Retrotec USACE - Manual (Page 349)

Appendix G G19
Air leaks decrease or increase internal surface temperatures very quickly.
In spite of these data and information, we hardly know how the surface tem-
peratures vary in well-constructed, high-quality buildings.
The crucial problem is: What are the lowest temperatures of internal struc-
tures and structural elements that could be considered “normal” under par-
ticular weather conditions?
The interpretation problem could be divided into three parts:
The limitations of the method (the thermography practice) ■
The performance of the structures under various conditions ■
The conclusions that are obtained by considering the above mentioned ■
factors
One way to analyze the results is to use thermal index (TI), as described in
section G10.
The surface temperatures should not decrease below the dewpoint, and
should be at RH = 70–80% at the lowest temperature that equals the indoor
air temperature when it is cooling from the current level to the level at which
RH is 70–80% (the limit for biological growth to occur).
If the temperature distribution differs from expectations, defects can be
assumed to take place. The problem is that in addition to simply detecting de-
fects, their quantitative evaluation should be done. In the ideal case, this would
mean evaluating a U-value or fi nding an approximate estimate of moisture con-
tent. In theory, this would be possible under dynamic conditions, when a struc-
ture with different thermal characteristics cools down or warms up slower or
faster than its surroundings.
The thermal scanning method can be classifi ed as quantitative and qualita-
tive. Presently, IR thermography is mainly used as a qualitative technique for
locating insulation defects or searching for objects inside structures without
trying to determine the absolute U-value or evaluate an impact of detected
defects. The quantitative methods would give better information, but they re-
quire more time and expense. For instance, a straight comparison of insulation
defects on the basis of thermal images can be quite misleading if the object
emissivity is varying, or if measurements are done under different thermal
conditions.
Thermography is also divided into a passive and active method. The pas-
sive method means imaging objects in natural conditions. The active, or dy-
namic, method requires an external heat source that is used to stimulate a
tested object in order to analyze its temperature response, and thus to deter-
mine its hidden features. IR thermographic inspection of buildings is mainly
considered a passive procedure that is often called “predictive maintenance”
or “condition monitoring.”
In its active mode, IR thermography is typically used in the evaluation of
building material properties and in some other cases in which a natural (sun)
or artifi cial (electric bulbs, fans) heat source can be used.
In many applications, there is no principle difference between active and
passive IR thermography. In maintenance also active methods can be used. For