2.2.3. Method 3—Infrared Thermography Method (ITM)

In the case of this method, the determination of thermal resistance performed as in Method 1 should be supplemented with radiometric measurements of the mean radiation temperature in the external environment and the mean radiation temperature in the room. This is because the formulas describing the unidirectional steady heat flow through building enclosures were derived assuming that the external air temperature and the mean external environment radiation temperature (representing the influence of the thermal radiation from the nearest surroundings of buildings) are equal. This assumption is a considerable simplification as a complex heat exchange occurs on the boundary surfaces of a building enclosure so that the heat flux density on a given surface of the building enclosure is equal to the sum of the densities of the heat fluxes transferred through convection and radiation. This means that the resultant heat flux proceeds from the building enclosure's external surface via radiation towards ambient radiation temperature, *Tr*, and via convection towards air temperature, *T*. The temperature, being a total thermal rating index of a physical environment, taking into account the radiation temperature of the surroundings and the air temperature, is known as operative temperature, *Top*, in environmental physics and is expressed by the formula:

$$T\_{op,i} = \frac{h\_{ci}T\_i + h\_{ri}T\_{ri}}{h\_{ci} + h\_{ri}}\tag{7}$$

or the relation:

$$T\_{op, \varepsilon} = \frac{h\_{c\varepsilon} T\_{\varepsilon} + h\_{r\varepsilon} T\_{r\varepsilon}}{h\_{c\varepsilon} + h\_{rc}} \tag{8}$$

where:

*Ti*, *Te*—air temperature on respectively the internal and external side of the building partition (K),

*Tri*, *Tre*—mean radiation temperature on respectively the internal and external side of the partition (K),

*hci*, *hce*—the convection heat transfer coefficient on respectively the internal and external side of the partition (W/(m2K)),

*hri*, *hre*—the radiative heat transfer coefficient on respectively the internal and external side of the partition (W/(m2K)).

Hence, the total thermal resistance of a building partition can be calculated from the relation:

$$R\_{tot} = \frac{\left(T\_{op,i} - T\_{op,\mathcal{E}}\right)}{q} \tag{9}$$

where:

*q*—the heat flux flowing through the building enclosure (W/m2).
