Estimation of the surface thermal resistances and heat loss by conduction using thermography

The thermal insulation of a construction is established by the resistances of the materials employed and the thermal surface resistances, which may vary significantly from one building to another because of its location and climatic conditions. The importance of the surface resistances has been underestimated probably due to its complex calculation and variability since they depend on the differences of temperature and humidity, wind, shape of the envelope, surrounding obstacles such as trees, and other buildings. In this work, infrared thermography is employed to obtain the internal and external surface temperatures of the façades of a representative construction as part of a methodology to quantify the heat loss by conduction through the building envelope. U-values that encompass the characteristics of the envelope components and the thermal surface resistances are used. It is found that the thermal resistances of the air boundary layers vary during the day and, in most cases, are much greater than the values currently suggested by local regulations, thus significantly reducing the effective thermal conductivity of the envelope with respect to the design value. The findings show how important it is to quantitatively determine the heat flux through the building envelope on site for an already existing construction and corroborate the value estimated in the design stage.