dynamic models, rather than models that average data over a considerable time (say using 5 minute data instead of weekly or monthly data) we can reduce the error in the measurement of the thermal properties of a building element (e.g. a wall) and drastically reduce the time taken to estimate it at the same time (from e.g. 2 weeks to a few days). This makes tests easier to fit in to building and QA schedules, it also allows us to investigate short term things that could affect properties, such as wind.
Another, but quite different, thing that we have looked at is the airtightness test results of dwellings in the UK. New homes are tested to check that they meet certain airtightness levels, to meet energy targets, and these results are stored in a national database. Analysis of this database surprised us initially – we saw huge numbers of houses with airtightness just within the target they were aiming for. The result shows what we know from other aspects of life – how something is measured and incentivised always impacts on what people do to meet their targets. Here we think there is a great deal of last minute leak sealing going on after most of the build is complete, probably when the airtightness testing is underway. That enables the builders to know that they have hit the target. However, these late methods to seal up leaks often don’t endure well, so in the long term this may not be good news. More research is required to unpick this further.
Is that something you are going to be looking at more in your CREDS work, or is that leading you elsewhere?
The CREDS work will build on our research on applying dynamic models to understand thermal properties. We’ve already started extending the work where we looked at the thermal properties of a building element to the properties of a whole house, and through CREDS we’ll take that further. We are moving towards developing a method to produce an Energy Performance Certificate for a building that is based on real in-use data so captures the construction method, build quality and state of conservation.
So how are the Energy Performance Certificates currently produced, and why is that a problem?
There is currently a significant gap between the expected energy use of building, based on the current method for an EPC of a survey followed by running a computer model, and that observed in practice; this is a barrier to the market for more efficient homes. We, and the wider research community, hope to develop a method to estimate the thermal performance of dwellings more reliably, so this might be used to check build quality, prove that retrofitting houses with insulation delivers the expected energy and cost savings and give a mechanism for feedback to improve such building works.
The work is quite challenging because of the number of different factors that come into play, not least the fact that houses are for living in, and that means constantly changing conditions to try and characterise: appliance use, heating schedules, window and door opening etc. In fact, one of the more surprising results of our research in recent years is that the dynamic models we apply seem to be capable of coping with this in many cases. But not all, and not without a lot of expertise to analyse each dwelling separately. We need to get beyond this for methods to be applicable more widely.
So when you aren’t at UCL researching energy performance, what do you like to spend your time doing?
I’ve a 7 year old daughter and 4.5 year old son who keep me pretty busy. We have lots of fun together playing games, craft and chasing about. And I also seem to do a lot of tidying up. If I get any further free time, I like to cycle, but I seem to be getting slower.