Postoccupancy Performance Of Passivhaus Homes in the UK
An intrinsic part of the UK government’s net zero carbon strategy is reducing energy demand from buildings. While improvements in building codes are designed to achieve this on paper, there needs to be confidence that these improvements translate to actual energy savings in use. Unfortunately, it is becoming increasingly clear that there is a difference between the predicted energy demand of a building and the measured energy demand once the building is occupied—the so-called energy performance gap.
Any building constructed to meet a prescribed energy standard may exhibit a performance gap, described as the difference between the predicted thermal and energy performance derived from computer simulations and the actual measured building fabric and energy use once the building is occupied. Some variations in measured energy performance appear naturally, due to differences in household sizes, occupation patterns, and chosen internal comfort temperatures. It is therefore unsurprising for some buildings to use more energy than predicted, and others less. However, emerging research suggests that many buildings use much more energy than predicted—in some extreme cases, 250% more.
This energy performance gap is a concern both for the construction industry and for consumers. If homes consistently use more energy for space heating than predicted, this gap affects carbon emissions reporting at a governmental level, contributes to climate change, and may place more people in fuel poverty. Therefore, it is vital that homes built to a certain standard will meet that standard, both for improving energy efficiency in our homes and for managing carbon emissions reductions nationally.
Many reasons have been identified for this performance gap. These reasons include the quality of the design and building modeling, the quality of the construction and commissioning, occupancy patterns, user behavior, and robustness of postoccupancy testing, among others. One of the challenges in teasing out the contributions of various factors is the lack of postconstruction monitoring, especially at scale. We at Greenbox Associates undertook research to fill in the understanding of certified Passivhaus buildings in the UK by examining their actual performance relative to the modeled estimates.
There are several in-depth case studies of UK-certified Passivhaus homes that typically focus on individual sites and provide a forensic analysis of the performance of the building fabric. The results of this postconstruction building testing show small variations in heat transfer coefficients, in situ U‑values, and air permeability, but in general the measured results were close or very close to design predictions. However, as the sample sizes are small, it is hard to draw general conclusions about the delivery of the Passivhaus standard in the UK.
Hence, the aim of our research was to compare predicted and observed space heating energy consumption at scale, from certified Passivhaus dwellings in the UK. There are around 1,300 certified Passivhaus units in the UK. We gathered heating and temperature data from 97 certified dwellings through a combination of (1) monitoring programs by consultants, (2) publicly available Innovate UK data from the Building Performance Evaluation program, and (3) self-reported data from homeowners. The main requirement for inclusion in the study was the availability of at least one year’s heating data as well as indoor temperatures. Predicted space heating demand was obtained from the Passivhaus certificate for each dwelling. The data were spread across multiple sites and collected by different actors, and did not follow the same measurement procedure. Hence, special methods were created to account for varying data quality, but in such a way as to avoid biasing results in favour of the standard. Table 1 gives a summary of the data collected.
The results reveal that UK homes built to the Passivhaus standard do not show the same space heating performance gap seen in other research into non-Passivhaus homes (see Figure 1). Average measured space heating demand is 10.8 kilowatt-hours per square meter per year (kWh/m2a), which is about 1 kWh/m2a below the mean predicted space heating (11.7 kWh/m2a). This difference is not statistically significant. Just over half of the dwellings (52 out of 97 homes) used less energy for space heating demand than was predicted on the Passivhaus certificate.
Occupant behavior is a known contributor to the performance gap; however, our results show that this effect can be limited though Passivhaus design. Ten homes had no space heating demand at all, while 83 of the 97 homes (86%) used less than 15 kWh/m2a. Only five homes (5%) used more than 30 kWh/m2a, which is still below the predictedperformance of a new-build UK home—and in reality, these homes will probably use a lot more energy. These results demonstrate that Passivhaus homes are being consistently delivered in the UK, not just on individual projects, but also from large sites, with a mixture of tenures.
The UK government is currently consulting on the Future Homes standard, which not only sets out new building requirements, but also is designed to address the performance gap. However, without major changes to current building practices, the performance gap may not be eliminated and may even increase—and poor performance will be locked in for a long time to come. Therefore, it is imperative that homes built to today’s standards meet design expectations, through a program of monitoring and testing, to ensure that any future improvement in regulation translates into a similar improvement in actual building performance.
Our results provide clear evidence that compliance with the Passivhaus standard delivers low-energy homes that are affordable to heat, with, on average, no statistically significant performance gap. The standard deviation for heating demand in our sample is small, about 1 kWh/m2a—suggesting that the effect of varying occupancy or other factors can be controlled through this type of design; these homes are delivering in practice a 5 times lower demand than that predicted for a new UK home built to current standards. The Passivhaus approach is thus a solution that could be adopted directly, or one whose design and verification process should be closely emulated, to ensure that a similarly robust performance is achieved in the Future Homes standard.
— Rachel Mitchell leads Greenbox Associates, an architectural engineering consultancy based in Hampshire, UK.