Mould behind a sofa or a bookcase is rarely a coincidence. Furniture stops air reaching a cold wall, and moisture settles there. Here is what to move, what to check and what to leave alone.
Furniture pushed flat against an outside wall stops warm room air reaching the wall surface, so the wall behind stays colder than the room and moisture condenses on it. Pull beds, sofas and bookcases a few centimetres away from outside walls, keep wardrobes and shelving off cold corners, and give the room a little background heat. If mould keeps returning even with a gap, look for a cold bridge, a leak or a ventilation fault before buying anything.
A dark patch of mould on the wall when a sofa is moved. A bed headboard that has left a rectangle of damp paint behind it. A bookcase with a musty smell, books that feel soft, or a dark tide-line on the wallpaper at the height of the top shelf. Sometimes the room itself feels fine, with clear windows and no smell, and the problem only shows up when you move something.
GOV.UK guidance describes condensation damp as moisture generated inside the home that cools and condenses on colder parts of the building, naming window frames, corners and low points of walls behind sofas or wardrobes as typical places. It also recommends leaving a gap between furniture and external walls. The reason is simple: a wall behind a large piece of furniture gets less warm air and less movement of air than the rest of the room.
An outside wall is cooler than the room because the outside of it is cold. Normally the warm room air touches it and keeps its surface just warm enough that most moisture stays in the air. Put a sofa or a bookcase tight against that wall and the warm air cannot reach it. The wall behind the furniture is shielded from the room and exposed to the cold, so its surface drops further. The air trapped in the gap is still the room air, with all the moisture it carries from cooking, showers, laundry and breathing, but it is now sitting against a surface colder than the rest of the room. That is the recipe for condensation, and then for mould.
The Property Care Association lists breathing, cooking, showers and baths, and drying clothes as the usual sources of that moisture. It says the main reason it causes problems is typically a lack of ventilation, and that homes which are poorly heated, poorly ventilated and poorly insulated are particularly prone to it. Furniture against a cold wall adds to all three.
Work down the steps in order. You can do the first four this weekend.
Fitted wardrobes and built-in shelving are harder, because you cannot pull them out. If a fitted unit stands against an outside wall, leave its doors open now and then in cold weather, avoid packing it to the back, and consider a small gap behind the back panel at the next refit. Check the back wall with a torch if you can reach it. If you find mould, treat it as you would any other patch: clean it, find the cause, and re-check.
Chatham and the Medway Towns have a wide mix of homes, from older terraces with solid outside walls to later houses and flats. In an older terrace, an outside wall with no cavity can be markedly colder than the room, so furniture against it is more likely to collect moisture. In a newer, more airtight home, moisture builds up in the air and finds the coldest spot, often the same wall behind the same sofa. Either way the rule is the same: look at where the furniture sits, and give the wall some air. Our pages for Brompton, Lordswood and Strood cover local housing in more detail.
A dehumidifier will not fix a leaking gutter or a wall that is wet from outside, and it is no substitute for leaving a gap behind a sofa. If the wall behind the furniture is wet to the touch in dry weather, or a stain grows after rain, deal with the building first. A Dryzen unit needs a loft to mount it, and it is not suitable for rentals unless the landlord authorises the changes. If you rent, move the furniture, photograph the mould and tell your landlord in writing.
A Dryzen system is a loft-mounted dehumidifier with insulated ductwork to the central area or stairway of the home. Each unit is rated for 150 square metres and has an extraction rate of about 12 litres a day, and it uses about 20 pounds a year in energy. It has an activated carbon filter, and the smart app, which shows temperature and humidity, is included. Parts and labour are covered for 10 years, with 1 year on the compressor. It suits owner-occupied homes with a loft. It is not suitable for rental properties unless the landlord authorises the changes. If you have moved the furniture and the walls are sound but mould keeps returning through the winter, a free survey can measure humidity through the day and tell you honestly whether a system would help.
GOV.UK guidance advises leaving a gap between furniture and external walls. A few centimetres is a sensible starting point, and more if the wall is very cold or mould keeps returning. Check the wall after a few weeks.
The wall behind the sofa is shielded from room warmth and still exposed to the cold outside, so it is the coldest surface in the room and the first place moisture condenses. It does not mean the rest of the room is dry.
Paint may hide the patch for a while, but it does not remove the moisture or the cold surface. Clean the mould, leave a gap behind furniture, add some heat and air, and find the cause.
Sources: GOV.UK: Understanding and addressing the health risks of damp and mould in the home; Property Care Association: Condensation help and advice for homeowners
Last reviewed 2026-10-18. General guidance only; this is not a survey of your home.