At 15 prominent churches and cathedrals in 7 European countries the environmental corrosive stresses at the stained glass windows were assessed by glass sensors. Special emphasis was given to the protective properties of different types of protective glazings. The obtained total stress level data, based on Ml-glass sensor measurements, help to conclude on the appropriate choice of the ventilation systems and give information about specific environmental impacts. The results indicate that internally ventilated protective glazings have a higher protective effect than externally ventilated systems. THe nonventilated type of protective glazing even raised the corrosive stresses for the stained glass window. -- CAL