Mass, in the form of a dense material, absorbs heat during the daytime to prevent overheating. It then stores the heat until the air temperature of the room drops when the sun goes down. Then the heat is naturally released from the mass material, warming the interior throughout the cool night. This same natural process occurs in the passive solar home, except that the heat is trapped by the walls or floors of the house and used to warm its occupants. A mass material's effectiveness is measured by its ability to absorb sunlight, conduct surface heat into its mass and hold the resulting heat.
Mass materials vary greatly in the amount of heat they retain.
Frequently, older structures are not designed to support the weight of additional thermal mass. Lightweight, efficient mass is suggested for many installations. The percentage absorption varies according to material, color, and finish or texture. The best thermal mass materials would seem to have a dark-colored, rough, matte surface. Of equal importance is the need to place furniture so that it shades the mass floor or wall as little as possible.