Mold and condensation: if the solution is worse than the problem

We cover here two real examples of mold and condensation problems, with thermal insulation installed on the inside of existing masonry walls.

In both cases, the solutions were chosen on an empirical basis, and turned out to be worse than the problems they were trying to mitigate.

The examples come from two separate residential buildings, both located near Reggio Emilia, Italy.

The first case is an habitable attic, showing condensation problems: in this article, it is shown only with details and finite element calculations. The second one is a bedroom, with mold problems, here illustrated with photographs (including the title picture).


The first example – the attic – has a condensation problem, with water drops forming on the internal surface of the wall, along the perimeter concrete beam at the junction between the external wall and the wooden roof.

Detail of the wall-to-roof junction, showing the concrete perimeter beam.

The wall-to-roof junction represents per se a geometrical thermal bridge: all edges of the thermal envelope are thermal bridges. On top of this, you need to add the thermal bridge effect caused by the material change in the junction, where the concrete “loses” more heat than the brick masonry. The external wall is made with standard baked clay, load bearing bricks, without any insulation (in this phase); the concrete beam is required due to seismic compliance.

The higher heat flow at the wall-to-roof junction, due to the geometry of the envelope and to the concrete perimeter beam.

As you can see from the image, the combination of geometry and material change, generates a higher heat flow between inside and outside: a thermal bridge.

Isotherms at the wall-to-roof junction.

This higher heat flow causes lower internal surface temperatures, with a lower thermal comfort level inside the room.

Simulation of surface condensation conditions according to ISO 13788.

From a health point of view, lower internal temperatures allow for mold and condensation to form on the surface of the wall. This is often the case also in energy efficient buildings that are not provided with a mechanical ventilation system.

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An example of mold forming on the interior surface of walls and ceiling – photo by Damiano Chiarini.

Although they are two separate phenomena, mold and condensation both depend on a combination of low internal temperatures and high relative humidity.

If you do know what causes these phenomena, the solutions are relatively straightforward. If you don’t, and of you try and fix the problem with empirical solutions, the solution may end up making things worse.


In the case of the attic, the contractor suggested to install a cavity wall on the inside of the existing masonry wall, with insulation in it. The reasoning line was: “whenever we have condensation, we always to this”.

Let’s analyse this option.

The solution suggested by the contractor, with insulation on the inside.

The cavity wall is supposed to be 5 cm (2”) thick, with 4 cm insulation, and drywall finish.

Heat flow at the wall-to-roof junction, in the case of internal insulation with a cavity wall.

The heat flow at the wall-to-roof junction would in fact be lower, thanks to the insulation included in the cavity wall.

Isotherms in case of internal insulation.

If the heat flow at the junction is lower, the surface temperature at the junction is higher, with a higher thermal comfort.

Finite element analysis of the risk of condensation, according to ISO 13788.

The cavity wall with insulation eliminates the risk of condensation on the inside surface of the drywall, at the junction with the roof. Same result for the risk of mold: no apparent problem on the surface of the drywall.

Glaser diagram of the external wall, including the cavity wall insulation, according to ISO 13788.

If the problem seems to be solved at the junction, the truth is that it is only hidden. With a Glaser diagram analysis (which allows to assess the risk of condensation within an assembly), you can see that now the problem is present not only at the junction, but throughout the assembly. In this case, the insulation in the cavity wall causes the temperatures on the masonry wall to be too low, causing a widespread condensation problem.

Luckily enough, in this case the home owner followed our advice, and decided not to install the cavity wall.


In a very lucky way for the intention of this article, the second example demonstrates the validity of the analysis described above.

In this case, the object is a bedroom, where an internal cavity wall with insulation was installed along the external wall.

The cavity wall in the bedroom, before its demolition – photo Emu

At the time of our site visit, the bedroom had a strong smell of mold, although it showed no visible signs of it.

The cavity wall was demolished, unveiling a widespread mold problem on the plaster of the masonry wall – photo by Thomas Ficarelli

The demolition of the cavity wall brought to light a substantial mold problem, with the entire internal surface of the masonry wall affected by it.

Mold had thoroughly covered the entire internal surface of the wall, demonstrating the validity of the Glaser diagram illustrated above – photo by Thomas Ficarelli

Coincidentally, the situation we discovered in the second case, confirms the Glaser analysis done for the first building.

The second case was also was caused by an empirical approach (“we always do this”), lacking a correct approach to the problem under a hygrothermal and air tightness point of view. This does not mean that insulating on the inside is always bad: it means that you need to know what you are doing.

We also want to point out how transpirability of building assemblies is a parameter often overrated – we’re going to cover this topic in a specific article.


Healthiness of the buildings we live in must be a priority, above other goals such as energy efficiency.

We spend one third of our life in one single room – the bedroom: if this room is not healthy, it is going to make us sick, whether in the short run or in the long one.

Saving money and skipping appropriate technical analyses at first, is going to get back to you in the form of medical bills. It’s your choice.

Far too often, empirical solutions are selected with the motto “we’ve always done it this way, and we never had any problems”. Problems are there, they may just be hidden, as in the case of the bedroom above.

Physics is not democratic: your case is no exception.

To avoid mold and condensation problems, you need to address the problems with proper technical skills. The alterative is to face the consequences later, both economically (by redoing the work), and with your health.


The Glaser analysis and the finite element calculations have been carried out with Dartwin software.


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