The subject of thermal mass can be confusing, as the effect of thermal mass is often equated with thermal insulation. However, thermal mass is not insulation. Insulation limits or restricts the flow of heat; thermal mass freely and easily absorbs and releases heat. The confusion between thermal mass and insulation can arise because, under certain conditions, the thermal performance of a massive material can lend itself well to occupant comfort.
Consider the two scenarios pictured on page 82. The empty tent has almost no thermal mass, being constructed of lightweight material and containing nothing but air (and an occupant). The tent containing the boulder has a huge amount of thermal mass.
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The empty tent will quickly cool down on a cold night. Then, when the sun hits the tent in the morning, it will quickly get warmer. The lack of thermal mass means that the interior temperature can change very quickly because only the air in the tent must change temperature.
The tent with the boulder will remain warmer longer, because the heat stored in the mass of the rock will take some time to be lost to the night air. Then, when the sun hits the tent in the morning, it will take longer to warm up because the mass of the boulder will absorb a lot of thermal energy before its surface temperature rises.