Thermal mass
Thermal mass, in the most general sense, describes the ability
of any material to store heat.
For a material to provide a useful level of thermal mass, a
combination of three basic properties is required:
- High specific heat capacity - to maximise the heat that can be
stored per kg of material.
- High density - to maximise the overall weight of the material
used.
- Moderate thermal conductivity - so that heat conduction is
roughly in synchronisation with the diurnal heat flow in and out of
the building.
Heavyweight construction materials such as brick, stone and
concrete all have these properties.
They combine a high storage capacity with moderate thermal
conductivity. This means that heat transfers between the material's
surface and the interior at a rate that matches the daily heating
and cooling cycle of buildings. Some materials, like wood, have a
high heat capacity, meaning that their thermal conductivity is low,
which limits the rate at which heat is absorbed. This cominbation,
which results in low thermal mass, can be useful in other ways.
Steel can also store a lot of heat, but in contrast to wood, steel
possesses a very high rate of thermal conductivity, which means
heat is absorbed and released too quickly to create the lag effect
required for the diurnal temperature cycle in buildings.
Concrete and masonry steadily absorbs heat that comes into
contact with its surface, conducting it inwardly, and storing it
until the surface is exposed to cooler conditions and its
temperature begins to drop. When this occurs, heat will begin to
migrate back to the cooler surface and be released. In this way,
heat moves in a wave-like motion alternately being absorbed and
released in response to the variation in day and night-time
conditions.
The ability to absorb and release heat in this way enables
buildings with thermal mass to respond naturally to changing
weather conditions, helping to stabilise the internal temperature
and provide a largely self-regulating environment. When used
appropriately, this stabilising effect helps to prevent overheating
problems during the summer and reduces the need for mechanical
cooling. Similarly, the ability to absorb heat can help reduce fuel
usage during the heating season by capturing and later releasing
solar gains and heat from internal appliances.
Benefits of thermal mass
Exploiting thermal mass on a year-round basis is not difficult,
but does require consideration at the outset of the design process
when requirements for the building form, fabric and orientation are
being established. Providing this is done sympathetically, a more
passive approach to design can realise benefits which
include:
- Enhanced energy efficiency and carbon savings over the life of
the building.
- Improved daylighting.
- Improved ventilation and air quality.
- Optimal decrement delay (time lag) and decrement factor (heat
flow) for reducing heat gains in summer.
- Good summertime comfort and a reduced risk of overheating.
- A measure of future proofing against the effects of a warming
climate.
- Reduction in the need for more expensive low and zero carbon
technologies to meet CO2 targets.
- Enhanced property resale value.
Concrete proof
Oriel High School, Crawley features an in-situ concrete frame
with precast concrete wall panels, insulated and rendered
externally. The underside of the floor slabs at both levels, which
are exposed concrete cast in situ, provides thermal mass that can
be cooled overnight via opening windows to help to moderate
internal temperatures when daytime external temperatures are
high.