Thermal mass is the ability of a material to absorb and store heat energy. A lot of heat energy is required to change the temperature of high density materials like concrete, bricks and tiles. They are therefore said to have high thermal mass. Lightweight materials have low thermal mass. Lightweight construction responds quickly to cooling breezes.
Thermal mass, correctly used, moderates internal temperatures by averaging out diurnal (day – night) extremes. Thermal mass acts as a thermal battery. During summer it absorbs heat during the day and releases it by night to cooling breezes or clear night skies, keeping the house comfortable. In winter the same thermal mass can store the heat from the sun or heaters to release it at night, helping the home stay warm. Thermal mass is particularly beneficial where there is a big difference between day and night outdoor temperatures.
Thermal lag: The rate at which heat is absorbed and re-released by uninsulated material is referred to as thermal lag. Lag is dependent on conductivity, thickness, insulation levels and temperature differences either side of the wall. In moderate climates, a 24 hour lag cycle is ideal.
High volumetric heat capacity (VHC): The amount of useful thermal storage is calculated by multiplying the VHC by the volume of material that has its surface exposed to a source of heating or cooling.
Water has the highest VHC of any common material. It takes 4186KJ of energy to raise the temperature of one cubic meter of water by one degree C, whereas it takes only 2060KJ to raise the temperature of an equal volume of concrete by the same amount. The VHC of any material is reduced or even eliminated if the material is covered with linings such as carpets, plasterboard, timber.
Some thermal mass materials, such as concrete and brick, have high embodied energy when used in the quantities required. Consider the lifetime energy impact of thermal mass materials: will the savings in heating and cooling energy be greater than the embodied energy content over the life of the building?
Phase change materials
There is growing interest in the use of phase change materials (PCMs) as a lightweight thermal mass substitute in construction. All materials require a large energy input to change state (i.e. from a solid to a liquid or a liquid to a gas). This energy does not change their temperature — only their state. Phase change temperatures vary enormously between materials. Commonly used PCMs include paraffin. They are an ideal way to install mass in lightweight buildings where cost savings are often achieved.