Thermal mass refers to the ability of the building structure to absorb and store heat, and to release it at some later time. Wood has significant heat capacity and has historically been used to provide thermal mass to buildings such as log homes.
Thermal mass works by absorbing and storing heat from the building space during the day, providing passive cooling, and releasing it during the following night(s), providing passive heating.
Mass is most effective when it is thermally ‘connected’ to the building space.
This means insulation should be located on the external side of thermal mass material in order to maximise its effectiveness.
Increasing the level of thermal mass by constructing with solid wood reduces temperature fluctuations within a building space.
This reduces the incidence of overheating if the space is not air conditioned and reduces cooling energy use if it is.
Provided the heat stored in the mass during the day is ‘free’ heat (solar heat or heat generated by occupants or equipment) then thermal mass also reduces heating energy use.
The energy savings due to thermal mass depends on the climate and design of building, and is typically 0-20% in New Zealand.
Greater savings are possible in buildings located in the warmer parts of the country, especially if they are prone to overheating when designed with a lightweight construction.
The thermal mass in buildings constructed with solid wood and concrete are similar provided the constructions differ in the following ways:
The volume of solid wood has to be approximately double that of concrete, since the volumetric heat capacity of wood and concrete are approximately equal to 1000 kJ/m3K and 2100 kJ/m3K respectively.
The surface area of solid wood exchanging heat with the building space has to be approximately fives times that of concrete, since the thermal effusivity of wood and concrete are approximately equal to 350 J/m2Ks½ and 1800 J/m2Ks½ respectively.
Thermal effusivity is a measure of a materials ability to exchange heat with the environment.
The storage and release of heat in thermal mass is due to the time-varying state of the internal and external environments. This dynamic behaviour makes it difficult to determine the effect of thermal mass on building thermal performance, without the aid of a building simulation tool.