There are some rules to follow while sizing the thermal mass. - The optimum thickness of a masonry wall increases as the thermal conductivity of the wall material increases. For instance, to compensate a rapid heat transfer through a highly conductive material, the wall needs to be thicker. - Accordingly, since the thicker wall absorbs and stores more heat to use at night, the efficiency of the wall increases as the conductivity and thickness of the wall increase. - There is an optimum thickness range for the masonry materials. - The efficiency of the water wall increases as the thickness of the wall increases. However, it is hard to notice a considerable performance increase as the walls get thicker than 6 inches. Likely, a water wall thinner than 6 inches is also not enough to act as a proper thermal mass that stores the heat during the day. • In the early Trombe wall design, there are vents on the walls to distribute the heat by natural convection (thermocirculation) from the exterior face of the wall, but only during the daytime and early evening. Solar radiation passing through the glass is absorbed by the wall heating its surface to temperature as high as 150 °F. This heat is transferred to the air in the air space between the wall and the glass. Through openings or vents located at the top of the wall, warm air rising in the air space enters the room while simultaneously drawing cool room air through the low vents in the wall. In this way additional heat can be supplied to the living space during periods of sunny weather. However, it is now clear that the vents do not work well in either summer or winter. It becomes more common to design a half Trombe Wall then combine it with a direct gain system. The direct gain part delivers heat early in the day while the Trombe wall stores heat for the nighttime use. Moreover, unlike a full Trombe wall, the direct gain part allows views and the delight of winter sunshine. • To minimize the possible drawbacks of the Trombe wall system, there are additional thermal control strategies to employ to the wall design. For instance, the minimum 4-inch distance between the glass and the mass allows cleaning the glazing and the insertion of a roll-down radiant barrier as needed. Adding a radiant barrier or night insulation between the glazing and the thermal mass reduces nighttime heat losses and summer daytime heat gains. However, to prevent overheating in summers, combining this strategy with an outdoor shading device like shutter, a roof overhang, or an interior shading to block excessive solar radiation from heating the Trombe wall would be the best.