Tom Gwilliam explains what Energy Performance Certificates (EPCs) are, how they are created and why they are unlikely to be a useful measure of how energy efficient a building is.
Interview with Tom Gwilliam
Tom is a sustainability consultant at Etude, with a background in structural engineering, energy assessment and whole life carbon analysis. This episode was inspired by a previous episode, Should we celebrate architecture that performs poorly?, which relied heavily on EPC data.
EPCs are a legal requirement when selling or renting a home
Energy Performance Certificates give a rating between A and G, and a number between one and 100, of how energy efficient the home is. This is the main output, but there’s also other information about what you might do with the home and how the home is performing at the moment.
A domestic energy assessor carries out a SAP assessment on new-builds, or an on-construction domestic energy assessor carries out a ‘reduced data’ SAP (RdSAP) assessment on an existing building. The two are different, but both rely on entering information into a computer model, which outputs the EPC.
SAP assessments make assumptions about how a building operates
SAP stands for the Standard Assessment Procedure. It measures heat loss, domestic hot water use, energy associated with pumps and fans, and the energy associated with lighting. It also estimates what people might use for appliances (‘unregulated energy’, because it varies by number of occupants and their behaviour).
The assessment makes various assumptions and boils them down to a single number for total energy use in kilowatt hours. On top of that, it superimposes carbon emissions, primary energy, and other metrics used for building regulations compliance.
EPCs also measure the cost of operating the building
The EPC process takes the total energy figure and puts a cost on it for the resident. But the accuracy of that cost all depends on the accuracy of the assumptions made earlier in the calculation.
SAP is used to drive policy decisions
There's an aspiration for every property in the UK to be at least EPC C by 2035. Part of the problem is that studies show the EPC rating doesn’t correlate well with the energy use of a building. You’d expect that moving from A to G would see increased energy use, but there isn’t much change in general.
Even if all homes are a C by 2035, we’re unlikely to see much change in energy use in buildings. We’ll still have high energy homes and it will be too late in 2035. There’s a real incentivisation issue, and no strong grasp of how to improve buildings through the EPC process.
Five different assessors would probably come up with five different EPCs
Another major problem with EPCs is consistency – there is none. The EPC industry is not well-moderated, and assessors are paid quite poorly. Plus, there’s pressure at government level from house builders not to increase the cost of assessments and EPCs.
The process relies on a person to interpret a physical characteristic of a building. People might measure things differently or inaccurately, or even completely wrong. Assessors also have to make a judgment on how old a building is, and how it’s constructed. They don’t have the time or money to investigate what’s actually in the walls, so a lot of their work is guessing and being as quick as they can.
There's proven evidence of getting an EPC for a new-build house, then sending an RdSAP assessor to do an assessment the next day, and the two producing completely different results.
Example of an Energy Performance Certificate
EPCs aren’t really about your home, even though they look like it
An EPC is presented in a way that makes you think that it's about your home, but it isn’t. If you had identical buildings in Cornwall and Durham, and carried out identical assessments, you’d get the same EPC. But we know the space heating demands for buildings in those two locations will be very different. That fundamentally undermines the EPC process.
SAP doesn’t have the ‘as built’ check that Passivhaus does
It's possible to game the system, particularly with new-builds. SAP assessors don't get paid well and they have to take the word of the developer or people on site. Assessors don't have the required understanding of how a building is going to perform in reality. They don't see all the detail, so they have to simplify things and make assumptions.
When you have that disconnect between what’s happening in reality and what the computer model says, there's a big incentive for developers to be unrealistic with what they're saying. By contrast, two people carry out the Passivhaus process. There’s a designer and a certifier, and a peer review process going on all the time. It makes it a better-quality process.
Simply adding a requirement for the SAP assessor to go to site at the end of construction would make a big difference.
Thermal bridging is a particular issue
Thermal bridging is a big source of heat loss in new-build properties. SAP has categories for thermal bridges, but they don’t cover everything. It’s hard to have a 3D understanding of a building when working off plans, so if you’re not on site and seeing where thermal bridges are occurring then they’re not getting picked up at all. There's no way of quality checking that process because nobody is overseeing the SAP assessor.
There are lots of assumptions about heating system performance
The technology behind boilers and, in particular, heat pumps has lots of nuances. SAP doesn’t require the level of detail that influences whether heat pumps perform really efficiently or very poorly. The model doesn’t ask for a lot of information about the system, so it can only make a simplified judgement about how it will perform.
A way to improve it would be to break the system down into more component parts, understand each part better, and reflect that in the calculation with evidence. If we’re going to shift to low carbon buildings, SAP assessments should reflect a need for more information and understanding to incentivise developers as part of the building regulations compliance process.
Some people advocate for adopting Passivhaus
It’s a proven system. With Passivhaus and PHPP, there’s a clear link between designed on-paper performance and in-use performance. It’s a tick box exercise in some ways: being detailed in how you assess the building, understanding what is built on site, and making sure that's reflected in the calculation.
Doing more testing would reduce cost for the whole industry
We need to be able to survey our building stock at a level that gives us a better understanding of the energy performance of buildings, and not accept RdSAP as a minimum requirement. There is testing available, but it’s costly – and the only way to bring the cost down is to do more of it.
You can’t judge a building’s quality by its EPC
There is no information about indoor air quality, or other sustainability metrics. This is another area where Passivhaus excels. A Passivhaus is a good quality building that is pleasant to be in. We can't tell from an EPC how good a building is going to be, so it’s impossible to compare one A-rated building to another A-rated building.
Studies are highlighting the issues with EPCs
Etude carried out a research project looking at EPCs versus metered energy use. And UCL Energy Institute has just published a paper looking at buildings they’ve been monitoring over a long period of time. The conclusions all come back to the issue of assumptions made in the SAP assessment.
In addition, there are formulas within the SAP calculation that have been ‘locked in’ for years and they don’t change. The development of SAP over the last 20 years has been slow and it doesn't reflect modern buildings. People know there's an issue, but there's no process for SAP to be regularly updated. When changes are proposed they have to be consulted on, so it’s all very slow.
The Future Homes Standard is expected to change things
Because people know there are issues, an overhaul of SAP is expected alongside introducing the Future Homes Standard in 2025. If SAP changes, that obviously changes how EPCs are generated. It’s not clear how the government would reconcile the difference between existing EPCs and the future EPC system.
Home buyers can try to get hold of the SAP assessment behind the EPC…
…but it requires a bit of knowledge in order to sense check it.
- Make sure ventilation systems have been entered correctly.
- If a default value has been used for thermal bridging, you could try and work out if that's a fair assumption for your kind of building. Maybe ask on forums if it is a suitable way of measuring thermal bridging.
- You could dig into the heating system and make sure what has been selected is what is actually in the building. Has the assessor selected all the right drop down boxes? Has a weather compensator been installed, for example.
- Also look at what the assessment assumes for the occupancy of the building, because that drives a lot of energy use. You can see if it's related to how many people are going to live in the building.
If the numbers in the assessment don’t seem right then it's up to you whether you appoint someone else to look at it. PHPP, or an open-source version of SAP, is available so it’s an option to try and do it yourself, if you want to.
You’ll always need a SAP assessment because of building regulations…
…but it’s interesting to do a PHPP calculation as well and compare the two. Etude has worked with local authorities to help them understand how SAP differs from reality.
The local authority can’t mandate PHPP calculations, so Etude helps them to convert the SAP result to a more realistic number that can be measured against a policy target. The SAP inputs are still limited, but Etude can correct for known errors in the assessment.
Getting EPC ratings wrong is a problem for retrofit policy
Something needs to be done with RdSAP to make sure ratings are in line with what is expected for a building. As more homes are retrofitted, assessors will need to understand the age of the building, how it’s constructed, and where retrofit measures have already been put in place, in order to have confidence that improvements being made are actually going to make a material difference.
Find out more
Visit the website of Etude
Check out the UCL Energy Institute's research
Download the Passivhaus Trust's research report on EPCs
What are the UK Passivhaus Awards?
If this award ceremony is not on your radar then you need to invest some time following the competition and delving into the archive. There have been some outstanding self build and retrofit projects in the hall of fame, so we recommend a lazy Sunday morning checking out the fact-files on the Passivhaus Trust website and watching some of the videos.
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