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Heat transfer by air movement
Mass transfer of air through the building envelope will carry
heat energy into or out of the building.
The rate of heat transfer is proportional to the rate of air
leakage as follows:
Where:
Q
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is the rate of heat transfer, in W/m2
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q
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is the rate of air leakage, in m3/s/m2
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is the density of air, in kg/m3
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Cp
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is the specific heat capacity of air, in J/kg/K
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Ti
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is the internal temperature, in K
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Te
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is the external temperature, in K
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The rate of air leakage depends on the air permeability of the building
envelope and the pressure difference across it, and is given by:
Where:
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is the air leakage coefficient;
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Pe
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is the external pressure, in Pa;
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Pi
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is the internal pressure, in Pa.
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Although
the wall is tested at a pressure difference of 50 Pa the average pressure
difference over time will be less. In the absence of further information
of the actual pressure difference the advice given in CIBSE TM23 should
be followed.
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