Linear response theory is widely used in science and engineering but its use in fire sciences is rare. This communication reviews shortly the Duhamel superposition technique for solving problems in fire sciences where the response of a material maybe assumed linear but the boundary conditions (BC) are non-linear. The method can be used as an alternative to e.g. finite element methods for problems where analytical solutions are not available. Examples include temperature distribution in solids with time-varying and non-linear heat flux boundaries using a simple spreadsheet solution technique. Supplementary material contains an Excel spreadsheet solving problems with non-linear BC.