FLIR InfraCAM User Manual page 124

12 – Introduction to building thermography
The surface temperatures used are the averages of surface temperatures on the
■
same material in an area near the anomaly on the inside and the outside of the
fabric. Together with the temperature of the anomaly, a threshold level can be set
dependent on these temperatures using the critical surface temperature factor.
These arguments do not obviate the need for the thermographer to beware of re-
■
flections of objects at unusual temperatures in the background facing the building
fabric surfaces.
The thermographer should also use a comparison between external faces facing
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different directions to determine whether there is residual heat from solar gain af-
fecting the external surfaces.
External surveys should not be conducted on a surface where T
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is more than 10% greater than T
For a defect that causes a failure under the 0.75 condition of IP17/01 the critical
■
surface factors are 0.78 on the inside surface and 0.93 on the outside surface.
The table below shows the internal and external surface temperatures at an anomaly
which would lead to failure under IP17/01. It also shows the deterioration in thermal
insulation that is necessary to cause this.
Example for lightweight built-up cladding with defective
insulation
Outside temperature in ℃
Surface factor from IP17/01
Outside surface temperature in ℃
Critical external surface temperature factor, after IP17/01
Insulation thickness to give this level of performance, mm
Local U value W/m
Inside surface temperature in ℃
UKTA TN1 surface factor
UKTA TN1 surface factor outside
12
Notes to the table
1 Values of surface resistances taken from ADL2 2001, are:
Inside surface 0.13 m
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Outside surface 0.04 m
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These originate from BS EN ISO 6946 (BN EN ISO 6946:1997 Building components
and building elements - Thermal resistance and thermal transmittance - Calculation
method).
110
– T
si
2
K
2
K/W
2
K/W
on the north or nearest to north face.
so
Good area
0
0.95
0.3
80
0.35
19.1
Publ. No. 1558299 Rev. a200 – ENGLISH (EN) – February 12, 2007
– T on the face
si so
Failing area
0
0.75
1.5
0.92
5.1
1.92
15.0
0.78
0.93