part. At a certain height there is a neutral zone where the pressures on the inside and
outside are the same, see the figure on page 116. This differential pressure may be
described by the relationship:
Air pressure differential within the structure in PaΔp
9.81 m/s
2
g
Air density in kg/m
3
ρ
u
Thermodynamic air temperature outdoors in KT
u
Thermodynamic air temperature indoors in KT
i
Distance from the neutral zone in metersh
If ρ
u
= 1.29 kg/m
3
(density of air at a temperature of 273 K and ≈100 kPa), this pro-
duces:
With a difference of +25°C (+77°F) between the ambient internal and external tem-
peratures, the result is a differential pressure difference within the structure of about
1 Pa/m difference in height (= 3.28 Pa/ft.).
Publ. No. T559597 Rev. a554 – ENGLISH (EN) – September 27, 2011 115
27 – Introduction to building thermography
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