Thermal imaging does not detect water per se. It has been seen that energy interaction on the surface of a material is complex. Furthermore damp surfaces have an even more complex relationship with energy transfer due to the increased variable such as time, spatial factors and air movement.
There are two areas that thermal imaging can detect the moisture. Firstly is the detection of cooling by evaporation (D Jenkins, n.d.). As the thermal imaging camera has less than 0.050 C sensitivity (FLIR, 2010), the evaporative effects of the water can theoretically be seen. This effect can either work with the thermal capacity of water, or against it.
The second area is, as mentioned, the thermal capacity of water. It is so much higher that the dry material that it leads to a temperature differential. This effect is a function of the energy flux over time and the spatial distribution of the water over the dry substrate (E Grinzato, 1999).
The porosity of the substrate allows water to enter the pores which increases its weight as well as creating a greater “thermal inertia” as the water increases the thermal capacity of the material. (E Grinzato, 1999).
The camera will pick up the different heat signatures and is capable of effectively mapping out the moisture. Some texts, mainly based in the Mediterranean countries imply that the mapping can lead to a feel of how deep the moisture has penetrated the substrate. This in turn will give an indication as to the physical properties the material. (N. Avdelidis, 2003).This form of detection mapping, the qualitative approach, is a useful tool and has arguably a greater breadth of use than the more specialised quantitative approach. This is a specialist area that involves ‘high end physics’ (E. Grinzato, 1998) (P. Bison, 1994) and many additional measuring instruments as there are a great deal of variables such as air movement, temperature, porosity and humidity.
The qualitative mapping surveys enable the surveyor to theoretically “see” areas of moisture that is invisible to the naked eye. However most damage from moisture that is seen initially visually and can then be remedied without the need for IR. Therefore a potential cost by having the survey has been removed. However it is arguable that the damaged area could be rectified sooner and at less cost if an IR survey was conducted initially (Lyberg, 1990). Lyberg suggests that a survey would take 1- 2 hours and this is supported by the author. However a skilled operator is required especially when identifying moisture and therefore is a barrier for the wide uptake of thermal surveys.