Seal a window reveal


Every building needs a vent and must carry moisture away. One weakness here is quickly identified with the reveal. How and why you have to seal a window reveal, you will find in the following guide.

Comparison of a building with a cooking pot

Even if nothing and no one cooks or cooks in a house, you can better understand the characteristics of a cooking pot, which is why sealing the window reveal is so important. Take an ordinary saucepan or pressure cooker, which should be brought to a boil filled with water.

Like in a pot, pressure builds up

In the pot creates a different temperature as opposed to outside. The pressure conditions change inside the pot opposite the outside. With the saucepan, this pressure escapes through the raised lid, with the pressure cooker over the valve. Even in a building, pressure and temperature are always slightly different from the outside. Even in a building, a slight overpressure is created.

This pressure escapes where weak points are

This overpressure is derived where it finds a way out. Previously, the window frames were mounted with foam in the soffit. This mounting foam is gas permeable. So succeeds the pressure on this vulnerability to the outside. The air, which is highly enriched with moisture, strikes ever cooler layers within the masonry.

In the masonry, the air then cools down with fatal consequences

This also decreases the relative saturation, ie the amount of moisture that the air can take. Therefore, the air flowing outwards on its way more and more moisture to the environment, in the form of condensation or condensation. Almost every homeowner knows the devastating consequences of moisture in the building fabric.

Sealing of the window reveal over the course of time

So it is a real concern of the modern house, to seal the windows perfectly. Previously, the following materials were used:

  • hemp twine
  • building foam
  • with mortar (plaster) directly connected to the frame

Today, a sealing tape, often referred to as Kompriband used. This sealing tape is drawn in between the window reveal and window frame. Outside, explicitly in areas that are known for driving rain, often a veneer is attached, which is also sealed. Since the seal lies deep inside between frame and soffit, a butt joint can be seen from the outside.

Sealing the butt joint between window frame and soffit

This butt joint is now often grouted with silicone or acrylic. If the butt joint is even larger, construction foam is often even sprayed in before it is sealed with silicone. This injection of construction foam has nothing to do with the widespread installation technique of windows with construction foam at the end of the 20th century.

Joints with grout are working or maintenance joints

However, silicone and acrylic are sealing materials that are also inferior to aging. Therefore, so executed joints are working joints. This means that you have to grout the joints again and again to ensure the tightness of the grout. But spraying foam is not enough, because the moisture-enriched air must be prevented from penetrating into the building fabric in order not to condense here.

Inside always denser than outside

For modern windows where RAL window and door installation has been carried out, it is not sufficient to seal between window frame and soffit with construction foam or mortar (plaster). In addition, especially because of the risk of condensation: from the inside out more dense than from outside to inside.

Tips & Tricks

A complete and complete sealing of the window reveal presupposes, however, that an alternative house ventilation is present. For this reason, separate vents are installed in low and passive energy houses. However, the high degree of sealing is often the reason for mold, if there is no corresponding alternative venting and the residents should compensate by appropriate ventilation.


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