A passive solar design generally incorporates a way to
store heat during the short days of winter and to release this
heat to warm the room during the long night. Objects with
high thermal mass can provide this storage. They are materi-
als with high mass and high heat capacity. For our room we
poured a 0.30 rn thick slab of concrete for the floor. We in-
sulated under this slab to minimize heat loss to the ground.
The back wall of the plant room is limestone, which also pro-
vides a large thermal rnass. Water storage barrels can also help
to moderate the temperature swings.