Roof insulation without vapor barrier: Diffusion-open systems improve moisture transport

As part of the roof insulation, the vapor barrier fulfills an important task. It prevents the ingress of moisture into the structure and prevents water damage. The problem: Vapor barriers only work if they are completely vapor-tight. In practice, this is almost impossible to accomplish. As an alternative, therefore, often vapor-permeable constructions are used, which only partially prevent the water vapor.

Roof insulation with vapor barrier - problems and risks

Ingress of moisture is a danger to any roof construction: If the insulation material becomes full of condensation, it is difficult to release this water. Mold forms, and also the insulation effect decreases with time.

The vapor barrier is installed as a thin layer - usually foil or kraft paper - on the inside of the roof. The diffusion-proof layer should prevent water vapor from entering the insulation from the interior. However, there are several problems:

  • The vapor barrier is only effective if it is completely free of damage. Moisture can get into the insulation over any crack, and the vapor barrier loses its effect for the most part.
  • An airtight vapor barrier is almost impossible to set up. Especially at cable openings and similar passages easily enters moisture.
  • Although the vapor barrier prevents the ingress of water vapor, once entered moisture is not released.

For this reason, instead of diffusion-tight vapor barriers often open to diffusion systems are used. These constructions allow moisture to escape and thus improve re-drying.

Diffusion-open construction instead of vapor barrier

For roof insulation without vapor barrier so-called vapor barriers are used, which in contrast to the vapor barrier are open to diffusion and thus fault-tolerant. Steam brakes do not completely prevent the ingress of water vapor, but limit it to an uncritical level. Entered condensation water is released gradually, so that no moisture can collect permanently in the construction.

As a vapor barrier can either serve the insulation itself or an additional layer of material. Often, polyethylene films and other plastic films are used. It is only important that the insulation is installed airtight, to prevent additional moisture transport via the air flow.

The "intelligent vapor barrier"

An alternative to the conventional vapor barrier is the climate membrane or "intelligent vapor barrier". The climate membrane is a breathable and at the same time waterproof construction film that changes its diffusion density depending on the humidity. It adapts flexibly to the prevailing humidity and temperature conditions:

  • At low humidity, the membrane acts diffusion-inhibiting. The ingress of moisture is prevented.
  • At high humidity, the membrane is permeable. Excess moisture, z. B. Residual moisture in the construction, is delivered to the room.

Climate membranes encounter a significant disadvantage of vapor vapor-permeable vapor barriers: In winter, there is a risk that moisture will enter the interior of the room. From closed vapor barriers in turn can escape even during the safe summer months no moisture. Due to the variable diffusion density, the climate membrane is able to release excess moisture and at the same time to prevent penetrating moisture.

Tips & Tricks

It makes sense to use a climate membrane, especially in new buildings, where there is still residual moisture in the roof beams. The risk of rot can be significantly reduced in this way.

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