Problems of Air Leakages
The most common reason (over 95%) of mould growth and rot within structural timber is caused by convection (air flow) as a result of air leakages. The air which penetrates into construction transports moisture into the insulation which, results in condensation when cooling down.

Up to 5g/m² (64grain/square yard) of moisture pass through a correctly installed internal vapor/vapour retarder with a s_d-value (mvtr, permeance) of 2.3m (11.5MNs/g, 1.43 US perms) and into the insulation system per day. By comparison, 800g/m² (10,226grain/square yard) of moisture can penetrate the thermal insulating material through a 1mm wide and 1m long gap in the vapor/vapour retarder.
This means: Performance down by factor 160.*

Development of Moisture Traps
The use of an impermeable vapour barrier e. g. polyethylene films reduces the diffusion of moisture into the insulation system, but this also means that the ability to dry out the insulation system is greatly diminished.

In constructions with a diffusion-tight sub-roof, flexible metal sheet, flat- or greenroof, moisture that has penetrated cannot evaporate away by passing through to the outside or to the inside. Instead the impermeable layers actually become moisture traps. This results in a considerable decrease in the safety potential of the insulation system.

The Answer to the Problem
The optimum solution to airtightness is a vapor/vapour retarder with a diffusion permeability that varies with the humidity conditions, i. e. permits lower permeability in cold winter conditions and higher permeability in warm summer conditions. In wintertime it reduces moisture ingress into the insulation system and in summertime it provides the right conditions for drying-out.

To Put it into Figures
The diffusion permeability of an intelligent vapor/vapour retarder like pro clima INTELLO^® is high in summer conditions and low in winter conditions.
This offers optimum safety for the insulation system even if there is an unexpected increase in moisture.

The difference between the quantity of condensation and quantity of evaporation (calculated with WUFI 4.0 pro) is only 10g/m² (128grain/square yard) in a period of one year when using a vapor/vapour barrier with an s_d-value (mvtr, permeability) of 50m (250MNs/g, 0.07 perms). By contrast when a pro clima INTELLO^® vapor/vapour retarder is used, the relevant figure is 4000g/m² (51,333grain/square yard) - almost 400 times as much.**