First Repeatability and Reproducibility Indicators for the <scp>ISO</scp> 13785‐1/ <scp>NPR</scp> 6999 Intermediate‐Scale Façade Test

ABSTRACT This short communication reports the preliminary repeatability and reproducibility (R&amp;R) results for the intermediate‐scale façade test in accordance with ISO 13785‐1/NPR 6999. A three‐laboratory round‐robin was conducted on two cladding systems (a phenolic foam system, K15B, and a wood panel), and complemented by an intra‐laboratory repeatability series on a second phenolic formulation (K15N). Three output metrics were assessed: the 60‐s running‐average peak heat release rate (HRR 60s av, max ) and total heat released at 5 and 30 min (THR 5 , THR 30 ). Data were treated under the ISO 5725 and ISO 13528 frameworks with a limited dataset; inter‐laboratory standard deviations were pooled across sites, Z‐scores were used to screen for outliers, and, where multi‐site repeats were not feasible, the within‐site repeatability standard deviation was proxied by a single intralaboratory series. Across both products, THR‐based metrics exhibited lower repeatability than HRR 60s av, max , with repeatability consistently tighter than inter‐laboratory reproducibility, and no | Z | ≥ 2 observations. Reproducibility is better for HRR 60s av, max , but further data will be required to refine these trends. Practical guard bands derived from the pooled coefficients of variation are proposed to support method qualification and acceptance criteria. The findings also inform the ongoing revision of ISO 13785‐1, which now contemplates explicit HRR measurement, and highlight the role of intermediate‐scale testing as a cost‐effective bridge between small‐scale screening and full‐scale façade performance assessment, providing a tool for extensions of applications. Limitations due to small sample sizes are noted, and recommendations are given for future r&amp;R campaigns.