The Low-Carbon House: Thermal Mass

• Thermal mass acts as a ‘thermal battery’
• Thermal mass plays an important role in the performance of a building by moderating fluctuations in space temperature. This role becomes more important as summer temperatures in the UK increase.
• The use of heavyweight construction materials with high thermal mass can reduce total heating and cooling requirements.

• The diagram shows the effect of thermal mass on indoor temperature. Whilst external temperatures in summer fluctuate between wide extremes, internal temperatures are moderated by thermal mass to within an acceptable comfort zone.

• There is no necessary correlation between thermal mass and structure. Both traditional masonry and more recent timber frame methods of construction can accommodate thermal mass.

• Research carried out by Arups (UK Housing and Climate Change – Arup Research and Development, 2005) reveals the likely failure of conventional and particularly lightweight forms of construction to meet with the demands of increasing temperatures in the UK. Arups demonstrate that thermal mass reduces the need for air conditioning whilst also reducing the consumption of winter heating fuel.

• In concluding their research paper ‘Thermal Mass, Insulation, and Ventilation in Sustainable Housing’ (University of Stratchclyde, 2004), Tuohy et al concur with Arups: ‘Thermal mass, ventilation, shading and shuttering are shown to have a large influence on summer peak temperatures with high thermal mass construction having a consistent beneficial effect.‘ They also noted that the IEA Sustainable Solar Housing demonstration houses reflected an increasing use of thermal mass in buildings towards southern Europe, ‘apparently driven by summer cooling’. The role of thermal mass in northerly locations was observed to be more marginal.

• Materials characterised by the expression ‘Thermal mass’ (aka ‘Thermal storage capacity’) are those that absorb heat, store it, and at a later time, release it.

• Thermal mass is measured in terms of ‘Volumetric heat capacity’. Volumetric heat capacity is the quantity of heat per unit mass per degree of temperature change or kJ/m³K.
• The effectiveness of Thermal Mass to absorb and emit heat is measured in terms of thermal conductivity. High conductivity implies a more rapid ability to absorb and emit heat. Conductivity is the quantity of heat transmitted in time through a thickness due to a temperature difference or Wm^-1K^-1

• High heat capacity (the ability to store large amounts of heat)
• Moderate conductance (must be able to transfer heat fairly well through conduction)
• Moderate density (cannot be too heavy or too light)
• High emissivity (must be able to easily emit, or give off, heat)

Thermal lag is a term describing the amount of time taken for a material to absorb and then re-release heat, or for heat to be conducted through the material.

Thermal Lag times are influenced by:
• Temperature differentials between each face.
• Exposure to air movement and air speed.
• Texture and coatings of surfaces.
• Thickness of material.
• Conductivity of material.

Thermal Admittance is a useful factor in assessing the likely performance of different materials during the design process. Thermal Admittance describes the controlling property of a material to exchange heat with the internal space due to a change in temperature over a period of time (usually 24 hours). It is measure in W/m² K, where temperature is the difference between the mean value and actual value within the space at a specific point in time.
It is high for heavyweight construction, and low for insulation

Thermal Admittance is influenced by:
• Thermal capacity
• Conductivity
• Density
• Surface resistance
• Ultimately admittance has an upper limit determined by the rate of heat transfer from the material’s surface to the adjacent air – though this can be increased through ventilation providing convective heat transfer.

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.

1 Heat is radiated through the surface of the mass by a warmer object (such as sun, lights, people, or equipment).

2 Heat is conducted from the warmer surface of the mass to the cooler interior of the mass, effectively "storing" heat in the mass.
3 When the mass surface becomes warmer than other objects surrounding it, the mass radiates heat to these objects (meaning the mass radiates heat back into the house).
4 Heat from the warmer interior of the mass is conducted to the surface of the mass as the mass cools (a reversal of step 2).

• The most effective construction materials are those with the highest volumetric heat capacity. In general, dense materials will generally have a higher thermal mass than less dense products. For example, dense concrete blockwork, rammed earth and mud bricks have a high effective thermal mass when compared to lightweight blockwork or wood.
• For thermal mass to be effective there must be minimal thermal resistance between the occupied space and the mass of the structure. The temperature fluctuations within the building fabric are greatest at the surfaces. Relatively thin layers of plaster can have a significant effect on the thermal mass by providing thermal resistance.

• In summer, thermal mass absorbs heat that enters the building. In hot weather, thermal mass has a lower initial temperature than the surrounding air and acts as a heat sink. By absorbing heat from the atmosphere the internal air temperature is lowered during the day, with the result that comfort is improved without the need for supplementary cooling.
• At night the heat is slowly released to passing cool breezes (natural ventilation), or extracted by exhaust fans, or is released back into the room itself.

• In winter, thermal mass in the floor or walls absorbs radiant heat from the sun through south, east and west-facing windows. During the night, the heat is gradually released back into the room as the air temperature drops. This maintains a comfortable temperature for some time, reducing the need for supplementary heating during the early evening.
• The most difficult period in winter is the early morning. The heat released during the night has dissipated, temperatures have dropped and the sun has yet to begin the heating process. During this time it will probably be necessary to use supplementary heating to warm the thermal mass before the air temperature rises.

• Thermal mass is most effective where exposed to direct sun radiation.
• Where not exposed to direct radiation, thermal mass relies on efficient convection.
• Comfort is improved if the mass is distributed evenly within a room.
• Thermal mass should be insulated from external temperatures for maximum effectiveness.
• Materials that make for effective thermal mass usually perform badly as insulators.

• The most important location for thermal mass is in south-facing rooms. To heat thermal mass effectively in winter, it should be optimised for exposure to direct winter sun.
• As the area of south-facing window increases, the more thermal mass is required to maintain a stable temperature.
• Thermal mass located within north-facing rooms is relatively un-important. It is frequently argued that thermal mass should be avoided altogether in bedrooms, so reducing an associated nocturnal rise in temperature.

• Summer conditions can lead to overheating to eastern and western facades. Consideration should be paid to locating thermal mass in these locations.
• Locate additional thermal mass near the centre of the building, particularly if a heater is positioned here.

• Area: The amount of useful thermal mass is determined by multiplying a material’s volumetric heat capacity (above) by the total accessible (that surface area exposed to the heat source) volume of the material.
Example:
A living room has 20 m² of thermal mass walling comprising exposed 100mm brickwork.
Volume of brickwork = 20 x 0.1 = 2m³
Volumetric heat capacity of brick = 1360 kJ/m³K
Therefore the amount of useful thermal mass = 2 x 1360 = 2720 kJ for every increase in degree of temperature

• Thickness: The effectiveness of thermal mass is conditioned by the volumetric thermal capacity and its thermal conductivity. In practice walls acting as thermal mass should have a thickness of no more than 150mm. Performance variation between 100 – 150mm is small. Marked transition occurs between 50-100mm and below 50mm performance improves rapidly.