ᐅ Moisture Issues with Isover Vario Climate Membrane KM Duplex UV

Hello everyone,

We used the product Isover Vario Klimamembran KM Duplex UV, 1.5 m x 40 m (5 ft x 131 ft), which is considered better than a polyethylene (PE) foil. In winter, no moisture rises into the insulation, and in summer, any moisture can be released back into the indoor air.

The construction is as follows:

Uninsulated attic (with underlay membrane)
21 mm (0.8 inches) tongue and groove boarding
240 mm (9.4 inches) insulation wool with thermal conductivity WLG 035 (between the rafters; rafters are 240 mm (9.4 inches) high)
Isover Vario Klimamembran KM Duplex UV
80 mm (3.1 inches) wood fiberboard with thermal conductivity WLG 038
21 mm (0.8 inches) air gap
12.5 mm (0.5 inches) drywall

Now a friend mentioned that this setup might cause moisture problems. Moisture could form beneath the tongue and groove boarding. With a PE foil, this would supposedly not happen because it does not allow moisture to pass through.

Is this really possible, or is the construction okay as it is?

Thank you very much for your opinions.

Best regards

Simply add an air gap beneath the tongue-and-groove boards. This allows air to circulate and prevents moisture from accumulating too quickly. Tongue-and-groove boards are vapor-permeable, meaning that as long as the moisture is not excessive within a short period, it can also pass through.