FLIR i5 User Manual page 89

16 – Introduction to building thermography
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Figure 16.4 Stress concentration factor (C) distributions for various wind directions and wind velocities
(v) relative to a building.
Wind conditions can vary substantially over time and between relatively closely situ-
ated locations. In thermography, such variations can have a clear effect on the mea-
surement results.
It has been demonstrated experimentally that the differential pressure on a façade
exposed to an average wind force of about 5 m/s (16.3 ft/s) will be about 10 Pa.
Mechanical ventilation results in a constant internal negative or positive pressure
(depending on the direction of the ventilation). Research has showed that the negative
pressure caused by mechanical extraction (kitchen fans) in small houses is usually
between 5 and 10 Pa. Where there is mechanical extraction of ventilation air, e.g. in
multi-dwelling blocks, the negative pressure is somewhat greater, 10–50 Pa. Where
there is so-called balanced ventilation (mechanically controlled supply and extract
air), this is normally adjusted to produce a slight negative pressure inside (3–5 Pa).
The differential pressure caused by temperature differences, the so-called chimney
effect (airtightness differences of air at different temperatures) means that there is a
negative pressure in the building's lower part and a positive pressure in the upper
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Publ. No. T559382 Rev. a358 – ENGLISH (EN) – June 23, 2009