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and concrete at room temperature are in the region of 1,007 Jkg-'K-1 and 880
Jkg-'K-1 respectively, whereas their volumetric heat capacities are in the region
of 1,170 jM-3 K-1 and 2,024,000
jM-3 K-1 respectively.
In considering the heat storage capacity of building materials in situ, it is the
volumetric heat capacity of materials which is of interest, that is the heat storage
capacity of any particular building element at its particular density. It should be
noted that if the volumetric heat capacity of the material can be found directly,
there is no need to establish the density of the material to understand its heat
storage capability. This means that samples may not need to be taken, weighed
and measured, thus avoiding potentially destructive testing.
Thermal diffusivitv
Thermal diffusivity, a (M2S-1 ), describes the ratio of thermal conductivity to
volumetric heat capacity:
a
PC
Equation (3)
Materials with a large thermal diffusivity respond quickly to changes in their
thermal environment, while materials with small thermal diffusivity values will
take longer to reach equilibrium with their surroundings (Incropera &
DeWitt, 1996). It is of note that where a and X are known, volumetric heat
capacity, pC, can be established immediately. The literature review, chapter 2,
will show that this feature has frequently been relied upon in thermal probe
analyses.
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