Moisture risk in walls in apartment buildings!
August 27, 2015

Moisture risk in walls in apartment buildings!

The construction used in the simulations was chosen since it is a typical construction of the building envelope in Swedish multifamily houses from 1950-1975 [5]. Also due to vapor diffusion through the building envelope, the construction represents the worst case scenario for these houses regarding moisture in general. From the exterior to the interior, the material layers in the chosen building envelope are:

  • 1 ½ stone solid masonry
  • Air gap
  • 90 mm Mineral Wool + Wooden Studs
  • Vapor barrier (PE-foil)
  • 13 mm gypsum board

The air gap is usually very narrow (15 mm) and unventilated, and is thus almost insignificant for the dry-out potential in the wall. However, it does serve the purpose of separating the studs and the insulation layer from the solid masonry, which might absorb rain and transport water inwards by capillary suction. There are, of course, different versions of the construction in focus. Another example of a poor version is the same construction but without the air gap, letting suction occur between the studs and the solid masonry, which increases the risk of mold growth on the wooden studs. The same risk is posed in the case of a previously placed and later neglected wallpaper on the masonry wall – when internal insulation is added in a renovation. Also, the vapor barrier is in some cases  non-existent, or it has degraded through time to what can best be described as a white powder. The state of the vapor barrier in such a case means that the vapor can freely diffuse through the construction and further increase the risk of mold growth. The worst case scenario for moisture damage caused by the moisture supply might therefore be the combination of the chosen construction and the three observed examples. These together form case A, where the moisture supply adds to the risk of mold growth.

Arguably, the mold growth on the wallpaper is caused by the combination of absorbed precipitation and a low temperature, and not due to the vapor diffusion from the interior. The fact that vapor diffusion only accumulates a small amount of water compared to rain-water suction supports this argument. However, a counter-argument is that if the vapor diffusion from the interior is left uncontrolled, it might add to the accumulated rain water and increase the risk of mold growth. The risk due to this phenomenon has therefore been worth investigating. In addition, the risk for mold growth depends on a sufficient relative humidity (RH) in combination with a preferable temperature, sufficient time and sufficient nutrition. The RH may be high even though the water content in the material may not be. This means that the risk does not require as high water content as might accumulate from rain absorption. The vapor diffusion alone should therefore be sufficient to cause mold growth since it might increase the relative humidity to a point where the conditions are preferable for mold growth. In some houses, case A has been modified and become even more sensitive to vapor diffusion from the interior. An example is when vapor resistant paint is applied to the exterior surface of the facade. The idea is that such application will repel rain water, which is true, but it also limits the dry-out possibility for vapor diffusion from the interior. As a consequence, water is accumulated in the wall, making the moisture supply the the sole moisture source causing mold growth on the wallpaper. This example will hereby be referred to as case B.


[5] Björk C, Kallstenius P, and Reppen L, 2013. Så byggdes husen 1880-2000: arkitektur, konstruktion och material i våra flerbostadshus under
120 år. Svensk Byggtjänst, Stockholm, Sweden.

Excerpt from the paper on “Moisture supply Set Point for avoidance of moisture damage in Swedish multifamily houses” by Abdul Hamid A., Wallentén P., Johansson D.