Thermal bridges: a two-dimensional and three-dimensional transient thermal analysis.

Thermal bridges are parts of the building envelope where, due to the two-dimensional or three-dimensional character of the heat conduction, either the inside surface temperatures are rather low, which can cause condensation, or the heat losses are rather high. In this paper thermal bridges are analyzed by numerical methods, shortly described in the first section. They are based on energy balance techniques. Features of these models are their implementation on personal computers, the simple use and the graphical output by means of plots of isothermals and streamlines, permitting a direct evaluation of both aspects of thermal bridges. As shown in the next section, a two-dimensional steady-state analysis is sufficient to quantify both thermal bridge problems. The importance of thermal bridges in uninsulated or insulated cavity wall& or massive walls is deduced. For example, for traditional insulated cavity wall constructions, the conductive heat losses exceed the one dimensionally calculated heat losses with 30% to 40%. The application of cavity insulation will not affect the condensation risk; it is also shown that with regard to this condensation risk, outer insulation of massive walls is much favorable than inner insulation. In a third section further study based on three dimensional and transient calculations shows some particular points of interest: the symmetry between an observed mold growth pattern and the calculated isothermal field, the influence of inside surface conductances, transient surface condensation, and the relativity of one dimensional heat loss calculations for complex-shaped buildings.