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The effect of equilibrium moisture content (ranging from 4 to 12.5% moisture, corresponding to 20 to 90% relative humidity) on piloted ignition times of single wall and tri-wall corrugated paperboard samples has been measured over a range of external radiant heat flux (10 to 60 kW/m2) in the Fire Propagation Apparatus (ASTM E-2058). It has been shown for heat fluxes up to 50 kW/m2 corrugated paperboard samples behave as thermally thin solids and satisfy a simple fundamental equation based on energy required for ignition: ( ) (1 ) e cr ig w q???? ? ? q???? t =? + ? m where ? and ? are: ( ) 0 c m T T pc dry ig ? = ?? ? and ( ) ( ) 0 0 c T T c T T L pc ig pw b w ? ? + ? = This thermally thin model provides the piloted ignition time for a given net radiant heat flux and at a specific moisture content, consistent with experimental data. The value of dimensionless,? is predicted to be 5.14 for corrugated samples initially at 23oC, whereas, the measured value is 5.0. Measurements of the tested tri-wall and single wall corrugated paperboard yield ? = 332 kJ/m2 and 230 kJ/m2, respectively. Based on moisture dependent property values obtained from the literature, the model has been extended to piloted ignition under thermally thick conditions.