Greengauge Director Toby Cambray explores new in situ building performance measurement tools that could revolutionise the way we quality assure buildings and help eliminate the performance gap.
Many of the people reading this are in my community of building performance nerds. If you’re in this club, you probably believe that the construction industry can and must be made more sustainable and we can, in part, do that by working out what sort of insulation to use, how thick it should be, along with lots of other design parameters that we can control on the drawing board. This is not a new idea, the tools we now know as the Home Energy Model, SAP and DEAP are evolutions of BREDEM, a model created in the 70s which itself is really just a refinement of the heat loss calculations engineers have been doing since heating systems were engineered.
PHPP shares the same fundamentals, but the Passivhaus methodology (as distinct from the calculation tool alone) addresses a significant issue that dogged SAP for years – the performance gap. The Passivhaus certification process ensures that the calculation and design are well-aligned, and that the real building and the design are well-aligned; therefore the calculation and the building are aligned. There has long been talk of Passivhaus as an accepted alternative route to Part L compliance, or even to replace SAP with Passivhaus Certification wholesale. The new kid on the block, HEM, closes some of the building physics gaps between SAP and PHPP, and Part L does now require some as-built evidence, though not to the same degree as Passivhaus.
Passivhaus has arguably been punching above its weight in better performance and smaller performance gaps. On the other hand, it sometimes feels like there’s still a long way to go. But what if we have been barking up the wrong tree for a decade or two?
Imagine you could wave a magic wand and measure the thermal performance of a newly built (or newly retrofitted) house.
As a building physicist, if I could have one number, it would probably be the heat loss coefficient – the sum of all the bits of heat loss we work out in SAP, DEAP, HEM and PHPP. It’s a cool metric: If you multiply this by your inside-outside temperature difference, you can size your heating system, and if you multiply it by some degree-days, you can estimate annual energy demand.
If we could measure it directly, it would unlock a whole new way of doing building energy regulation, basing it upon how well a building is not just designed – but constructed. We could measure the performance gap on every building, and do something about it. In Passivhaus we go to great lengths in the QA process to achieve this, and various studies show this works well – on average, no performance gap.
Friends: this magic wand exists.
For several years, a few technologies have been quietly maturing, which could be described as Co-heating 2.0. The Co-heating test was only ever practical in academic realms – it was expensive, necessary to have an empty building for several weeks, involved quite a lot of kit and manual data processing, and struggled to account for solar gains. The new generation of technologies (confusingly called SMETERs – they’re not directly to do with smart meters) use various machine learning approaches to solve most of these issues, and are gaining momentum. A government validation scheme is being developed – raising interesting questions around how we validate something which essentially can’t be accurately measured by conventional means – and discussions are ongoing about integrating these into EPCs. This would have significant implications not just on Part L but on the various regulations that use the EPC as a proxy for energy, cost and carbon savings. It also raises interesting questions about the purpose of Passivhaus.
This technology won’t replace the Passivhaus methodology, much as the laser measure didn’t take away the vocation of builders.
It’s a tool for measuring a building’s thermal performance, and the Passivhaus community is really good at making buildings with amazing as-built thermal performance.
To coin an analogy, currently, applied building physics works by what mariners would call dead reckoning – projecting where you’re at based on speed and direction. Passivhaus does a good job of this by making more precise estimates of speed and direction, but the industry is about the get GPS, telling us exactly where we got to.
Most of the industry has got away with poor performance for decades because it was easier not to bother, and there was no means of really holding their feet to the fire.
The technology exists to change that, and if its adopted, the job of building physicist suddenly got a lot more important.
This technology is exciting, it has the potential to help do what I got into this for – create buildings that work really well; but it can’t doit on its own. It’s a measurement: a good, 21st century one, but it can’t dimension the insulation any more than a ruler can. The clever clogs with the SMETERS can measure all they like, but someone still has to design and deliver high performance buildings. Under the anorak, (and under the roll-neck, if that’s your thing too) building physics is smoking hot.
Toby Cambray is Director of Moisture at Greengauge.
This article first appeared in Passive House + magazine, Issue 52, 2026
