Help! Mould on the walls! What you need to consider before starting your wallpapering project

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First things first: Don’t panic if you find traces of mould on the wall after stripping the old wallpaper, or even if they already show on top of it - it’s not the end of the world. Mould isn’t to be taken lightly, natch, but as mould on walls can have a number of reasons, the first thing to remember is: Keep calm and go through the steps one by one.

Reasons and extent of mould growth on walls

Condensation as a result of inadequate airing or ventilation is one of the main reasons for mould growth in rooms. Bathrooms are a prime example. But putting furniture too close to the wall, thus creating a lack of air circulation, can also lead to moisture build-up. As a result, traces of mould are often only discovered during renovation projects or when furniture is moved.

Water damage within the wall, for instance as a consequence of faulty pipes or when a room wasn’t dried out properly before wallpapering, is another possible cause for mould. Thermal bridges on outer walls can also lead to the formation of condensation water, and his can have an effect on internal walls, too. If water cannot drain properly, rain and condensation run along the wall and will penetrate it over time. A leaky roof can also lead to permanent water damage in the rooms underneath it. The formation of mould is a slow process and doesn’t happen overnight. The longer the

As you can see, there are many different reasons for mould and they can be difficult to recognise for laypeople. Once you strip the old wallpaper, you are a lot closer to a solution, as you will be able to discover if and to what extent the wall is affected by mould. If traces of mould are visible on the old wallpaper, you should wear a protective mask and gloves. The same safety precautions are recommended when you eventually treat the mouldy wall.

If you are just dealing with a small dark spot, especially behind furniture and in window recesses and alcoves, the reason is fairly clear: Condensation water. If you find a larger mould problem in various colours like black, brown, yellow, and red, it is advisable to contact a specialist who will be able to evaluate how the moisture penetrated the wall. It makes no sense to just clean the mould off the wall and then put your wallpaper up without tackling the root causes first. This will only delay the problem temporarily - and the mould will be back in no time. Using anti-mould paint and sealants or dehumidifying plaster is particularly risky, as it simply encourages the mould to spread and multiply because it stops the air flow. It creates a kind of vacuum.

Furthermore, it is important to determine whether the mould is only on the surface or if it has spread to deeper layers of the wall. If the latter is the case (e.g. as a result of water continuously penetrating the wall due to water damage or a lack of draining), the plaster needs to be removed generously. It is out of the question to simply disinfect and seal the wall before wallpapering over it.

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How to treat a mouldy wall - different solutions

You can buy a number of anti-mould products which can be categorised as follows:

  • Mould remover, disinfectant
  • Mould-stopping products
  • Anti-mould paint, anti-mould plaster
  • Barrier primer to prevent moisture/anti-mould primer

There are also a number of “household remedies”, especially to clean mouldy surfaces, for instance methylated spirit, high-proof alcohol (isopropyl alcohol)and hydrogen peroxide. However, as these substances are highly flammable there is an increased danger of fire, notably in closed rooms and when used on large surfaces. Commercial mould removers are available as biocides or fungicides. As a general rule, it is important to ensure appropriate ventilation, avoid ignition sources, and wear protective masks and gloves when using anti-mould products of any kind.

Mould removers and disinfectants are generously applied to the affected areas with a brush or spray bottle, left to soak, and then brushed off thoroughly or washed off with plenty of water. A mould-preventing product can then be applied. Depending on further treatments, the surface should be prepared with a moisture-stopping barrier primer or an anti-mould primer. When it comes to replacing plasterwork, dehumidifying plaster variations have proven to be very effective; however, applying them should usually be left to specialists.

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How to proceed

So you have discovered mould on top of or after removing the old wallpaper? Here is our step-by-step guide on how to tackle the issue:

  • Before stripping mouldy wallpaper, always treat the affected areas with mould remover in order to kill the fungus.
  • Check to see if the affected area of the wall is damp - either by conducting the tin foil test described in How to prepare the wallpapering surface or by using a moisture meter. Moisture meters (as used by specialists) will give you exact measurements regarding the moisture situation. They are readily and relatively cheaply available in DIY and hardware shops and very user-friendly. Measurements above the norm indicate a constant moisture load.
  • How large is the affected surface? Is the mould just on the surface or has it penetrated deeper into the wall? To find out, simply remove a bit of plaster from the area.
  • If you cannot identify the root cause yourself, get some expert advice pronto! Without tackling the root causes, the most thorough removal of mould will come to nothing in the long term.
  • You can treat small surface areas (no larger than half a square meter) yourself, but anything exceeding that should be sorted out by an expert (painter/decorator, plasterer with experience in mould control).
  • Apply the anti-mould product according to instructions, observing correct soaking and drying times. Before you can proceed with the next step, the area needs to be completely dry.

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Preventing mould on walls

Everyone is familiar with the saying: Prevention is the best cure. This adage rings particularly true when it come to mould on walls. We cannot stress it enough: “Correct and frequent airing is key!”That means quick bouts of opening the windows, balcony and patio doors wide - don’t worry about a bit of short-term draught. This should generally be done in the entire flat or house (including basement and loft), repeatedly and on a daily basis, for about 15 minutes at a time. During the hot summer months, airing should happen in the cooler morning or evening hours. Otherwise the warmer outside air will cause condensation on the cooler interior walls.

  • You can use a hygrometer to determine if the air humidity level is too high, but steamy or misty windows and doors are also an indication. For modern windows with insulating glass, the condensation will usually settle on the outside. If you still have single-glazing, you will notice the moisture on the inside of the window.
  • In wet rooms like bathrooms and kitchens, steam produced by showering, bathing or cooking should be able to escape immediately, for instance by leaving the windows wide open during the process, but at the very least directly afterwards. Make sure you choose the right wallpapers for wet rooms and and apply them correctly. You can find all the relevant information in our instructions: How to wallpaper in the bathroom.
  • Be sure not to place furniture too close to the wall so that the air can circulate behind.
  • After a case of water damage in or on the wall, sufficient time must be allowed to make sure it is completely dry. Furniture should only be put back once this has been confirmed.
  • For homeowners: Take moisture issues on, in, and around the house seriously and act swiftly if any problems are discovered. A damp basement or loft will permanently damage the structure of a building - if not immediacy then certainly long-term. The quicker you react, the sooner you will have tackled the root cause and prevent moisture from spreading in the rooms.
  • When performing insulation measures, be sure to avoid thermal bridges. We recommend that you consult a specialist when replacing windows, doors, or adding insulation to a roof, basement or facade.