In January 2018, a revision of DIN EN ISO 10077-2 was published, that now provides for two different methods to calculate the thermal performance of profiles. What does that mean for the representative Psi-values stated on the BF data sheets? This question was discussed during a meeting middle of 2017, when the Warm Edge Working Party decided to clarify this question with another project order to ift Rosenheim.

DIN EN ISO 10077-2 specifies a method for the calculation of the thermal transmittance of window frame profiles Uf and of the linear thermal transmittance of their junction with the glazing Ψg. The Psi-values for façade profiles are calculated according to this standard as well. A two-dimensional cross section of the window- resp. façade profile is modeled in appropriate software and rasterized with a resolution sufficient for the numerical simulation. Air-filled cavities are consigned with prescribed key figures, so-called equivalent thermal conductivity values („single equivalent thermal conductivity“), considering all three mechanisms of thermal transmission – conduction, convection and radiation.

The revised version of the standard dated January 2018 now includes an alternative approach for the treatment of air cavities. This new method is considerably more complex. It still considers the influence of thermal conduction and convection with an equivalent thermal conductivity value, but the heat transport through radiation inside the cavities is calculated separately according to the radiosity method.

According to DIN EN ISO 10077-2:2018-01 the simulator now can choose which of the two valid methods he wants to use for air cavities.

To determine the values of the BF data sheets, firstly, several test specimens made of stacked spacer profiles are measured with the guarded hot plate method according to ift guideline WA-17/1. Then, the equivalent thermal conductivity value of the spacer system is calculated with statistical evaluation from the measured results. Finally, with this declared conductivity value, the representative Psi-values stated on the data sheets are calculated according to DIN EN ISO 10077-2 and the prescriptions of the ift-guidelines WA-08/3 and WA-22/2. Aim of the project was to clarify whether if the choice of the calculation method has any influence on this determination of representative Psi-values. Based on the results it was planned to specify one of the two methods as a uniform procedure in the regulations of the Warm Edge Working Party.

Within the framework of the project to compare the methods, the cross sections of the representative window- and façade profiles and glazing designs were calculated with three spacers of different thermal performance. As expected the method only has minimal influence on the Psi-values, which is within the calculation accuracy. In nearly all cases of Psi-values for windows, a difference could only be recognized in the fourth decimal place. For façade profiles, the differences were slightly higher, up to 0.002 W/(m∙K).

During the meeting of December 5th, 2017, the BF Warm Edge Working Party decided, that from January 1st, 2018, all newly established data sheets shall solely be calculated with the new radiosity method. This will continue to allow for a real comparability. Because of the very minor impact, data sheets for windows published before January 1st, 2018 will remain valid, whereas the data sheets for façade profiles were recalculated according to the new method and updated end of October 2018.

The Warm Edge Working Party

The Warm Edge Working Party is a subcommittee of the Technical Committee of Bundesverband Flachglas. The participants in the working group are members and sponsoring members of the BF. Scientific support for the working group is provided by Mr. Norbert Sack, ift Rosenheim.

This text was a press release of the BF Bundesverband Flachglas of November 2018.