Preventing and fighting damp issues in your house

28 Sep 2021

Damp patches, mould, condensation: if you’ve been faced with any of these problems at some point, you’ll be well aware that damp is a stubborn foe to beat in the home. Issues like these are not just detrimental to your health; they are detrimental to your home too. As such, it’s important that you don’t give damp a chance, and that you take action at the first sign of any problems. In doing so, you’ll need to tackle the underlying cause, not just the symptoms. When mould starts to form, for example, people will often simply scrub it off without looking for the root cause: a (complete) lack of ventilation in the home.

Damp can be a complex issue, so it’s always a good idea to get a specialist involved. One thing should be abundantly clear though: you can’t simply tiptoe around the problem. It needs to be tackled straightaway — and we’re more than happy to help you get started.

In this article:

How does damp form in the home?

Moisture, the root cause of damp, is everywhere. First of all, there’s our very own bodies. Whenever we breathe out, we release moisture particles into the air. That by itself is enough to release around 300 to 400 ml of water into the air per person per day. Human beings also sweat, even when we’re not moving. We each produce around eight litres of water a day through perspiration, all of which evaporates and also ends up in the air around us. Then there are the activities we all carry out every day: having a shower in the morning, cooking some food at lunch, washing up afterwards and cleaning also add moisture particles to the air.


Vochtproblemen in huis kunnen leiden tot schimmelvorming op de muur van een woning. Om dit te voorkomen is ventilatie noodzakelijk.

Moisture can also originate from outside of the home. Porous external walls are often no longer able to stop rainwater, and some of it will find a way indoors through the walls. Groundwater can also seep into (old) walls and find its way up. These issues only add to the moisture levels in our homes.

How harmful is damp in the home?

Damp issues often creep into the house under the radar. Most people only take action once the symptoms become visible, but by that point, the problem has often been around for a while, and a significant amount of damage could already have been done. Symptoms of damp issues can come in the form of mould in the house, damp patches on the wall, condensation on the windows, or an unpleasant, musty smell that just keeps lingering...

If mould starts to form, our health could be at risk. Mould often releases toxic substances that can harm our respiratory system. In people with asthma, bad allergies or breathing difficulties, the consequences of this can be severe. Mould can also affect our immune systems, and when combined with other conditions or illnesses, that can have a major effect on older people or people in poor health.

Damp can also damage our homes. Once it starts to spread on walls and ceilings, serious problems can occur. Wood can start to rot, wallpaper and paint can start to peel and so on — and in severe cases, issues can appear in the basic structure of the home. One extreme consequence of damp in the home is dry rot. The fungus that causes dry rot can penetrate deep into masonry or grow behind plasterwork, meaning it can eventually make its way into wooden structural elements. Dry rot can severely weaken these structural elements, causing unrepairable damage to your home. Humid and relatively warm environments are the ideal breeding ground for the fungus that causes dry rot.

Where do damp issues most commonly occur?

Damp issues most commonly occur in the so-called ‘wet rooms’ around the house. This includes toilets, the bathroom, the laundry room and the kitchen. Due to the activities that take place in these rooms, this is where the highest volume of moisture is released into the air. Once that moisture starts to settle on the walls, issues as mentioned above can start to appear. It’s important that spaces like these are well ventilated, so that the excess moisture in the air is extracted outside. You can do so by opening multiple windows, by installing window or wall louvres in these rooms that draw in plenty of fresh outside air (= natural ventilation) or by installing a mechanical extract ventilation system (MEV).

Vochtproblemen op de muur van deze keuken.

Damp issues also commonly occur in bedrooms. Most people don’t realise that we lose a huge amount of moisture at night: just through sweating and breathing, we can lose between one and 2.5 litres of water every night. As bedrooms tend to be a little smaller than other rooms and we spend eight hours at a time there on average, the amount of moisture in the air is highly concentrated. This moisture can settle as condensation on the windows, ceiling or walls, which in turn causes mould and damp patches to form. Once again, proper ventilation is key: open the doors and windows every morning, or simply let your ventilation system do what it does best.

Fighting and treating damp issues

Once you’ve got a damp issue, it’s important to act as quickly as possible to prevent it from getting worse. As the causes can often be complex, it might be a good idea to get a specialist involved to tackle the root cause of the problem instead of simply fighting the symptoms. That said, the symptoms can provide clues to give us some idea of exactly what the root cause might be. Below, we’ve listed four types of problems you might encounter:

Damp patches on the outside and inside of walls: penetrating damp

In this case, damp is penetrating through the façade from the outside. Damp patches will appear both outside and inside. Penetrating damp occurs when the external wall has become porous and is no longer able to stop rainwater coming in from outside. In the long term, mould may also start to form alongside damp patches. The solution for penetrating damp is to apply a (colourless) protective layer to the external walls to make them less porous.

Damp patches at the bottom of walls: rising damp

In modern homes, a moisture barrier is placed between the walls and the foundation. The purpose of this membrane is to stop rising groundwater. Many older homes don’t have this kind of barrier, meaning damp patches at the bottom of walls (up to around one metre high) are a common occurrence. Over the course of time, these patches can cause problems to the masonry, plasterwork, paint and wallpaper. The best solution for this problem is to inject a moisture-resistant liquid into the walls. This chemical damp proof course forms a barrier against groundwater, meaning it can no longer penetrate into the walls.

Moisture settling on windows and walls: condensation

This type of moisture doesn’t originate from outside; instead, it is present in the air in the form of water vapour. In fact, we produce most of this moisture ourselves through breathing and sweating, and through everyday activities like cooking, washing and showering. All of these activities can contribute to excessive air humidity levels in the home. If the humidity level is too high, the moisture in the air will start to settle on the coldest surface areas it can find, such as windows, walls and ceilings. If this happens on a regular basis, mould may form on the wall as a result. As the walls are damp, this mould can spread very quickly, with disastrous consequences for masonry, paint and wallpaper — and that’s before we mention the impact mould can have on our health. Preventing condensation is possible by ventilating your home properly and sufficiently. Opening windows and doors every day for long enough or installing a smart ventilation system that extracts humid air outside can both help.

Vochtproblemen in huis door condensatievocht op de ramen.

Damp patches in the wall: leaks

The walls of your home hide lots of pipework to supply and remove water. If these are not properly joined together or if they start to wear, they can start leaking over the course of time. This can cause water to seep into the walls, causing damp patches as a result. One clue to a possible leak is a sudden rise in your water bills. In this case, an expert will need to track down and stop the leak using specialist equipment.

Preventing damp damage through ventilation

Condensation and the issues this can give rise to are the most common form of damp. There’s no way of avoiding moisture in the air, but you can prevent the harmful effects it may lead to. Proper ventilation plays a key role in this.
Our modern homes are very well insulated, meaning humid and dirty air often remains trapped inside. The moisture in the air can find no way out and starts to settle on cold surface areas like windows, walls and ceilings. Regularly opening windows and doors is one way to prevent any issues. That said, there are two major drawbacks to this method: opening the windows only has a temporary impact. As soon as you close the windows, the problem will return. The other drawback is that in the colder months, you’re ‘kicking out’ the heat in your home, leading to higher energy bills.

What you need is a structural solution such as window vents installed above the windows. These vents ensure a constant supply of fresh air and prevent any draughts from occurring. Of course, the amount of fresh air that comes in through window vents is relatively limited. Consequently, the best solution is to combine window vents with a ventilation system that mechanically extracts dirty and/or humid air. That way, dirty air is removed and systematically replaced by fresh outdoor air.

Renson ventilatiesysteem C, de Healthbox 3.0

Better still, when you opt for a smart C+ ventilation system (system 3 - MEV) like the Healthbox 3.0 shown above, you get a system that only ventilates when necessary. On the one hand, a system like this monitors and analyses the indoor air for the presence of CO2 and VOCs (volatile organic compounds), as well as moisture. Once these substances are detected and exceed the permitted value, the system will automatically extract the dirty indoor air. When you take a shower, for example, Healthbox 3.0 will notice that air humidity levels are too high in the bathroom, and it will provide extra ventilation for that room. Healthbox 3.0 is capable of doing so for every room in the home. When you’re cooking, for example, the system will only ventilate the kitchen, and not the living room, which is empty at the time. As a result, no heat and energy are needlessly lost. 

Renson waves, de ideale oplossing voor badkamerventilatie

If you’ve already applied the last few finishing touches to your home and you don’t quite fancy installing a centralised ventilation system (with all the demolition and cutting works this involves), you can also opt to install a decentralised ventilation system like Renson Waves in your wet rooms (in your laundry room, for example, or as a bathroom ventilation system). Just like Healthbox, Waves monitors the indoor air and will start ventilating fully automatically when certain substances, like moisture or VOCs, exceed a critical threshold. The system will keep ventilating for as long as necessary to return the indoor air quality to normal levels. The benefit of Waves is that is can be directly connected to existing air ducting. As a result, Waves is the perfect solution for renovation projects.

Detecting moisture: Renson Sense

If you’re still not quite sure whether you actually need a ventilation system, Renson Sense can help you find out. Above all else, Sense is a CO2 monitor, but it can also analyse the other parameters required for a healthy indoor climate. Moisture is one of these parameters. When air humidity levels start rising too high, Sense will let you know. Sense also keeps track of historical data through a connected app, so you can check at any time whether a problem might be structural. In that latter case, it’s best to install a centralised or decentralised ventilation system to prevent any future problems.

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