A non-linear model of flashover, FLASHOVER A1, which has been described in earlier work, has been used to explore the dependence of the critical heat release rate (Qfc) for flashover on the properties of the boundary of the enclosure. The compartment is assumed to have a single ventilation opening stretching from floor to ceiling. Specifically, the dependence of Qfc upon the thermal inertia of the boundary has been calculated for several different compartment sizes. The boundary materials considered are: marinite, gypsum, brick and concrete. Variations in Qfc with thermal inertia of the boundary and compartment size have been computed. The findings should be tested by experiment. © 2010 Elsevier Ltd. All rights reserved.