In the UK, there is mounting evidence that the measured in situ performance of the building fabric in new build dwellings can be greater than that predicted, resulting in a significant building fabric ‘performance gap’. This paper presents the coheating test results from 25 new build dwellings built to Part L1A 2006 or better. Whilst the total number of dwellings reported here is small, the results suggest that a substantial ‘performance gap’ can exist between the predicted and measured performance of the building fabric, with the measured whole building U value being just over 1.6 times greater than that predicted. This is likely to have significant implications in terms of the energy use and CO ₂ emissions attributable to these dwellings in use. Practical application: This paper describes an aggregate approach (coheating test) that has been applied to a small sample of dwellings to quantify the size of the ‘performance gap’. The results suggest that this ‘gap’ can be large (>100%) and can vary depending upon form and construction type. The results also illustrate the importance of developing a larger, more representative whole dwelling heat loss data set. This could be used by industry to apply a suitable performance factor (note 1) to the nominal performance of new build that accounts for the ‘performance gap’. This factor could be specific to form and main construction type.

It is recognized that there is often a discrepancy between the measured fabric thermal performance of dwellings as built and the predicted performance of the same dwellings and that the magnitude of this difference in performance can be quite large. This paper presents the results of a number of in-depth building fabric thermal performance tests undertaken on three case study dwellings located on two separate Passivhaus developments in the UK: one masonry cavity and the other two timber-frame. The results from the tests revealed that all the case study dwellings performed very close to that predicted. This is in contrast with other work that has been undertaken regarding the performance of the building fabric, which indicates that a very wide range of performance exists in new-build dwellings in the UK, and that the difference between the measured and predicted fabric performance can be greater than 100%. Despite the small non-random size of the sample, the results suggest that careful design coupled with the implementation of appropriate quality control systems, such as those required to attain Passivhaus Certification, may be conducive to delivering dwellings that begin to ‘bridge the gap’ between measured and predicted fabric performance.