Thermal mass

ALF and thermal mass in a house design.

Thermal mass describes the ability of materials to store heat. Heavyweight building materials can absorb the sun’s heat during the day (even if not directly sunlit) and release it slowly at night. In an insulated house they moderate daily temperature swings.

Exposed concrete floors are a common type of thermal mass in houses. Floors intended to provide thermal mass should not be covered by any insulating materials such as carpet and underlay (which have an R-value of approximately 0.4 m2 °C/W). Other floor coverings such as ceramic tiles and vinyl have small R-values and can therefore be included in the floor area.

Thermal concrete floor

Uncarpeted concrete floor contributes a large amount of thermal mass to a house. If the slab is laid on top of the ground, the ground also contributes to thermal mass. Slab-on-ground floors have a thermal mass of 300 Wh/m2°C.

If the concrete floor is suspended or the full area under the slab is insulated, the amount of thermal mass provided depends on the slab thickness.

Suspended concrete floors and fully insulated floors have thermal mass of:

  • 50 mm slabs – 28 Wh/m2°C;
  • 100 mm slabs – 56 Wh/m2°C;
  • 150 mm slabs – 83 Wh/m2°C.

Intermediate thicknesses can be interpolated.

Thermal timber floor

A timber floor not covered by carpet also contributes to thermal mass, but to a much lower degree than concrete floors. The type of floor covering determines the amount of thermal mass per m2:

  • a tiled timber floor – 20 Wh/m2°C;
  • exposed and vinyl-covered timber – 10 Wh/m2°C.

Carpeted floor has no accessible thermal mass.

Exterior walls

The thermal mass of exterior walls depends on their construction. The following thermal mass values are used for the available constructions:

Construction Thermal mass [Wh/m2°C]
Any internally-lined construction 9
Solid timber wall (44 mm) 7
Solid timber wall (62 mm) 10
Concrete blocks (unlined, inside exposed) 42

The area of external walls is counted only once – do not count the internal side of the walls again.

Interior walls

The thermal mass of internal wall construction types is the same as listed in the table figures for exterior walls. In this case too, only count one side of each wall.

Total thermal mass

The total thermal mass is the sum of the four individual thermal mass components plus a contribution for furniture and ceiling. ALF automatically assumes that ceiling and furniture contribute 4.5 Wh/m2°C and 2.5 Wh/m2°C respectively. This value is multiplied with the total floor area as an approximation of ceiling area and furniture density.

Effective thermal mass

Not all thermal mass takes part in daily heat flows – it is 'effective thermal mass' that actually makes a contribution. Once the total thermal mass reaches a certain threshold the effective thermal mass stays almost constant. The estimation of thermal mass in the building is therefore particularly important for low thermal mass houses because small changes in the total thermal mass can lead to significant changes in effective thermal mass.

Thermal mass background

The consideration of both the 'mass warm up' energy as well as the thermal storage of free heat in the calculations allows a specific evaluation of the disadvantages and benefits of thermal mass as a function of building design and heating habits. The method allows a decision on the appropriate amount of mass.

Wall or floor area, construction type and finish details are needed to calculate the thermal mass of a house. The effective thermal mass is calculated by adding all the significant contributors of thermal mass together (such as floors, external and internal walls, ceilings and furniture). The thermal mass of each of the contributors is calculated by multiplying its area with its specific thermal mass. The area used to calculate the thermal mass of a component is always only the surface area of the material exposed to indoor air (wall, ceiling, and floor areas). The total thermal mass is then divided by the total floor area. The resulting thermal mass density provides an indication of the accessibility and effectiveness of thermal mass in the building.

This approach does not explicitly account for orientation or placement of the thermal mass.

Effective thermal storage is that exposed to the inside of the building. Some judgement is required to decide whether particular floor and wall areas of the building should be included in the thermal mass. This is more critical for low mass building designs (suspended timber floors, timber framed constructions) since small variations in available thermal mass have a larger impact on the usability of gains. In heavy mass houses the decision is less critical as there is usually a large surplus of thermal mass available which is not used for thermal storage. In particular, exposed slab-on-ground floors have such high thermal mass that the effects of walls and furniture may become negligible.

The effective thermal mass density depends on the applied heating schedule. The units of the effective thermal mass density are chosen to be compatible with the specific heat losses (W/°C). Multiplying the value of the total floor area and the ALF-value gives the energy needed to warm up the effective thermal mass from ambient temperature to the temperature defined in the heating level.

Houses with a larger thermal mass generally have more uniform temperatures and are thus less likely to have condensation problems.

In the case of insufficient free heat (small windows, unsuitable climate, etc.) thermal mass absorbs heat from purchased heating and thus increases the amount of required heating. For houses with low solar gains, thermal mass can be a liability.