The cooled potato store in this situation had approx. 8° C room air temperature with 95 - 98% relative air humidity it dripped in the winter frequently, in particular at the element longitudinal joint, this was criticized by the owner.
FischerTHERM® DL 120 sandwich panels with a U-value of 0.29W/m²K were used for the roof. Incidentally, the U-values contained in the company's documentation of sandwich elements do not take into account the minor thermal bridge influence of the longitudinal joint.
At 8°C indoor temperature and 98% relative humidity, the dew point temperature of 7.7°C is only 0.3°C below room air temperature of 8°C.
This means that condensation precipitates on all surfaces that have a temperature that is only 0.3°C lower than the ambient air. 
In order to avoid condensation water failure, the U-value of the roof elements must not exceed 0.342W/m²K at any point at 95% rel. air humidity and assumed - 10°C outside temperature. 

At 98% relative humidity, even a permissible U-value of only 0.128W/m²K is calculated.
With the FischerTHERM® DL 120 panel used, with a core thickness of 80mm and an average U-value of the surface of 0.29W/m²K, condensation does not yet occur at about 95% relative humidity, but in the area of the longitudinal joint condensation can already occur there due to the somewhat higher heat conduction.
Incidentally, if a roof panel with a core thickness of only 60mm and a U-value of 0.38W/m²K had been used, there would be more even dripping over the entire roof surface.
But even with the FischerTHERM® DL 140 element with a core thickness of 100mm and a U-value of only 0.23W/m²K, dripping would occur over the entire roof surface if the air humidity rose to 98%.
By improving the thermal insulation of the entire roof surface, e.g. by
bonding 60mm thick PUR insulation boards with aluminum vapor barrier on both sides, improves the thermal insulation, as the heat transfer coefficient U decreases from 0.29 to 0.17W/mK.
The U-value of only 0.128W/m²K required at 98% humidity would not be reached and it would still drip at this extreme humidity.
It would be better to air condition the storage room to lower the relative humidity.
There is no defect in the sandwich roof elements.
The formation of condensation is practically unavoidable under these extreme conditions. 
By sticking on additional insulation, dripping can at least be reduced, and even prevented by air conditioning.
Netphen, January 2004

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