Elsevier

Energy and Buildings

Volume 252, 1 December 2021, 111447
Energy and Buildings

In-situ U-value monitoring of highly insulated building envelopes: Review and experimental investigation

https://doi.org/10.1016/j.enbuild.2021.111447Get rights and content

Highlights

  • 309 In situ U-value tests from 14 past studies are reviewed and analysed.

  • A paucity of tests on highly insulated envelopes exists.

  • Out of 30 tests at U-values <0.3, 93% report underperformance.

  • 10 envelopes with very low theoretical U-values are measured in situ.

  • Underperformance was reported in 9 out of the 10 envelopes.

Abstract

In-situ U-value monitoring of building envelopes are investigated in this article by reviewing past studies and monitoring the performance of highly insulated building envelopes. Specific past research studies have concluded both over- and underperformance of the building fabric when measured in situ. This current research has found that when analysing all studies and results over the full range of envelope U-values together there is no evidence of over- or underperformance. However, when highly insulated envelopes (U-values <0.3 (W·m−2·K−1)) are investigated in isolation a clear trend in underperformance becomes evident with 93% of tests from past studies reporting underperformance. In-situ monitoring results from this study conclude that 9 out of the 10 tests on envelopes with very low U-values (<0.2 (W·m−2·K−1)) result in underperformance with an average deviation of more than 100%. The significance of this finding is two-fold. First, this means that some of the insulation is redundant and adds to the embodied energy of the building without offering a beneficial increase in thermal resistance. Second, under-sizing of heating systems in low energy buildings will result, which in turn will result in lower efficiencies and greater overall energy consumption.

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.

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