Professor Carsten Rode, DTU Byg, har ledet en undersøgelse af skimmelvækst ved indvendig efterisolering med faste plader. Foto: Jesper Kirkeskov.

All types of internal insulation may cause mould growth

Wednesday 23 Jun 21

Contact

Carsten Rode
Professor
DTU Civil Engineering
+45 45 25 18 52

How to prevent mould using rigid internal insulation boards

  • High pH value in the adhesive mortar inhibits mould growth. However, the durability of this effect is not known.
  • The best basis for interior post-insulation with solid insulation material is a dry, even, and clean wall. Mechanical cleaning—where all the plaster is removed from the raw wall—is the most effective method to remove mould residues and other organic matter.
  • It is an advantage to apply new render or fully putty the walls to avoid uneven surfaces and cavities where mould spores can grow if the pH value decreases.
  • It is generally not recommended to use internal -insulation if there is rising damp, moisture entry from the ground surface, a defective outer wall, or penetrating driving rain.

About the test

In a comparative study, four different systems with rigid internal insulation boards were mounted on test walls of solid masonry using the accessory adhesives. The test walls were placed in airtight chambers with controlled relative humidity. Measurements were made over a period of 12 months of the moisture content of the materials and the pH value of the adhesives. The presence of mould spores and mould growth were also examined. In addition, the effect of various methods to clean for old mould was tested.

It requires great care to avoid moisture and mould growth in connection with internal insulation of an external wall. A new study from DTU shows that this applies regardless of the type of insulation system used.

Many professional builders are aware that moisture and mould can develop if you install a stud wall with mineral wool and vapour barrier as internal insulation. Therefore, there is currently great interest in a number of new systems with rigid insulation boards without a vapour barrier, which are often marketed as more secure solutions.

DTU has studied how different solutions with rigid insulations boards work. The conclusion is that there is also a risk of mould growth with these boards.

"Systems with rigid insulation boards require great care when they are installed, just as is the case with a conventional stud wall with a vapour barrier"
Professor Carsten Rode, DTU Civil Engineering

“Systems with rigid insulation boards require great care when they are installed, just as is the case with a conventional stud wall with a vapour barrier,” says Professor Carsten Rode, who has headed the study.

No system is perfect

The study included four different insulation systems with rigid insulation boards that were glued directly on a number of test walls with an adhesive mortar.

Three of the systems in the test consist of diffusion-open, capillary-active materials of calcium silicate, aerated concrete, or polyurethane foam with capillary suction channels that can absorb condensation and transport it to the inner surface, where it evaporates to the indoor air. The fourth type was a so-called diffusion-tight solution of hard phenolic foam with a built-in aluminium foil that limits moisture penetration.

The study does not highlight one single best solution. There is a risk of mould growth in all insulation systems.

“There are pros and cons of the different systems, and there’s not one single system that is clearly better than the others. It depends on a number of factors, including indoor moisture load, whether the outer wall is impregnated, and whether there has previously been mould growth”, says Professor Carsten Rode.

Thorough pre-treatment

The results from the study are compiled in a new DTU Civil Engineering Analysis, which highlights a number of conditions that should be taken into consideration to prevent mould growth when using rigid insulation boards.

“The best basis for avoiding mould growth in connection with internal insulation is a dry, even, and clean wall. Mould spores are nourished by organic materials, and it is therefore important to remove any mould and other organic material from the original wall, such as remnants of wallpaper, glue, and paint, before attaching the insulation boards,” says Carsten Rode.

The study also showed that a high pH value in the adhesive mortar inhibits mould growth, but does not kill mould spores. A previous study has shown that if the pH value decreases to 9.5 or less, and if there is a high moisture level, there is a risk of mould growth.

Read more in the DTU Civil Engineering Analysis Forebyggelse af skimmel ved indvendig efterisolering med faste plader (Prevention of mould growth when using rigid insulation boards for internal insulation).

Read the whole report Kontrol og forebyggelse af skimmelvækst ved indvendig efterisolering (Control and prevention of mould growth when using rigid insulation boards for internal insulation) (BYG R-440).

The study has been funded by Grundejernes Investeringsfond (the Landowners’ Investment Foundation).

Learn more about internal insulation at International Building Physics Conference 2021

Internal insulation is one of the themes at the International Building Physics Conference 2021 that takes place online 25-27 August 2021. See the programme and register at www.ibpc2021.org 

Related videos  

video thumbnail image

video thumbnail image

video thumbnail image

video thumbnail image

Show more

News and filters

Get updated on news that match your filter.