Thermal conditions in a 3 m2 sunroom attached to a residence building were estimated from a vegetable production perspective. The set points of the air temperature were 12–24 °C, an appropriate temperature range for most vegetables. The air temperatures in the full scale model sunroom could be maintained over 12 °C with up to 417 W supplemental heating for the measuring period. In the sunroom, the averaged overall heat loss coefficients between the sunroom and the outside and between the sunroom and the residence building for 24 days during nighttime in February and March were 2.6 and 13.0 W/m2 °C, respectively. The tendency of the diurnal range of air temperature in the sunroom could be roughly simulated for the measuring period except in some daytime cases in February, although the simulation method was a simple and approximate energy balance model with the averaged overall heat loss coefficients.