Horizontal flame spread of flame-retardant-treated Japanese cedar (Cryptomeria japonica) exterior siding material

Abstract

The primary objective of this study was to determine whether in-house-developed flame-retardant-treated cedar could serve as an exterior material, not only by meeting the fire-resistance criteria of the cone calorimetry test, but also by demonstrating dependable performance under realistic fire conditions. The results of the cone calorimetry test indicated that the material met the established criteria for total heat release and peak heat release rate, validating its classification as a quasi-noncombustible material. Furthermore, the ASTM E84 Steiner tunnel test was conducted to evaluate the performance of the flame-retardant cedar siding under conditions that simulated real fire scenarios more closely. Flame propagation exhibited a delayed onset and progressed gradually from 2.5 ft to 5 ft over 10 min, indicating restrained flame spread. Concurrently, smoke development remained negligible throughout the test, which is critical for maintaining visibility and minimizing exposure to toxic combustion byproducts. Post-test examination revealed minimal structural degradation, with no evidence of cracking or penetration beyond superficial charring, suggesting that the material retained its physical integrity even under elevated thermal stress. Collectively, these findings confirm that the flame-retardant cedar siding meets the requirements for an ASTM E84 Class A rating and functions as a reliable exterior cladding material capable of enhancing occupant safety in fire scenarios.