While standards for quality performance of treated wood have been in place for many years, none have been developed for modified wood such as thermally modified wood (TMW). Only in one case has TMW been listed in 2018, by the Nordic Wood Preservation Council (NWPC). A key component of a quality system is its control parameters, i.e. the metrics that are to be measured and controlled. Existing metrics for treated wood are not meaningful for TMW. In this paper we review literature and present an analysis of the lab and field test performance data used by the NWPC to list TMW. We propose Equilibrium Moisture Content (EMC) as a valid and reliable metric for TMW quality performance at a threshold value of EMC=6. Thus, decay resistance of thermally modified wood is widely believed to be related to reduction of wood cell wall moisture capacity. We further explore the presence of a universal moisture threshold value, below which decay does not occur. The aim is to provide a scientific basis for quality performance measurement in industrial quality management systems and development of national standards. Laboratory data are based on 900 specimens of 5 species/qualities, thermally modified at low, medium and high modification intensities. EMCs are measured and durability performance according to EN 350 is determined by means of two different procedures, CEN 15083-1 (fungal test) combined with EN 84 (accelerated ageing – leaching) and EN 73 (accelerated ageing – evaporation), respectively. We also report data from 6 years of accelerated testing in Borneo, after CEN 12037 (horizontal lap-joint). Laboratory data reconfirm strong statistical correlations between moisture and durability performance as described in existing theory. Data also support the theoretical notion that a threshold EMC value exist, at which decay cannot progress. Surprisingly, the threshold, at same level of intensity, appear to be different for softwood and hardwood. Based on these insights, a threshold EMC value suitable for industrial quality control and national standards development is identified. 6 years of field data support and validate the statistically derived and proposed metric.

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