The direct gain system is the most common and simplest solution for a passive solar heating system: solar radiation enters the room through a glazed surface and directly warms it (Fig. 69). So, the living space works as a solar collector, but it must have the means and structures that are capable of absorbing and storing the intercepted thermal energy to keep the internal air temperature constant as much as possible. In this way, the daily overheating and the excessive decrease

in night temperature can be reduced. A direct gain system needs a wide glazed surface oriented southward to allow transfer of the winter solar radiation through direct communication with the living space. The southward orientation generally allows intercepting the greatest quantity of solar energy during winter, whereas in summer since the sun is very high, the transmitted radiation is less and it can be minimized by a suitably proportioned horizontal object (overhang). The choice of window components is very important in planning the solar heating system. Windows with a high heat transmission coefficient are preferred to maximize the quantity of intercepted radiation when the radiation is very poor and also to restrict heat losses.

A wide glazed surface oriented southward can cause overheating inside the house during the day and excessive inner air temperature fluctuations when there is no direct solar radiation (during the night or when the sky is cloudy). To solve these problems, it is important to use a thermal mass that is connected with the walls and floors whose surfaces and thermal capacities are well proportioned and also well positioned to intercept solar radiation and store thermal energy. During the day, the heat produced by the intercepted radiation is not completely released into the room; it is partially stored and released later after a delay of a few hours to stabilize the air temperature of the house. The storage thermal mass is generally made of masonry. Masonry materials for thermal storage are characterized by a high thermal capacity: cement blocks, concrete, bricks, stone, etc. To restrict dis­persion of stored energy inside the thermal mass, the brickwork walls are insulated on the outside, while floors are realized with a perimeter or an extrados insulation [1, 3,4].