thermal mass

Introduction | Thermal Mass 2

In architecture, thermal mass is any mass building material that absorbs and stores heat during one part of the day, and then releases that heat several hours later or during the night.

Thermal mass is used in two main ways in desert architecture. For much of the year, the thermal mass of exterior walls is used to absorb and store the summer heat, keeping the internal mass at a lower temperature than outside. Then, for perhaps 3 months of the year, the thermal mass of internal floors and walls can be used to absorb heat (typically direct solar radiation through glass) and assist with heating during cold winter nights.

While often confused, thermal mass and insulation are different properties and are used for different purposes. Insulation blocks or prevents the transfer of heat, while thermal mass is about absorbing and storing heat and "smoothing out" temperature changes.

At some point during the day, the uninsulated thermal mass of exterior walls start radiating the heat they have absorbed into the interior of the building. Ideally, the thermal mass would delay this point until around sunset, when doors and windows can be opened up for ventilation of interior heat to the exterior.

Like evaporative cooling, the large scale use of thermal mass makes the most sense in hot arid climates. In tropical and temperate climates, it is best used sparingly. In tropical climates, the need for lightweight, very well ventilated buildings precludes its use on any scale, while in temperate climates it can cause heating problems in winter (too much heat goes into heating the thermal mass). On the other hand, the use of large-scale thermal mass for external walls appears almost universal in the traditional architecture of hot, dry regions (eg. middle-east, africa, mediterranean).

An interesting question is the extent to which the traditional use of large scale thermal mass in hot arid climates was driven by conscious selection, as opposed to the fact that in many dryland and desert areas stone and earth are the only building materials available in relative abundance. Whatever the case, whether by design or necessity, thermal mass is dominant feature of desert architecture over the centuries.

The best thermal mass construction materials have a high specific heat capacity, high density, and low thermal conductivity (but not too low). Typical materials include mud brick, earth, stone, concrete, and water, which are all able to slowly store, and slowly release, relatively large quantities of heat per unit mass compared to other materials. Insulation materials typically have much lower thermal conductivity than materials used for thermal mass. Materials with high thermal conductivity do not work well as thermal mass because they release stored heat too quickly. Conversely, materials with extremely low thermal conductivity (such as insulation materials) take too long to absorb and store heat.

Concrete and stone have a typical density of around 2500 kg/m³, with adobe and rammed earth slightly less dense at around 2000 kg/m³. With a specific heat capacity of around 850 J/kg K, stone or concrete require about 2.125 megajoules of heat to raise the temperature of one cubic metre of thermal mass by one degree. To raise the temperature of the thermal mass of stone or concrete building (10m by 10m by 2m high with 0.5m thick walls) by 10 degrees Celsius, around 850 megajoules of heat would be required.

Egyptian architect Hassan Fathy suggests that in hot arid climates, an appropriate level of thermal resistance for outer walls is a coefficient of thermal transmittance (U-value) of about 1.3 W/m²K. Table 1 provides details of wall thicknesses in different materials which equate to this value (after Fathy 1986).

Table 1: Building materials and dimensions giving U-values of 1.3 W/m²K