Thermographic analysis of the thermal properties of wood for wooden windows

摘要

The frames of windows are typically made of wood in Italy, even though aluminum, PVC and other materials are more and more utilized in the building manufacture. On the other hand, the growing attention on the problem of energy saving makes more stringent the attention to the insulation properties of any component of the building envelope. Therefore, it is paramount to evaluate the thermal properties of wood that will be utilized in the windows frame manufacture. Wood is a material characterized by a high anisotropy due to its characteristic growing. Mechanical properties, and thermal as well, are very different if considered along the direction of grain or perpendicular to it. In manufacturing the frame for windows, the fiber or grain direction must be selected in such a way to maximize the thermal resistance along the inside to outside direction, that means the inside/outside direction of frame (i.e. inside/outside direction of window) must be perpendicular to the grain direction. Indeed the grain direction is the one with the maximum thermal conductivity while the perpendicular one (crossing the fiber direction) owns a lower conductivity value. The anisotropic characteristics of wood made it a challenging material for the measurement of thermal conductivity. Three types of wood have been measured: oak, larch and spruce. Two instruments have been utilized: a) the hot disk apparatus; b) the IR thermography equipment in transmission (a variant of the Parker's method) and reflection scheme complemented by density and specific heat measurements. In particular, IR thermography gives the possibility to evaluate by images the preferential direction of heat propagation by looking at the deformation of a localized heat source released on the surface (i.e. a circular shape can become an ellipse as heat diffuses on the surface). Results coming from different kind of measurements are compared and critically considered.