Protection from wind and weather

The wind­proof­ing lay­er pro­tects the thermal in­su­la­tion from rain, snow and wind from the out­side. It also en­sures that the in­su­la­tion will not be per­meated by cold air and hence can fully un­fold its ef­fect­ive­ness.

This means:

The windproofing layer is critical for the optimal effect of the insulation. Mounted on the outside of the thermal insulation, it prevents cold outside air passing through the outer insulation layers as well as ventilating the insulation layer.
Fixed air pockets in the material are required for the insulating effect of cellulose, wood fibre, hemp, wool, mineral fibres, etc. The wind seal thus ensures the effectiveness of the thermal insulation and prevents the localised cooling of the surfaces facing the inside of a room.
The windproofing layer provides ventilated constructions with bottom-ventilated roof seals with an additional level of protection from any dripping secondary condensation, rain and driving snow.
A carefully executed windproofing layer increases the protection level to avoid convective air flows.

Unprotected insulation

Insulation by stationary air

Unprotected insulation:
Air movement in the porous structure reduces the insulating effect.

Protected insulation

Protected insulation material

Protected insulation:
No air movement possible in the porous structure, full insulation effect.

An example:
The thermal insulation effect of a woolen jumper is based on the stationary air inclusions in the fibres: as soon as a cold wind starts to blow, the insulation effect decreases.
However, the effect is restored if you wear a thin windbreaker, which itself has no significant heating function, over the jumper.

Airtight on the inside, windtight on the outside

For this reason, the insulation material is sealed on all sides in the ideal insulation structure: outside with the windtightness layer, e.g. an underlay or facade membrane that is open to diffusion, and on the inside with an airtightness layer, e.g. a vapour retarder.
The windtightness stops cold outside air flowing through the insulation. The airtightness provides protection against the entry of humid indoor air and thus against condensation and mould.

Note: Faultless installation work is important when installing air sealing, as leaks in surfaces and at joints will have consequences.