


Water is one of the best thermal mass materials.  Water has a very high heat capacity (specific heat), which means it can hold a lot of heat or cold before releasing it to the surrounding air.  It was common in early solar buildings to see 55 gallon drums filled with water and painted black.  Stored solar gain water walls or seats can be fabricated out of metal and incorporated into the south wall of the building, or placed near a wood burning stove.  Water storage has the disadvantage of possible leaks.

Although its heat capacity is less than that of water, stone can also be used for thermal mass.  An easy way to create an instant rock wall is to use gabions or heavy gauge wire baskets filled with rock.  They make nice partitions, and can also be used for cooling in the summer if water is set up to trip down the rocks.

There is one other thing I need to mention here: thermal mass is useless if not surrounded by properly insulated exterior shell. It would reduce the rate of temperature drop inside the house when you turn the heaters off, but would not reduce the total heat loss!