To explain this common building defect, we will summarize the most common cases and provide a concrete example. This deficiency basically consists of the growth of plant organisms in the interior part of the envelope of our buildings. This can lead to health problems and harm the health of building occupants, leading to respiratory and allergy problems.


Mold is a symptom of a constructive pathology and not a constructive pathology in itself. That is why the treatment of mold will be based first on finding the cause of its appearance. It is usually due to persistent surface condensation caused by these deficiencies:

  • Thermal bridges or absence of thermal insulation
  • Insufficient ventilation, excessive tightness of the system
  • Excess steam production

A very common mistake is to believe that mold on facades appears through leaks and apply waterproof products to facades

Concrete example:

It is observed that these lesions manifest themselves in the two bedrooms studied in a sensibly symmetrical way: we found them both on the ceiling, in the area where the slab (roof) meets the dividing wall (separation wall between neighboring houses). They both have a similar “constellation” shape and greenish color.

It is observed that in the East bedroom, the aforementioned lesion becomes larger and more intense in terms of size and color in proximity to the façade. In the West bedroom, the aforementioned lesion is located along the entire wall as well as in the East bedroom, on the ceiling where it meets the dividing wall. We can distinguish three types of shapes: a linear concentration on the edge between the facade and the dividing wall, isolated isolated appearances on the roof, and a linear appearance on the perimeter of the window.

It is observed that the texture, shape and color of the stains is characteristic of the presence of mold.

Figure 1. Mold under window

Figure 2. Linear mold on the edge between roof and facade

Figure 3. Mold in specific spots on the ceiling [/ caption]


The cause of surface condensation is in most cases an insulation problem that makes the wall so cold that the vapor from the environment condenses. Just like the windshield of your car when it’s cold outside, on a wet and rainy day.

Theoretical calculation of the condensations can be carried out easily. Figure 3 shows in the line “Saturation vapor pressure” the condensation pressure of the vapor inside the wall, calculated as a function of the vapor permeability of the different layers of the wall and the presence of vapor in the interior environment i Exterior. The other line “vapor pressure” will be the pressure of the vapor that we actually have in each layer of the wall. Normally at the intersection of these two lines there are condensations.

[caption id="attachment_447" align="alignnone" width="792"] Scheme 3. Temperatures inside the wall. [/ Caption]

Concrete example:

We consider that the most probable cause of this injury is the presence of humidity due to condensation on the underside of the slab, which has caused molds to appear. This condensation occurs on the interior face of the slab as its surface temperature is lower than the dew temperature of the interior environment. In the case at hand, due to the porosity of the surface finish (plastered and painted) and, therefore, to its suction capacity, the symptom of the injury is first the damp stain and later the detachment of paints and molds.

The most probable causes of the low temperature of the walls in the aforementioned locations are:

– A low temperature of the space under the roof, as can be seen in diagram 1, there is no layer specifically dedicated to insulation on the front of the façade, with the hollow brick and the concrete band as an enclosure only.

– Existence of thermal bridges. The thermal bridge of the dividing wall represented in diagram 2 (this explains the location of the lesions in the proximity to the dividing wall) and the thermal bridge of the upper floor of the bedroom represented in diagram 1.

This sum of factors causes the surfaces affected by the lesions to have surface temperatures lower than the dew temperature of the indoor air.

[caption id="attachment_443" align="alignnone" width="792"] Scheme 1. Detail section and thermal bridges

Scheme 2. Detail plant and thermal bridges. [/ Caption]


Normally, the repair proposal will involve properly insulating the envelope and breaking or treating the thermal bridges. Even if it is convenient, vapor barriers should be used.

[caption id="attachment_450" align="alignnone" width="792"] Figure 3. Insulation from the inside

Concrete example:

After studying all the sources, the technician is preparing to issue some technical conclusions:

Figure 4. Space under roof specific case

This technician considers that the appearance of moisture from condensation and mold on the ceilings of the East and West bedrooms on the upper floor of the building affect the minimum required health and habitability conditions and are due to the conjunction of these factors:

  1. The space under the roof, the vertical facing of the facade is insufficiently insulated.
  2. The slab in the meeting with the facade configures a thermal bridge that does not favor good thermal conditioning.
  3. The dividing walls in the meeting with the façade configure thermal bridges that do not favor good thermal conditioning.
  4. In windows the support piece of the sill probably sets up a thermal bridge. The insulation of the facade does not have continuity with the piece of carpentry and the piece of windowsill does have continuity to the interior.