Understanding Thermal Mass
Thermal mass is a term used to describe the ability
of building materials to store heat (thermal storage
capacity). The basic characteristic of materials with
thermal mass is their ability to absorb heat, store it,
and at a later time release it.
Adding thermal mass within the insulated building
envelope helps reduce the extremes in temperature
experienced inside the home, making the average
internal temperature more moderate year-round
and the home more comfortable to live in.
Building materials that are heavyweight store a lot
of heat so are said to have high thermal mass
. Materials that are lightweight do
not store much heat and have low thermal mass
.
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The use of heavyweight construction materials with
high thermal mass (concrete slab on ground and
insulated brick cavity walls) can reduce total
heating and cooling energy requirements by up to
25% compared to a home built of lightweight
construction materials with a low thermal mass
(brick veneer with timber floor).
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Thermal mass is particularly important for comfort
in climates where
summer temperatures are high and there is a large
difference between daily average maximum and
minimum temperatures. Thermal mass is less
important, but still beneficial, in climates with lower
summer temperatures. However, in situations
where solar access is poor, thermal mass could
increase winter heating requirements.
Seasonal Effects of Thermal Mass
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Summer
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 |
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During the 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. Inside temperatures at night time
will be slightly higher than if there was low thermal
mass, however with the cooling night effects,
temperatures are still within the comfort zone
(unless a long spell of consistently hot days and
nights is experienced).
The ability of thermal mass to even-out fluctuations
in indoor temperatures is illustrated below.
The solid line represents the air temperature in
summer inside a double-brick house with a
concrete slab-on-ground floor.
The dashed line
represents the air temperature inside a lightweight
timber building. Note that the temperature variation
in the brick house is much smaller and temperatures
are almost always within the comfort zone.
Winter
In winter, thermal mass in the floor or walls
absorbs radiant heat from the sun through north,
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.
For good winter performance, thermal mass should
be exposed to direct sunlight and is best located in
areas with unobstructed north-facing windows. |
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An additional benefit is that some of the heat from
lengthy periods of internal space heating can be
stored in the thermal mass. Long after the heating
is turned off, the slow release of heat from the
walls or floor will maintain comfortable internal
temperatures.
Negative Winter Effects
In some cases thermal mass can actually increase
winter energy requirements. Where there is little
possibility of solar gain, either because north
windows are too small or are overshadowed (poor
solar access), the benefits provided by the use of
thermal mass will be minimal. Each time
supplementary heating is used, the thermal mass
needs to be heated before the air temperature
rises, increasing the heating energy needed.
Increasing the area of north-facing glass can help
offset this effect.
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Design 
