Mold under the windowsill - what can this be?

Mold often forms in dark, cool corners in the room, even in cavities. Nevertheless, mold can occasionally occur even below window sills. What this may mean, which physical conditions can be responsible for it, and how best to remedy the situation can be found in detail in this article.

Formation of mold

The basic cause of mold is always moisture. On dry components in low humidity in the room no mold can form. The sources of this moisture can be varied:

  • Moisture from the outside (leaky windows, etc.)
  • thermal bridges
  • dripping condensation
  • "Sweaty" walls

Moisture from the outside

If windows are leaking, or if they are installed incorrectly, moisture can penetrate from the outside. This is more common than generally thought. Due to the leaks so much water penetrates that the area under the windowsill remains constantly wet. If mold spores are added, mold forms in these areas.

thermal bridges

A thermal bridge always arises where well-insulated components meet less well insulated ones. Often this happens after an energetic renovation, if the walls are insulated, but not the windows are replaced.

Dripping condensation

The U-value of the windows is then significantly higher than the U-value of the surrounding wall surface. If the warm indoor air hits the poorly insulated area, the dew point will be undercut and condensation will form continuously. If it drips down, the area below the windowsill may also be affected by the mold.

Sweaty walls

When a wall - or wall section - "sweats", the same thing happens as with a thermal bridge. The inside of the wall cools down faster in this area than in other places, and a small amount of condensation forms due to the impact of the warm indoor air. In the heating niche below the sill, this can be done if there is not enough heating to compensate for the heat loss.

Other causes

Unconnected or improperly connected vapor barrier tracks and other insulation faults (such as faulty or not performed dew point calculations) can also lead to the formation of condensation, which can then lead to mold growth below the window sills.

remedial measures

Basically, the cause should be determined once and corrected immediately. Without the elimination of the cause, the fight against mold will not be successful in the long term. A professional refurbishment only has to be carried out at infestation points that are larger than about half a square meter in total. In smaller places, it is sufficient to remove the mold, for example with home remedies such as alcohol or hydrogen peroxide. If the area remains dry afterwards, mold can no longer form at this point.

Tips & Tricks

In mold-prone areas, always make sure that the relative humidity in the room stays below 55% at all times. This can be regulated by proper and targeted ventilation, the control is best done with a simple hygrometer. On the other hand, you can control the humidity inside the wall section with special humidity meters for components.

Video Board: How To Remove Mold From A Window Sill (Without Harmful Chemicals)