Automatic U-value measurement by local thermographic analysis

The measurement of U-value is an important issue for energy budget evaluation, but conventional techniques are not satisfactory. To date, IR thermography has barely been used for a qualitative measurement or to support other standardized measuring. The proposed method is able to very accurately detect the temperature difference between the air and a surface. The instantaneous heat flux through the inner surface of a room is monitored starting from a local mapping of such data. The improvements detailed in this paper include the evaluation of both convective and radiative heat flux on a local basis with a space resolution of 1 cm. In particular, the surface heat exchange coefficient is evaluated with a fast algorithm. A steady thermal state, with the help of the outside air temperature, allows for the thermal performance evaluation of the building envelope. In the case of a transient condition, an averaging process or a system identification algorithm must be applied on time-series of such data. An automatic procedure has been set up in order to accomplish this recording. In either case, it is shown that a conventional technique based on heat flux metering for the U-value measurement is much less informative about critical points than quantitative IR thermography. Tests in situ are reported and compared with results obtained both by the standard methods for measurement and computation.