When the roofing envelope is functioning properly, the vapour pressure on the inside of the envelope equals the vapour pressure on the outside.
By comparing these values, one can deduce the moisture content and the dry conditon of the envelope.
When a significant difference in vapor pressure is measured over time, it means that there is a problem with the roofing envelope.
The roofing envelope will ultimately become saturated, damaging both the envelope and possibly the building structure.
Here's an example from the headquarter of PROGEO in Großbeeren.
The green section shows no anomalies, but the red roof section displays an accumulation of water in the roof package.
