In order to preheat a particle mat having upper and lower faces and edges to a predetermined desired temperature for pressing into a pressed board air is conditioned to a temperature below 100.degree. C. and a moisture content together imparting to the conditioned air a dew point generally equal to the desired temperature. This conditioned air is simultaneously introduced from above and below at the same rate into the respective faces of the mat to produce in the mat a pair of horizontal and vertically approaching fronts at which moisture condenses out of the conditioned air. Introduction of the conditioned air into the mat is ended when condensed moisture and the conditioned air are forced out of the edges of the mat.