Thermal mass materials have the ability to conduct and store energy, both heat and cold, and to release that energy back into the living space when it's needed.
Heat always moves to colder surfaces.In the solar home, the free solar energy first heats up the air. Since the mass floors and walls are cooler, the heat is absorbed and conducted into these materials. Later, when the sun has set and the room air temperature falls, it will reach a point where the mass materials are warmer than the room air temperature.Since heat seeks out cold, the stored energy will now return to the room. The more mass in the home, the more energy that can be stored.
The amount of south facing glass to be installed is related to the amount of thermal mass in the home, and the reverse is also true. In the sun tempered design, the home does not have the mass needed to absorb the amount of solar energy delivered when glass amounts exceed 7%. With slab construction, the mass is built in, but when carpeted, it can't work for you. When you tile your slab, you have added thermal mass and you can increase the amount of south glass accordingly.
The Direct Gain System: Glass and Mass
The direct gain system is the easiest and most cost effective way to use solar energy. The structure itself is the solar system. The south windows are the collectors. The walls and floor are the storage. (mass). Orientate the structure and windows as close to true south as possible.
The ideal thickness for mass materials is 4 to 5 inches.
Use mass materials in the construction, for floors and walls. (adobe, concrete, brick, rock)
Water is the best mass, storing far more energy than other materials, BUT it's not structural.
A masonry fireplace adds thermal mass but should be located on an interior wall.
An interior mass wall performs better than an exterior wall.
Insulate the exterior of your walls, keeping the masonry inside, protected from outside temperature extremes.
Provide for night time insulation on large glass areas. ( insulating curtains, moveable insulation, shutters. )
Provide adequate overhangs on the south side to prevent direct gain during the cooling season.
Use light colors on low mass construction. (ceilings, and partition walls)
Glass to Mass Ratios
Each design starts with 7% south glazing. (net) To increase beyond 7% we must also add thermal mass, usually starting with floor mass and then walls.
An additional 1 sq. ft. of south glass may be added for every:
5.5 sq. ft. of sunlit thermal mass floor *
40 sq. ft. of floor not in direct sunshine
8.3 sq. ft. of thermal mass wall
* The maximum amount of sunlit floor is 1.5 times the south window area
The recommended maximum amount of south glass for direct gain is 12-15%
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