What is thermal mass?
Thermal mass is anything in the greenhouse that absorbs solar energy and heat from the sun during the day and releases that heat at night. Using thermal mass can moderate greenhouse temperatures and may reduce energy costs.
Many things can act as thermal mass in the greenhouse, including wood, soil in raised beds, concrete, block, or brick foundations, or even the floor of the greenhouse. But the most effective and least expensive thermal mass is water. Water can store far more heat for a given mass than any other commonly available material. Water is also useful because of its latent heat; it release up a tremendous amount of heat energy as it freezes, and often this is just when greenhouse heat is needed most.
A common practice is to store water in containers along the back wall of a greenhouse or in 55 gallon drums used to support benches. A commonly recommended minimum volume of stored water for solar greenhouses is 2-3 gallons per square foot of glazing.
Information provided by: Stressbaby