Adapting ISO 9869-1 for post-retrofit low U-value walls: Evidence from 130 days of continuous in-situ monitoring

Du, H ORCID: 0000-0002-1637-0626 and Pilkington, A (2026) Adapting ISO 9869-1 for post-retrofit low U-value walls: Evidence from 130 days of continuous in-situ monitoring. Energy and Buildings, 369. pp. 1-21. ISSN 0378-7788

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Abstract

Large scale retrofit programmes require a practical means of verifying that newly insulated walls perform as designed. ISO 9869-1, the international heat flow meter method, permits test durations as short as 72 hours, a threshold established when uninsulated walls lost heat far more rapidly than today’s high performance retrofits. This study evaluates the suitability of the standard for modern post retrofit construction and proposes evidence based adaptations.
A recently insulated cavity brick wall in Liverpool, UK (design U-value 0.18 W/(m2 K)), was monitored at two adjacent points over 130 days. Heat flux and indoor and outdoor temperatures were logged at 10-minute intervals alongside on site weather data. The Average Method was applied to all 13,687 candidate time windows to examine the influence of test duration, starting hour, rainfall, and other environmental factors.
The 72-hour minimum performed poorly: only 5–10% of windows satisfied the convergence criteria, and among these the mean absolute error against long duration references was approximately 32%. A minimum of 14 days was required for indicative results, and 28 days for accuracy within  ± 5%. Starting hour was statistically irrelevant once windows exceeded three days. Moisture proved decisive: wet periods inflated measured U-values by 13–16%, with recovery of 4–6 mW/(m2 K) per consecutive dry day. Measured values of 0.206 and 0.188 W/(m2 K) exceeded the design value by only 0.026 and 0.008 W/(m2 K) in absolute terms.
Three adaptations to ISO 9869-1 are proposed: a wall-type-dependent minimum duration, mandatory dry-spell pre-conditioning, and a combined absolute and relative tolerance for design measurement agreement. Together these provide a measurement based quality assurance route for large scale retrofit delivery.

Item Type:	Article
Uncontrolled Keywords:	In-situ U-value; ISO 9869-1; Heat flow meter; Test duration; Wind-driven rain; External wall insulation; 33 Built Environment and Design; 3301 Architecture; 09 Engineering; 12 Built Environment and Design; Building & Construction; 33 Built environment and design; 40 Engineering
Subjects:	T Technology > TA Engineering (General). Civil engineering (General) T Technology > TH Building construction
Divisions:	Computer Science and Mathematics Engineering
Publisher:	Elsevier BV
Date of acceptance:	20 July 2026
Date of first compliant Open Access:	17 August 2026
Date Deposited:	17 Aug 2026 11:26
Last Modified:	17 Aug 2026 11:26
DOI or ID number:	10.1016/j.enbuild.2026.117983
URI:	https://researchonline.ljmu.ac.uk/id/eprint/29152

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