Dry damp walls with ventilation and heat

Bathrooms, kitchens and cellars are affected by moisture on a regular basis. A family with two children and a couple of indoor plants releases up to 12 litres of water vapour every single day. Add to that outdated thermal insulation along with insufficient ventilation and the vapour soon begins to condense on walls and windows. A high amount moisture also builds up in cellar vaults, particularly if there is no proper ventilation.

The following will explain various ways to ensure air circulation and how to keep rooms dry and free from mould.

Humidity and moisture are already inside the house?

⮩ Check out our guide about construction drying and dehumidification


  • Correlation of temperature, humidity and mould growth
  • Proper ventilation and heating (exception cellar)
  • Free and mechanical ventilation
  • How to ventilation the cellar
  • Measures and devices used for dehumidification
  • Preventive measures for dry brickwork

Correlation of temperature, humidity and mould growth

The vapour pressure of warm air is higher and binds more water molecules than cold air. This is an important fact for residents in a home because moisture from air in a heated room can be extracted.

In short: Warm air transports more moisture than cold air.
1m3 of air at 20°C can absorb a maximum of 17.5g water (100% relative humidity = dew point).
1m3 air air at 15°C can only absorb 13.0g and 1m3 air at 10°C absorbs 9.5g at most.

If warm air comes into contact with a cold surface like a wall or window glass, it cools down. The vapour pressure decreases and water molecules are released: dew is formed.

In order to prevent condensation, the accommodation needs to have a temperature of 20°, an average humidity level of 50% as well as a temperature of at least 12.6°C on the interior walls. However, this calculated optimum value is not very realistic. Exterior walls, windows and heating pipes inside the wall without insulation cause cold spots and corners. If you measure the surface temperature of the indoor walls with an infrared thermometer you will notice the different temperatures. Some spots on the wall will be at about 16-20°C whereas others might be below 10°C. Particularly in older buildings with draught caused by leakage these irregularities become noticeable. Walls can also cool down if windows are kept open for extended periods or because of small cracks in the brickwork. The result is water condensation around these cold spots. If this happens regularly (3-4 days consecutively) mould can already begin to form.

Mould spores are part of the breathing air by nature. Starting from a relative humidity of 70-80% and a temperature of 0°C, the amount of mould multiplies. The “normal“ indoor humidity in an occupied building ranges from 40% to 60%, after cooking or taking a shower this value increases. Mould can only be kept in check by ideal indoor climate conditions (if required in combination with HEPA filters).

That is why old buildings need systematic heating AND ventilation to keep damp walls dry. Cellars without heating call for individual solutions which we also offer in our shop.

Proper ventilation and heating (exception cellar)

Reducing the vapour content slightly at home is done easily with our tips in the last section of this text. Nevertheless you will continue to cook, shower and sweat, so there is no way to prevent moisture completely. That is why will begin with heating and ventilation that help discharge moisture regularly and prevent mould.

Heating makes a home feel cosy. The air temperature rises, the gas binds more walter molecules. Furthermore, the temperature along the walls and other materials susceptible to mould increases too, less vapour will condense in these areas. Particularly in damp rooms like the bathroom the heating should be on regularly. Air the room and extract the warm moist air from the room to the outside. The heater should be turned off if windows are opened.

Open doors mean that heat from one room will manage to get into other less heated rooms where it will cause condensation. It is not a good idea to warm up the bedroom through the open living room door. It is better to keep all doors closed and heat the second room individually.

Free and mechanical ventilation

Natural, free ventilation (in old buildings)
An opening in the building envelope, usually a window or a door lets in fresh air. Opening windows and walls or installing ventilators opposite each other, the air mass can be exchanged quickly and efficiently. Natural ventilation however is almost impossible to control.

open windows for a short time frequently and not every now and then for extended periods
open windows and doors
if possible open windows and doors that are opposite each other
10 minutes at least 3-4 times a day
workforce: at least 5 minutes in the morning and evening

Duration once every hour
Winter: 5 minutes
Transitional season (March, November): 10 minutes
Spring, autumn: 15 minutes
Summer: 20 minutes

Primarily in the winter frequent ventilation is required even though the outdoor air is cold. Fresh air comes into the building, quickly warms up inside the home and absorbs water molecules, which is then extracted again. That is why ventilation is much more efficient in winter than in the summer where the atmosphere tends to be warm and humid. For this reason it is enough to simply open a window in the winter for about 5 minutes, longer periods would cool down interior walls and the home. Make sure to turn off your source of heat if windows are being opened to prevent enormous heating bills. Repeat short ventilation periods regularly, every 1 – 2 hours.

The bedroom also requires fresh air in the morning: Breathing air contains moisture and even during peaceful sleep the body releases sweat.

mechanical ventilation system is recommended if residents do not have the possibility to open their windows. At certain times it is also not a good idea to open the windows, for example in the winter or transitional seasons where it is cold and draughty outside. Even in old buildings state-of-the-art centralised ventilation systems can be implemented.

Mechanical ventilation systems
Air supply and extraction is easily controlled and adjusted to current needs, even if residents are not at home. Decentralised fans individually supply fresh air in separate rooms. Some appliances do this fully automated without any human intervention at all. Humidity sensors for example can be set to react to a relative humidity of 60% and automatically switch on the fan.

Small rooms like bathrooms can often be supplied with air from adjacent rooms due to their small size. In this case it is enough to install a simple extractor fan. It is recommended to add or opt for a model with time delay to facilitate operation. Controlled residential ventilation offers a variety of benefits such as recovering heat energy, absorbing noise and particulate matter from the outside. The quality of air is kept at a good level consistently. There are also options that combine manual and automatic ventilation.

Wall extractor fans

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Bathroom fans

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Window mounted fans

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Inline fans

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How to ventilation the cellar

Well insulated walls give off less heat to the outside. However, refurbishing old buildings is often so uneconomical that residents are willing to put up with higher heating bills. Warm walls are more able to prevent dew from settling than cold walls. However, most cellars are old constructions with leakage and cracks, trying to use a normal heater is almost futile. Particularly if the room is used for storage, the room is kept cool, and therefore stays humid. Proper ventilation can help extract moisture and protect the masonry even in the winter.

A cellar or basement requires a different kind of ventilation than a normal room in the household. It is a common misconception that warm exterior air in the summer can dry a cold cellar faster in the summer. This possibly causes even more moisture in the building because the warm air from the outside concentrates more moisture than cool air. If this warm exterior air now enters the cellar vault, the air temperature decreases and water is being released. It will condense on the inside, because the floor of the cellar is the coolest place due to its proximity to the soil. It is on the ground where you can see the condensation best. The water then spreads through the brickwork, a musty odour becomes noticeable and mould begins to grow.

⮩ Ventilate at night or in the cool hours of the morning and evening during summer time. Laundry should be left to dry outside if possible.

In the winter there should not be constant and permanent ventilation. On one hand to prevent pipes from freezing over or heated rooms from cooling down. On the other hand, exterior air can be quite rich in moisture, particularly during winter time. However, if the outdoor air is colder than the temperature in the cellar and the air is dry instead of humid, ventilation can be turned on. Cool air is let into the building, is heated up slightly, absorbs water vapour and then leaves the building.

Indoor and outdoor air humidity can be measured with a thermo hygrometer that can be attached inside the celler and on the outside of the building envelope. Instead of using a thermometer you can also try a different method with less exact results. Take a glass bottle from the celler (must have temperature of the cellar) and put it outside. If condensation is produced on the bottle, the bottle (and the cellar) are cooler than the air outdoors. In that case, do not ventilate. Instead of manually monitoring your ventilation system, you can also opt for automatic ventilation with temperature sensors and automatic shutters that prevent cold air from infiltrating.

Measures and devices used for dehumidification

Normal to slightly elevated humidity levels:
Fan heaters and infrared heaters for domestic areas are not designed for drying damp brickwork. However, they do make staying in a damp room feel a lot more pleasant.

Fan heaters heat up the air that is already present in the room, meaning it also absorbs more moisture. Using a heater blower before opening windows and doors therefore means more moisture can be extracted from the inside.

  • Infrared heaters with an IP code can be used in damp rooms like a bathroom. They do not heat up the ambient air, instead they heat up surfaces directly, such as your skin. That is why windows can opened even though the heater is operating. Whilst being in the room it will not feel as though the room cools down and heat is lost.

Surfaces that became wet because of a one-time occurance, e.g. rain coming in through an open window, are easily restored with condensation dryers. In our creoven store we offer dehumidifiers that can be used in rooms with up to 70m2 size. Portable devices are a great choice for single rooms that are affected, like a laundry room, bathroom or kitchen. The Stadler Form Albert dehumidifier features a drainage hose that allows you to connect the device directly to the drainage system, meaning it can also operate without supervision during nighttime. Thanks to an agreeable weight of about 10kg and handy transportation wheels at the bottom the unit is moved from one place to the other effortlessly. An integrated hygrostat measures humidity levels and automatically switches into standby as soon as the indoor conditions reach an acceptable value.

If the source of dampness is unclear, it must be determined by a trained professional before other measures are taken. Is groundwater rising from the soil, is a pipe damaged or a window untight? Depending on where the water penetrates the building, customised measures of redevelopment must be taken. Simple dehumidification and new ventilation measures are not enough if the cause of leakage persists. Professional humidity measurement can determine the degree of damage so that the correct countermeasures can be taken.

Locate the cause
Check moisture / mould
Ventilation / drying / dehumidification
Remove mould

Check for leftover condensation water underneath the adhesive foil after the first drying process. Upon completed drying any possible cracks should be sealed and the facade should be plastered anew. Permeable plaster on the inside of the building can absorb humidity and release it when the air is dry. Before the plaster is applied, saline cristals are removed with a sponge or soft brush.

Some old basements cannot be dried out by ventilation alone. Damage on the building materials or flawed construction with thermal bridges can be the root of the problem, because they resurface time and again. In this case only a professional detection of leakage and sealing/refurbishment can help. Condensation often spreads extensively, whereas leakage occurs in single spots, for example around a leaky window. Moist insulation material also poses a problem in old buildings.

Seal leakage:
Renew plastering and gaskets
Replace damaged thermal insulation
Replace damp parts of the brickwork
Electrophysical procedures
Barricades (horizontal, vertical)

Preventive measures for dry brickwork:

Do not let water accumulate, empty out buckets, remove obvious moisture
Dry off steamed up tiles with a dry towel
Open the windows after waking up, taking a shower, cooking
Monitor relative humidity with a hygrostat
Dry towels, mats and laundry in an airy place, if possible outdoors
Drain washing machine once a month
Keep drains and sinks unclogged
Regularly check joints, corners, windows and cupboards for mould growth
Keep joints clean, if necessary seal them (silicone paste)
If necessary, install a fume hood and extractor fan in the kitchen and bathroom
Dry joints, tiles and the tub after taking a shower or bath
Leave the washing machine and dish washer open to dry after using them, if necessary wipe off moisture to prevent mould and odour
Bulky furniture should be placed at least 5cm away from exterior walls (sufficient air circulation)
Keep doors inside the house shut
Keep drapes, clothes drying racks and furniture away from the heater
Do not use the radiator to dry laundry
Do not shut off the heater completely in cold weather
Do not place too many plants in front of windows

In old buildings and cellars without modern retrofitting you should not be stingy with the heating during colder seasons of the year. Warm air binds more water molecules and can be extracted to the outside of the building simply by opening windows and airing the home. If you do not have the time or possibility to manually open windows and doors, you can prevent mould by implementing a mechanical ventilation system.

originator Ingo Bartussek, Fotolia.com