The amount of thermal mass in a building and its distribution in the envelope play an important role in the effectiveness of the heating and cooling discharge operations and the amount of the stored thermal energy. These are mainly night time ventilation during summer and radiant temperature during the whole year. For assessment of the effectiveness of indoor thermal mass, the concept of the 'diurnal heat capacity' was introduced. This is defined as the amount of energy per degree swing in temperature stored in the material and returned back to the indoor space during the diurnal cycle per unit surface area.
Modulation of heat gains can be achieved through the use of materials with high thermal - storage or thermal mass as it is also called - in the building's structure. High thermal mass materials, like brick and concrete, act as storage for both heat and cold as they heat up and cool down relatively slowly.
Thermal inertia is an important parameter especially during the summer when temperature fluctuations during a 24 hour period are very significant. It helps avoid the daytime heat and keep the night-time coolness inside the building for a longer period. The thermal capacity of the building's elements delays the heat transfer to the interior of the building, by soaking up excessive heat for several hours. During the night, when the external temperature is lower, the stored heat is slowly expelled to the environment by radiation and by convection.
During summer, this procedure provides an attenuation of peaks in internal temperatures, by delaying the discharge of heat until a later time, when outdoor ambient temperatures are lower. At the same time it reduces the heat flow reaching the interior of the building, as part of the stored heat in the envelope is radiated and convected back to the external environment during the evening hours. During the evening, the thermal mass acts as a storage of cold that is gradually recovered during the day. Exactly the opposite procedure occurs during winter time.
In warm climates such as those of the Mediterranean, materials of high thermal capacity should be used, in order to help the building operate as a 'thermal storage bank'. This will improve the building's performance during summer and winter.
Special care should be taken to provide night-time natural ventilation during the summer. This is essential for the dissipation of stored heat by convection. This is the best (if not the only) way of passive cooling during night. In regions with great temperature differences between day and night the night-time ventilation reduces the cooling load of the building, and also reduces the maximum daytime internal temperature by 1 to 2 degrees.
Bear in mind that increased thermal capacity helps reduce cooling loads during the summer and avoids excessive temperatures - overheating - caused by high solar and intermittent internal thermal gains. Thermal mass delays the appearance of the maximum temperature values (time lag) and also reduces their absolute values.
Thermal mass should be combined with adequate insulation of the external envelope of the building. This combination leads to reduced mean internal temperatures and to satisfactory thermal comfort conditions during the summer.
In order to place the thermal mass within the building, and distribute it correctly a series of parameters should be taken into consideration. These are: building and facade orientations, the thermal insulation of the building, the potential ventilation provided and its characteristics, the external climate, the operation schedule and the control mechanisms of the installed heating and cooling systems and the thermal characteristics of the building materials.
In existing urban buildings of a given design and mass distribution it is very important to combine the existing elements and their thermal capacity with the correct insulation of the envelope.
Thermal mass with no insulation effectively cancels possible solar gains during winter.
Thermal mass placed on northern facades or elements does not play a very important role, except in cases of urban buildings and especially in office buildings with large internal gains due to artificial lighting and office equipment.
One should not underestimate the role of thermal mass placed in the east west elements and facades, especially during the summer period. Both the external envelope of the building and the internal spaces become thermally charged by solar radiation, and can cause discomfort within the building if not properly insulated.
The contribution of thermal mass in combination with adequate insulation of the roof of the building should not be neglected. The roof receives solar radiation at an angle of almost 90o during the summer period, causing overheating problems in the spaces directly below.