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Gas analysis and temperature measurement using a room with controllable heat sources reveal the condition of smoke layer formation in fires in forced-ventilation and highly airtight enclosures. Three combinations were used. 1) An inlet opening located in a lower part and an outlet at an upper level, 2) both an inlet and an outlet located on the ceiling, 3) An inlet at the upper level and an outlet at the lower. Methane combustion lasted for longer than 30 minutes in the first case with an air supply rate of 50m3/h and a heat release rate of 100kW. In the other configurations, combustion lasted for much less than 30 minutes, and there was hardly any smoke layer formation. The stability of the smoke layer was also affected by the inlet air velocity. The mechanism of the smoke layer destruction due to increased inlet velocity was also investigated using CFD results.