Abstract
This paper presents new evidence from a nationwide cross-project meta-study investigating the magnitude and extent of the difference between designed and measured thermal performance of the building fabric of 188 low energy dwellings in the UK. The dataset was drawn from the UK Government’s national Building Performance Evaluation programme, and comprises 50 Passivhaus (PH) and 138 non-Passivhaus (NPH) dwellings, covering different built forms and construction systems. The difference between designed and measured values of air permeability (AP), external wall/roof thermal transmittance (U-value) and whole house heat loss were statistically analysed, along with a review of thermal imaging data to explain any discrepancies. The results showed that fabric thermal performance gap was widespread especially in terms of AP, although the magnitude of underperformance was much less in PH dwellings. While measured AP had good correlation with measured space heating energy for PH dwellings, there was no relationship between the two for NPH dwellings. The regression analysis indicated that for every 1 m3/h/m2 reduction in designed air permeability, the gap increased by 0.8 m3/h/m2@50 Pa. Monte Carlo analysis showed that likelihood of AP gap was 78% in NPH dwellings designed to 5 m3/h/m2@50 Pa or lower. The study provides useful evidence for improving the fabric thermal performance of new housing through in-situ testing.
Highlights
The domestic sector in the UK accounts for more than a quarter of the national energy use and associated CO2 emissions [1]
The data were derived from air permeability tests conducted to the Air tightness Testing and Measurement Association (ATTMA) standard [53], though the test had been extended to include both pressurisation and depressurisation with the final air permeability result represented by the average of the two
The designed air permeability was in the range of 0.4–0.6 m3/h/ m2@50 Pa for PH, while for NPH it ranged from 1.5 m3/h/m2@50 Pa in a one-off dwelling to 10 m3/h/m2@50 Pa which is the minimum requirement set in Part L of the Building Regulations
Summary
The domestic sector in the UK accounts for more than a quarter of the national energy use and associated CO2 emissions [1]. Under the scope of UK’s legally binding 80% greenhouse gas emissions reduction target to be met by 2050, various policies aimed at encouraging energy efficiency measures in domestic buildings have been put in place in the recent years [2]. Behaviours, lifestyles and socio-economic aspects of Abbreviations and acronyms: ACH, air change per hour; n50, air tightness @50 Pa; CSH, Code of Sustainable Homes; HLC, heat loss coefficient; ATTMA, Air tightness Testing and Measurement Association; TSL1, Technical Standard L1; SAP, Standard Assessment Procedure; ΔT, temperature difference between the inside and the outside of dwellings; BS EN, British version of European harmonised standard; DomEARM, Energy Assessment and Reporting Methodology for domestic applications; BPE, Building Performance Evaluation; PH, Passivhaus; NPH, Non Passivhaus; SIP, Structural insulated Panel; NV, Natural Ventilation; MEV, Mechanical extract ventilation; MVHR, Mechanical Ventilation with Heat Recovery; APm, Measured Air Permeability; APd, Design Air Permeability; APmp, difference between measured and design air permeability
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