In this study, models based on the regression analyses of the critical exposure time periods of humidity and temperature conditions needed for the development of brown rot decay in pine and spruce sapwood are presented. The growth conditions and decay development of a typical brown rot fungus Coniophora puteana (Schum. ex Fr.) P. Karsten (BAM Ebw. 15) was studied in Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies Karst.) by exposures at 0, 5, 15, 20 and 30 °C using RH of 86-88, 90-92%, 96-98% and 99-100% under laboratory conditions. A regression model is presented for the critical response time periods of temperature and humidity conditions allowing the initiation and development of decay as expressed by the mass loss of wood. The response time for decay development in wood is strongly dependent on the temperature: after 3 months' exposure at RH of 96 and 100% and at 30 °C, the wood was heavily attacked and decayed by C. puteana but after one year exposure in different humidity conditions, no decay was detected at 0°C. According to the present model, the lowest humidity conditions for decay development must lie above RH of 90-92% during 60 month exposure time at 30 °C. Between 0 and 5°C, the lowest humidity conditions for decay development lie around RH of 97% and the wood moisture content around the fibre saturation point.