Vapor barriers prevents the penetration of moisture into the thermal insulation from inside of the house.
Without the vapor barrier membrane the moisture contained in the warm air could diffuse out of the residential interiors into the construction and thermal insulation.
The diffusion of water vapor in the air can result in the formation of condensation, which could significantly affect the effect of the insulation. In addition, the condensation can lead to massive damage to the timber construction of the roof truss.
This is where the vapor barrier membrane comes into play: as an efficient protector, it ensures that the humidity in the living areas does not penetrate into the roof structure and does not damage it.
Vapor barriers does not only prevent the entry of moisture into the insulation, but also counteracts the flow of air through the thermally insulated roof construction.