In-situ U-value monitoring of highly insulated building envelopes: Review and experimental investigation
Introduction
The latest global status report for buildings and construction estimate that 38% of the total global energy-related carbon emissions are due to the operation and construction of buildings [1]. This disconcerting proportion of emissions contributing to global warming has been the focus for many nations across the world for over a decade. The European Parliament issued the Energy Performance of Buildings Directive in 2010 [2] (amended in 2018 [3]) which sets out guidelines and requirements for all EU member states to prepare their near-zero energy building (nZEB) plan. The directive allows member states to define nZEB differently. A concise summary of the different definitions has been described by the Buildings Performance Institute Europe (BPIE) [4]. Each member state has had to enforce their nZEB plans by the start of 2021 with approximately half of the member states defining maximum component U-values as part of their plan [5].
Reducing the maximum allowable theoretical thermal transmittance (U-value) through the building’s envelope reduces the energy consumption in both heating and cooling dominated climates. Thus, providing a logical part of the pathway for architects and engineers towards buildings with ‘near zero’ energy consumption. However, the precise limit on these maximum allowable U-values is subject to discussion [6], [7], with more insulation not necessarily resulting in lower lifecycle energy consumption and carbon emissions [8], [9]. This is because of the embodied carbon and energy associated with the insulation material itself (illustrated in Fig. 1) and the diminishing returns on U-value as the thickness of the insulation layer increases (illustrated in Fig. 2). Furthermore, there is a question on whether the performance of the building envelope in-situ matches the estimated performance. The objective of this research is to identify if building envelopes, and in particular highly insulated envelopes, underperform in practice.
Section snippets
In-situ U-value performance gap
Several non-invasive methods exist for monitoring in-situ U-values. These methods have been reviewed and compared in detail by other authors [11], [12], [13] and can generally be categorised as those that use a heat flow meter (A.1 Average heat flow meter method ISO 9869-1 A.2 Dynamic heat flow meter method B. Simple hot-box heat flow meter method) and those that don’t (C. The quantitative infrared thermography method and D. The temperature-based method).
The most commonly used method is the
Methodology
International standards for both theoretical (ISO 6946 [42]) and in-situ measured (ISO 9869-1 [14]) U-values are followed to compare the performance gap.
Results
Results from this study presented in Table 4 and Fig. 9 show that at lower U-values the deviation between theoretical and measured U-values increases considerably. Out of the 10 in-situ tests with theoretical U-values lower than 0.3 in Table 2 (Sites A-E) only one of the envelopes resulted in a better-than-expected performance. On average the measured thermal transmittance is 127% higher than the theoretical value representing an average deviation of 0.2 W·m−2·K−1.
The one test result that did
Conclusions
The results from this study show that the design U-values of highly insulated envelopes are not being achieved in practice with more than 90% of envelopes tested to date underperforming. An in-situ correction factor in the order of 2.0 would help bring the design U-value closer to what is observed in situ, statistically speaking at least. But a more fundamental question remains as to whether the risk of higher-than-expected U-values will be mitigated by simply adding more insulation and whether
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work forms part of the nZEB101 project which is supported by the Sustainable Energy Authority of Ireland under Grant Agreement 18/RDD/358.
References (43)
- et al.
Optimisation of retrofit wall insulation: An Irish case study
Energy Build.
(2021) - et al.
Review of in situ methods for assessing the thermal transmittance of walls
Renew. Sustain. Energy Rev.
(2019) - et al.
Review and comparison of current experimental approaches for in-situ measurements of building walls thermal transmittance
Energy Build.
(2019) - et al.
Laboratory and in-situ non-destructive methods to evaluate the thermal transmittance and behavior of walls, windows, and construction elements with innovative materials: A review
Energy Build.
(2019) - et al.
In situ measurement of façades with a low U-value: Avoiding deviations
Energy Build.
(2018) - et al.
A comparative assessment of the standardized methods for the in–situ measurement of the thermal resistance of building walls
Energy Build.
(2017) - et al.
A comparison of standardized calculation methods for in situ measurements of façades U-value
Energy Build.
(2016) - et al.
Comparison of characterisation methods determining the thermal resistance of building components from onsite measurements
Energy Build.
(2016) - et al.
Feasibility experiment on the simple hot box-heat flow meter method and the optimization based on simulation reproduction
Appl. Therm. Eng.
(2015) - et al.
A comprehensive experimental approach for the validation of quantitative infrared thermography in the evaluation of building thermal transmittance
Appl. Energy
(2015)
Quantification of heat energy losses through the building envelope: A state-of-the-art analysis with critical and comprehensive review on infrared thermography
Build. Environ.
U-value assessment by infrared thermography: A comparison of different calculation methods in a Guarded Hot Box
Energy Build.
U-value time series analyses: Evaluating the feasibility of in-situ short-lasting IRT tests for heavy multi-leaf walls
Build. Environ.
Thermographic 2D U-value map for quantifying thermal bridges in building façades
Energy Build.
Infrared thermovision technique for the assessment of thermal transmittance value of opaque building elements on site
Energy Build.
Evaluating in situ thermal transmittance of green buildings masonries—A case study
Case Stud. Constr. Mater.
Thermal performance of a selection of insulation materials suitable for historic buildings
Build. Environ.
A longitudinal building fabric and energy performance analysis of two homes built to different energy principles
Energy Build.
Thermal transmittance of historical brick masonries: A comparison among standard data, analytical calculation procedures, and in situ heat flow meter measurements
Energy Build.
Thermal transmittance of historical stone masonries: A comparison among standard, calculated and measured data
Energy Build.
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