Why is there black mould on the wall behind my wardrobe?
Why the coldest square metre in a bedroom is behind the furniture on the external wall, and the changes that raise that surface temperature enough to stop mould returning.
- Difficulty
- beginner
- Time
- 45 min
- Read
- 4 min
- Safety
- caution
Short answer
The wall behind the wardrobe is several degrees colder than the wall either side of it, because the furniture blocks the warm room air from reaching it. Mould grows on the coldest surface, not the wettest room. Pull the furniture 50 to 75mm off the wall, get some background heat into the room, and reduce the moisture going into the air.
Cleaning mould off the wall behind a wardrobe and putting the wardrobe back is a job you will do every year for as long as you own the house. The mould is not growing because the wall is dirty or because the wardrobe is damp. It is growing because that particular patch of plaster spends the winter below the dew point of the room air, and nothing you spray on it changes that.
Safety
What you'll need
- FFP2 or FFP3 mask, gloves, eye protection
- Fungicidal wash
- Disposable cloths
- Hygrometer (optional)
- Thermometer or an infrared thermometer (optional)
Step by step
- Pull the furniture out and look at the wall properly.Note the shape of the growth. Mould that is heaviest in the corner and fades outwards, or that stops exactly where the wardrobe ended, is confirming the diagnosis for you.
- Feel the wall with the back of your hand.Compare the covered area, the exposed wall beside it and an internal wall in the same room. The temperature difference is usually obvious, and it is the whole explanation.
- Check the wall is not actually wet.Look for a rain correlation, a level tide mark, salts, or a stain that comes from behind rather than sitting on the surface. If any of those are present you have a different problem as well.
- Clean it correctly.Mask, gloves, window open, door shut. Wipe with a fungicidal wash, working from the outside of the patch inwards, using disposable cloths that go straight into a bag. Do not brush, scrape or sand.
- Move the furniture off the wall.50 to 75mm of clear air behind it, and ideally onto an internal wall instead. Air movement behind the wardrobe raises that surface temperature and it is the single most effective change available.
- Give the room some background heat.A cold bedroom with the door shut and two people breathing in it all night is the classic setting. A low continuous background temperature keeps surfaces above the dew point far more effectively than blasting the heating for an hour.
- Reduce the moisture load.Washing dried on radiators, a bathroom without a working fan, unlidded pans and a bedroom door kept shut all night. These are what raise the dew point that the cold wall then meets.
- Ventilate the room properly, especially in the morning.Windows wide for a short period rather than ajar all day. Short and wide changes the air without chilling the fabric.
- Consider raising the wall's temperature permanently.If the wall is solid or the cavity is unfilled, insulation is the real answer. A thin insulated lining board on that one wall is a smaller job than it sounds and it removes the cause rather than managing it.
- Repaint only after a dry winter.Redecorating a wall that is still going below dew point wastes the paint. Change the conditions first, watch it through a winter, then decorate.
Tips
- Slatted or open-backed furniture on an external wall causes far less trouble than a solid-backed wardrobe. If you are buying, that is a real consideration.
- Mould on the clothes as well as the wall means the humidity inside the wardrobe is high, so leave the doors ajar when the room is being aired and do not pack the rail solid.
- An infrared thermometer costs very little and shows the cold spots in a room in a couple of minutes. It is the cheapest diagnostic instrument in this whole subject.
Common mistakes
- Treating it as a hygiene problem — It is a surface temperature problem. Cleaning removes the growth and changes none of the conditions, so it returns in the same place every winter.
- Sealing the room up to keep it warm — Less ventilation means more moisture in the air, which raises the dew point, which means the cold wall condenses sooner. Warmth and ventilation have to go together.
- Painting over mould with an anti-mould paint and putting the wardrobe straight back — The fungicide in the paint is exhausted within a few years and the surface is still cold. It buys time and nothing else.
Questions people ask
Is black mould in a bedroom dangerous?
For most people it is an irritant rather than a hazard, but for anyone with asthma, an allergy, COPD or a weakened immune system it is a genuine health matter, and small children and older people are more affected. If someone in the house has a respiratory condition, treat it as urgent and get the cause fixed rather than managed.
Would a dehumidifier fix it?
It helps, because lowering the room humidity lowers the dew point that the cold surface has to meet. It is treating the symptom rather than the cold wall, but in a house where insulation is not currently possible, it is a legitimate and effective measure.
I rent. Is this my responsibility?
Condensation caused by cold uninsulated surfaces, missing extract fans, inadequate ventilation or a heating system you cannot afford to run is a fabric problem and it is the landlord's responsibility. Report it in writing with photographs and keep a record.