The psi Value - Evaluation of the Thermal Performance of Windows and Facades

Franz Feldmeier

The thermal quality of windows has substantially improved during the last few years. The U-value of the window framework has improved, as has the U-value of the glazing, but what about the linking zone between the frame and the glazing? The condensation risk at the glass edges was well known and some attempts were made, known as "warm edge", to increase the interior surface temperature, but this improvement was not qualified and hence disregarded.
The German standard DIN 4108 differentiates only between five groups of framing materials. A table will give the U-value of the window depending on the group and the type of glazing - a pleasingly simple procedure. Today, this approximate distinction is no longer appropriate, with European legislation ever tougher. With few people knowing about these new European developments, this article intends to set out the most important standards for the thermal evaluation of windows and extend these ideas.

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