Effect of Thin Intumescent Coating Type and Thickness on the Charring of Mass Timber Under Varied Heat Flux Exposure

Abstract This study investigates how three types of thin intumescent coatings, applied at different Dry Film Thicknesses (DFTs), influence the charring behaviour of mass timber in fire, including the effects of varying heating conditions. Bench-scale timber samples of Cross-Laminated Timber (CLT) were tested for 60 min at a constant 25, 50, or 75 kW/m 2 incident radiant heat flux. Tests were conducted on timber samples painted with opaque coatings (solvent-based Coating A and water-based Coating B) and a transparent coating (water-based Coating C), as well as bare timber samples as a control. Three DFT thresholds were considered for each coating, spanning 0.98–2.60 mm for the opaque coatings and 0.18–0.40 mm for the transparent coating. The findings demonstrated that, although coated timber still chars, the onset of charring is significantly delayed compared to bare timber. For the opaque coatings, this was dependent on both thickness and heat flux, whereas for the transparent coating, the insulating efficacy was primarily governed by the heat flux – with its durability markedly compromised above 25 kW/m 2 . Compared to bare timber, opaque coatings A and B showed reductions in mean charring rates of up to 70%, 62%, and 56% under heat fluxes of 25, 50, and 75 kW/m 2 , respectively. In contrast, timber with transparent Coating C achieved mean charring rate reductions of up to 36%, 18%, and zero under the same heat flux conditions. The main conclusions from the study were: (1) opaque coatings are more effective than the transparent coating in reducing timber charring; (2) higher heat flux conditions reduce the effectiveness of intumescent coatings for all coating types; and (3) while increased DFT generally reduced charring of coated timber, particularly for opaque coatings A and B samples, it was insufficient to offset the effects of higher heat flux intensity in timber samples with transparent Coating C - especially at 50 and 75 kW/m 2 . Graphical Abstract