Photo: Bunch Bygningsfysik

The mystery of the weeping walls

Monday 17 Jun 19


Carsten Rode
DTU Civil Engineering
+45 45 25 18 52


Niels-Jørgen Aagaard
Head of Department
DTU Civil Engineering
+45 45 25 18 77


Kurt Kielsgaard Hansen
Associate professor
DTU Civil Engineering
+45 45 25 18 26


Wolfgang Kunther
Associate Professor
DTU Civil Engineering
+45 45 25 17 52


The MgO boards are imported from China and are primarily used as windbreak panels in new buildings and renovation work.

DTU Civil Engineering and Bunch Bygningsfysik receive the first enquiries about ‘weeping’ walls.

The MgO boards are analysed in DTU’s laboratories, and it is discovered that the salts in the boards absorb moisture from the air and discharges it as salty drops which can corrode metal, infect wood, and provide a breeding ground for moulds.

In March, The Danish Building Defects Fund issues a warning to the industry about the use of the boards. All MgO boards need to be replaced—and any damage to other building components must be repaired. But who is responsible and must pay the cost?

The Arbitration Tribunal delivers its judgment in the first case on the use of MgO boards and places the responsibility on the consultant of a construction project.

The Arbitration Tribunal’s second judgments places responsibility on the contractor of a construction project.
Magnesium-oxide boards, or MgO boards, were used in construction from 2010-2015 as windbreak panels until it turned out that the boards had a frightening ability to absorb moisture.

Classic horror stories often begin on a dark and stormy November night. This story, which turns into a true horror story and has been described in the press as one of the worst construction scandals ever in Denmark, also starts in November.

The year is 2014. Whether it was dark and stormy is uncertain, but according to DMI it was certainly a very wet November. Humidity levels were so high that a number of new buildings started acting strange: The walls were weeping. Water came through the surface and was discovered when the moisture had darkened the underlying foundation behind the façades or started to run down windows, where it left a peculiar greasy residue. Just like real tears, the drops seeping out of the buildings were salty.

At DTU Civil Engineering, both Professor Carsten Rode and Associate Professor Kurt Kielsgaard Hansen receive a phone call from a perplexed architect who tells them about the disturbing sight.

“At that time, no one had discovered or mentioned the MgO boards’ ability to draw moisture out of the surrounding air and then release it again as drops when the humidity reaches a very high level,” says Carsten Rode.

In Denmark, MgO boards were, at the time, primarily used as windbreak panels on outside walls in new buildings and for renovations, where the moisture problems are not directly visible because the boards are often hidden behind the facing.

The boards are produced in China and were introduced to the Danish market in 2010, where they became the dominant solution for windbreak panels over the next five years because of their advantageous properties such as good burning behaviour, easy handling, and low prices compared to the products used previously.

Data needs verification

In Vedbæk in November 2014, Tommy Bunch-Nielsen, owner of the consultancy company Bunch Bygningsfysik (building physics), also received a call, this time from a contractor who also described strange moisture problems from a building elsewhere in the country. Bunch Bygningsfysik has worked together with DTU Civil Engineering for many years, and a close collaboration was now once again needed. Because, as the MgO boards quickly became the main suspects, the data had to be solid, says Tommy Bunch-Nielsen:

“We performed the preliminary analyses of the MgO boards ourselves, but in order to achieve precise analyses we contacted DTU Civil Engineering which has the right equipment for the job. As it also appeared to be a major issue, it was important to us that DTU was involved so we were sure that our data from the analyses of the boards was completely correct before we came to any conclusions.”

On behalf of The Danish Building Defects Fund, Bunch Bygningsfysik started the process of uncovering construction with MgO boards in social housing.

Researchers stunned

Bunch Bygningsfysik then had four samples of MgO boards from different construction projects tested at DTU.

The tests took place in so-called climate chambers. A climate chamber is the size of a small refrigerator, and inside you can control the temperature and humidity and also measure the moisture retention of the material. The climate chambers operate at different humidity levels, so the behaviour of the boards can be analysed.

Data from the readings showed that the MgO boards are able to absorb enough moisture from the air to increase their weight by a staggering 160 per cent. In comparison, a building material such as wood typically increases its weight by only 30 per cent when it absorbs moisture from the air at relative humidity levels above 95 per cent, says Associate Professor Kielsgaard Hansen.

Even though the researchers had suspected that the MgO boards would absorb a lot of moisture, they were still stunned by the readings.

“You don’t believe your own eyes. We checked if we had read the readings wrong, or if there was some error with the equipment or the experiment. But there had been no mistakes.”

After a month inside the climate chamber, the boards suddenly began to lose a bit of weight. But unfortunately the explanation for this was not good news.

“The boards started growing moulds, which is what caused the weight loss,” says Kurt Kielsgaard Hansen.

Why the boards weep

"You don’t believe your own eyes. We checked if we had read the readings wrong, or if there was some error with the equipment or the experiment. But there had been no mistakes."
Kurt Kielsgaard Hansen, Associate Professor at DTU Civil Engineering

A third researcher from DTU Civil Engineering, Assistant Professor Wolfgang Kunther, then began to study the material composition of the boards through an electron microscope, where you can define the content of a material at the atomic level, as the Chinese boards did not come with very detailed product information.

The extreme moisture retention of the MgO boards is due to their high levels of different salts, of which the most dominant is magnesium chloride.

“However, the magnesium chloride is chemically combined through the content of magnesium oxide in the boards, which is where they get their name from (MgO). This is probably why no one were initially aware that the boards contain quite a lot of salt,” says Professor Carsten Rode.

And it is the salts that cause the problems. The salts absorb moisture from the air, and when the humidity reaches a certain point—the so-called weeping point, which is 84 per cent for some of the MgO boards—a chemical reaction takes place inside the boards that makes the moisture seep out in the form of drops: The boards weep.

And in November 2014, humidity levels in Denmark were above 84 per cent on most days. Once the salty drops have arrived, they wreak havoc.

In buildings where the MgO boards are surrounded by metal and fastened with screws, the metal parts start to corrode, creating a risk of collapse. And where the boards are surrounded by wood, the wood is infected by the salts, which in turn almost triples the wood’s moisture retention. This creates a risk of moulds in the structure.

After an extended period in a humid environment, the boards themselves start to break down.

 Kilde: DMI og Carsten Rode  

When humidity is above 84 per cent, some MgO boards start to release salty drops. This is called the weeping point and is marked with a red line in the figure. In November 2014, humidity levels in Denmark was above the weeping point on most days.

Source: DMI and Carsten Rode. 


Knowledge must be shared quickly

Once the data had been collected and verified at DTU Civil Engineering, the results were handed over to the construction industry as soon as possible. In 2015, The Danish Building Defects Fund issues a warning against the use of MgO boards, along with Bunch Bygningsfysik. Both DTU and Bunch Bygningsfysik still receive enquiries about weeping walls—now mostly from abroad.

“I estimate that most cases have been identified in Denmark, but the same problem is now occurring in several other countries. I have received a number of enquiries from England, as they have discovered that we know a great deal about the boards here in Denmark. We have become a kind of leading international experts in MgO boards,” says Tommy Bunch-Nielsen about the knowledge of him and his DTU colleagues.

New materials should be tested

The expertise of Tommy Bunch-Nielsen and Carsten Rode is also used as a basis for legal cases where it must be decided who is to pay for the replacement of the MgO boards as well as for any rectification of damage caused by the boards. Bunch Bygningsfysik estimates that around one million m2 of MgO boards has been used in Denmark and that the replacements and repairs will cost approximately DKK 2 billion.

“This is the first time that the Danish construction industry has seen such extensive damage from a single material. It will be extremely expensive to replace all MgO boards, and it’s a huge loss for the industry companies and for society,” says Tommy Bunch-Nielsen.

And according to him, it might have been avoided.

“When a new material spreads all over the market like this, a neutral party such as DTU should be asked to assess whether it’s the right material from a purely scientific point of view.”

Head of DTU Civil Engineering Niels-Jørgen Aagaard agrees and points out over the past few years, DTU has invested heavily in new laboratory and test facilities which can contribute with knowledge of new building materials.

“We have the knowledge and facilities to test and analyse building materials. We can help to ensure a knowledge-based basis for the choice of new materials or for new uses of known materials, so we can avoid cases like the MgO boards. This can contribute to ensuring that innovative products find their way to the construction industry without gambling with the industry economy or people’s health.”

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