This study was conducted to develop an automatic insulation system to reduce the heat loss through the wall of greenhouses during the nighttime in winter. A double covered model vinyl house was constructed, and blowers were used to fill or remove styrene pellets automatically the inside of the double-wall of the double covered vinyl house. The effects of insulation of the double covered vinyl house insulated with the styrene pellets were analyzed by comparing with the single covered vinyl house. The results obtained from this study can be summarized as follows : 1. The optimum thickness of the double-wall of the double covered vinyl house was found to be 5 to 10 cm. 2.When the outside temperature varied in the range of -8.3 to -1.5$^{circ}C$ during the nighttime, the inside temperature of the double covered vinyl house insulated with the styrene pellets showed 7.7 to 11.7$^{circ}C$ and the inside temperature of the single covered vinyl house showed -5.8 to 2.3$^{circ}C$. Therefore, the effects of insulation of the double covered vinyl house insulated with the styrene pellets were confirmed to be excellent. 3. Also, the excessive increase of the inside temperature in the vinyl house was prevented by the shading effect of the double covered vinyl house insulated with the styrene pellets during the daytime. 4. When the outside temperature varied in the range of 17.7 to 30.0$^{circ}C$ during the daytime, it was possible to keep the inside temperature at 30${pm}$4$^{circ}C$ in the double covered vinyl house by operating the insulation system. 5. The transmissivity of the double covered vinyl house with the styrene pellets removed was 52.4 % on the average.