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Thermal mass is a heat storage material, such as water, concrete or masonry, used in passive solar heating systems. With passive solar heating, the house itself acts as the solar collector and the storage facility. The essential elements of a passive solar system are excellent insulation, solar collection (with south-facing windows), and thermal storage facilities (thermal mass).
There are three categories that passive systems can fall into. One of which is a direct-gain system. In this system, large south-facing windows are used to admit the sunlight. and thermal mass is placed in the house to absorb the solar radiation. Another system is indirect-gain. Indirect-gain collects and stores the solar energy in one part of the building and uses natural heat transfer to distribute the heat throughout the building, such as the Trombe wall. Finally, the third category of these passive solar heating systems is an attached greenhouse on the south side of the house. The greenhouse acts as an expanded thermal storage wall. It will share some of its heat with the adjoining house.