Heat transfer through the roof of a test room has been investigated to ascertain the comfortability of a thatched house. The heat transfer coefficient of the roof was determined from the heat transfer rate through the roof and the temperature difference between both surfaces of the roof. The heat transfer rate was calculated from the amount of heat transfer from the test room, which was determined from the flow rate and the temperature difference between the inlet and the outlet of hot water circulated through the test room as the heat source, by taking into account the heat transfer through the room walls. Several types of roofs were examined, namely, a roof consisting of Japanese pampas grass packed between two sheets of thin plywood, a roof packed with lawn grass, a roof packed with fallen leaves and a roof consisting of alternating layers of plywood and air space. The heat transfer resistance of roofs with the packing materials is very large, and the heat insulating ability of the packing materials is comparable to that of glass wool or foam styrene. The heat transfer resistances calculated on the basis of the heat conduction and the heat radiation for the roof consisting of alternative layers of plywood and air space and the roof packed with fallen leaves agreed with the experimental values, which indicates that the large heat transfer resistance of the roof with packing materials (the thatched roof) comes from controlling the radiation with the packing materials.