All PUR rigid foams of sandwich panels have practically the same thermal insulation properties. The specific thermal conductivity l10tr measured in the laboratory is 0.21 to 0.23W/mK, depending on the foam quality and type of blowing agent.
According to German regulations, this value must be increased by 10% due to the expected ageing of the foam, in order to then be classified in the next higher 5-group. There are the groups 0.025; 0.030; 0.035 etc. All sandwich manufacturers selling in Germany have a calculation value lR of 0.025W/mK for calculating the U-values. The thermal conductivity l is constantly monitored by the authorities.
The U-values (formerly k-values) of the individual sandwich panels are calculated by the manufacturers themselves from the l-value and the respective hard foam core thickness.
There is no official control of these values.
Even in the case of products with a completely identical cross-section, there used to be considerable differences. Today, according to information provided by us to the IFBS, an association of component manufacturers, there are still small differences.
It is relatively difficult to determine the effective average foam core thickness for strongly profiled outer shells, such as roof sandwich panels.
One manufacturer "forgets" to consider the line depths of the shells, another is not able to determine the effective average foam core thickness correctly, or he deliberately does it wrong in his favor.
In 1995, a computer program was developed by the TU Cottbus under the direction of Prof. Dr. Liersch, with which the k-values of trapezoidal profiled sandwich elements could be determined exactly. 
A publication in the trade journal wksb, issue 35/1995, shows the k-values stated by the companies, those determined according to a calculation method of the IFBS and those correctly determined by means of a new computer calculation program. The results were partly surprising.
Elements by Hoesch, Thyssen, Romakowski, Fischer and Forges Haironville were compared.
According to this, only Fischer Profil had specified the correct U-values of their sandwich elements, in the case of FischerTHERM T 65 even by 0.017W/m²K to their disadvantage.
Except Fischer Profile, all others had more favorable, but also wrong values.
The record holder was the Promisol 1001 TS 40 element with a k value increased by 0.072 W/m²K.
It is known that in the case of leaky longitudinal joints of sandwich panels much more heat loss occurs by convection, designated by the a-value, than by transmission, designated by the U-value (formerly k-value).
Our FischerTHERM joint with the DUO seal and the additional EPDM sealing profile has clear advantages.
Even if the joint width is not completely installed according to the instructions, the a-value of all FischerTHERM sandwich elements is practically 0, no air passage can be measured. Recent measurements at the Fensterinstitut in Rosenheim have once again confirmed this.
Germany, August 2003

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