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 applicationThis 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 dataset. This could be used by industry to apply a suitable performance 2 factor 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.

In the UK, it is recognised 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 that were 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 of the case study dwellings tested 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 nonrandom 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. KeywordsAir tightness, building performance, housing, thermal barrier, coheating, heat flux. ContextOver a number of years, it has become evident that there is often a discrepancy between the predicted energy and thermal performance of a building and the measured performance of that building in-use. This discrepancy is often referred to as the 'performance gap'. The 'performance gap' affects domestic and non-domestic revealed discrepancies between the measured and predicted energy and thermal performance of whole buildings, or individual building elements. The reasons identified for the discrepancies in performance tend to be widespread and highly context specific. Despite this, they can be broadly categorised into three main areasthose relating to the thermal performance of the building fabric, those relating to the energy performance of the building services and those relating to occupancy. It is also important to recognise that all three of these areas are also influenced by the external environmental conditions. This paper is concerned with the thermal performance of the building fabric in new UK dwellings only. Despite this, the findings from this paper are likely to be equally applicable to other building types both within the UK and abroad. 2

Highlights  A 'performance gap' is found to exist between measured and modelled building fabric performance.  Highly accurate measurement of building fabric properties (i.e. U-values and air permeability) is possible under controlled conditions.  Calibration of building energy models using accurate measurements of the building's fabric properties reduces the observed performance gap.

ABSTRACT

The accurate assessment of buildings to assess their performance across a range of parameters is an essential part of understanding both new and retrofit buildings. The growing understanding of the performance gap in terms of its assessment and characterisation relies on effective methods of analysis. Here, we evaluate an experimental whole house method, known as QUB. As with many whole building approaches the method establishes heat loss through transmission and ventilation losses. This study compares QUB against an alternative, established, whole house test known as coheating. It was applied in a whole house test facility under controlled conditions. The test property, a solid wall pre-1919 UK archetype, was retrofit using a set of commercially available products and then the retrofit was removed in stages. At each of these stages a QUB test, which commonly takes one night, and coheating test, which can take few weeks, were applied. The objective of the study was to provide a comparison between the new method and more established method in terms of accuracy. The two methods showed close agreement in terms of results, suggesting that the quicker test has great potential as a more practical and economic test. There were higher levels of uncertainty with the QUB method due to shorter measurement periods. The lack of full boundary conditions within the test facility should be considered a limitation in applying the findings directly to the field. However, this study indicates the potential for QUB in validating performance, warranting further investigation.

No abstract

This paper presents the methodology, along with some of the initial findings and observations from tests performed on two dwellings, of differing construction and form, in which a coheating test was performed using the dwelling's central heating system; this method is referred to as integrated coheating. Data obtained during the integrated coheating tests using a dwelling's heating system have been compared with data obtained during electric coheating of the same dwelling. In one instance, integrated coheating test data from one dwelling was compared to a similar adjoining control dwelling that was simultaneously subject to an electric coheating test. The results show a good agreement between the heat loss coefficients (HLC) obtained using a dwelling's own heating system and those obtained through electrical coheating. Initial analysis suggests the HLC estimate obtained from integrated coheating is likely to be more representative of how a dwelling performs in-use. The findings question the appropriateness of comparing current steady-state HLC predictions to those derived from in-use monitoring data. Integrated coheating has the potential to provide a more cost-effective and informative indication of whole house heat loss than electric coheating, as it enables in situ quantification of both fabric and heating system performance.

Structured Abstract:Purpose: The coheating test is the standard method of measuring the heat loss coefficient of a building, but to be useful the test requires careful and thoughtful execution. Testing should take place in the context of additional investigations in order to achieve a good understanding of the building and a qualitative and (if possible) quantitative understanding of the reasons for any performance shortfall.Design/Methodology/Approach: The authors have access to more than twenty years of experience in coheating testing. This experience is drawn upon to discuss practical factors which can affect the outcome, together with supporting tests and investigations which are often necessary in order to fully understand the results. Findings:If testing is approached using co-heating as part of a suite of investigations, a much deeper understanding of the test building results. In some cases it is possible to identify and quantify the contributions of different factors which result in an overall performance shortfall. Practical implications:Although it is not practicable use a fully investigative approach for large-scale routine quality assurance, it is extremely useful for purposes such as validating other testing procedures, in-depth study of prototypes or detailed investigations where problems are known to exist. Social implications:Successful building performance testing is a vital tool to achieve energy saving targets.Originality/Value: The approach discussed clarifies some of the technical pitfalls which may be encountered in the execution of coheating tests and points to ways in which the maximum value can be extracted from the test period, leading to a meaningful analysis of the building's overall thermal performance.