Condensation, Damp and Mould: Keeping Your London Home Dry This Winter

October is when the trouble starts. The heating comes on, the windows stay closed and the air indoors suddenly has nowhere to go. Within a few weeks many London homes show the same signs: streaming windows first thing in the morning, a musty smell in the wardrobe and grey or black spots creeping across the bathroom ceiling. Damp and mould are not just unsightly. They damage plaster, paint and timber, and mould spores can aggravate asthma and allergies. The good news is that most cases are caused by condensation, and condensation is something you can control. This guide explains how to identify the type of damp you have, where the moisture comes from and the fixes that make a lasting difference.

1. Why damp problems peak in autumn and winter

Warm air holds far more moisture than cold air. When warm, moist air from a shower or a pan of pasta meets a cold surface such as a window pane, an outside wall or the back of a wardrobe, it cools and releases that moisture as water droplets. In summer, open windows carry the moisture away. In winter we seal our homes up to keep the heat in, so the moisture stays put.

London housing makes this worse. Victorian and Edwardian terraces often have solid brick walls with no cavity, so the inside face of the wall stays cold. Many flats have small kitchens and bathrooms with no opening window. Newer homes are much more airtight, which saves energy but means ventilation has to be designed in and kept working.

2. The three types of damp and how to tell them apart

Before you spend any money, work out which type of damp you are dealing with. The fix for one will not cure the others.

• Condensation: by far the most common. Look for water on windows and window frames, black mould in corners, behind furniture and around the bathroom ceiling, and damp patches that come and go with the weather and with how you use the room. It is usually worst on north facing and outside walls.
• Penetrating damp: water getting in from outside through a fault in the building. Look for a patch that grows after heavy rain, often below a leaking gutter, around a window or chimney, or where pointing has crumbled. It can appear at any height.
• Rising damp: ground moisture drawn up through a wall where the damp proof course has failed or been bridged. It is much rarer than many people are told. Look for a tide mark up to about a metre above the floor, salts blooming on the plaster and skirting boards that are soft or rotting.

If you are not sure, a simple test helps. Tape a square of kitchen foil tightly to the damp area and leave it for a day or two. Moisture on the room side of the foil points to condensation. Moisture underneath it suggests the water is coming through the wall.

3. Where all that moisture comes from

An ordinary household releases several litres of water vapour into the air every day without anyone noticing. The biggest sources are:

• Showers and baths, especially with the door left open
• Cooking without a lid or without the cooker hood running
• Drying washing on radiators or clothes airers indoors
• Tumble dryers that are not vented to the outside or properly emptied
• Kettles, house plants, aquariums and simply breathing, particularly in bedrooms overnight

Cutting down on these at source is the cheapest fix of all. Put lids on pans, close the bathroom door while showering and dry washing outside or in a closed room with the window open and the door shut whenever you can.

4. Get the ventilation right

Good ventilation removes moist air before it has a chance to settle. Building Regulations in England set minimum extract rates for new and replacement fans, including 15 litres per second for an intermittent bathroom fan and 30 litres per second for a cooker hood that extracts outside. Plenty of older fans fall short of this, or have simply stopped working.

• Test your fans: hold a single sheet of toilet tissue up to the grille. If the fan cannot hold it in place, it is not moving enough air. Clean the grille, check the duct is not crushed or blocked and replace the fan if needed.
• Choose a fan with an overrun timer or humidistat: a timer keeps the fan running for several minutes after the light goes off, while a humidistat switches it on automatically when the air gets humid. Both clear the steam that a fan linked only to the light leaves behind.
• Keep ducting short and straight: every bend and every extra metre reduces airflow. Rigid ducting performs better than long runs of flexible hose.
• Open trickle vents: the small slots at the top of modern window frames are there to provide background ventilation. Keep them open, especially in bedrooms.
• Use the cooker hood properly: switch it on before you start cooking and leave it running for ten minutes afterwards. A recirculating hood will remove grease and smells but not moisture.

Any new fan in a bathroom must have a suitable IP rating for its position, and wiring work in bathrooms should be carried out by a qualified electrician.

5. Heat your home steadily

Cold surfaces attract condensation, so how you heat your home matters as much as how much. Short blasts of heat followed by long cold spells allow walls to cool down, and moisture then settles on them. A steady, moderate background temperature is usually more effective and not necessarily more expensive.

• Avoid leaving unused rooms completely unheated with the door open, as warm moist air will drift in and condense.
• Fit thermostatic radiator valves so you can set each room to a sensible temperature rather than switching radiators fully on or off.
• Bleed and balance your radiators so every room warms evenly.
• Do not block radiators with sofas or long curtains.
• Leave a gap of a few centimetres between large furniture and outside walls so air can circulate behind it, and avoid overfilling wardrobes on outside walls.

6. Clean up mould safely

Small patches of surface mould can usually be dealt with yourself. Larger areas, mould that keeps returning quickly, or mould in a home where someone has a respiratory condition, are worth getting professional advice on.

• Wear rubber gloves, eye protection and a dust mask, and open a window while you work.
• Use a dedicated fungicidal mould remover and follow the instructions on the pack. Never brush or vacuum dry mould, as this spreads the spores.
• Wipe the area with a damp cloth afterwards, then bag up and throw away the cloth.
• Once the surface is completely dry, repaint with a mould resistant paint to help stop it coming back.
• Black mould in silicone sealant around baths and showers cannot be cleaned out properly. Cut the old sealant away and reseal with a sanitary grade silicone that contains a fungicide.

Remember that cleaning only treats the symptom. Unless you also reduce the moisture, the mould will return.

7. Check the outside of your home

Autumn is the ideal time to look for the causes of penetrating and rising damp before the worst of the winter weather arrives.

• Clear leaves from gutters and make sure downpipes are not cracked or blocked.
• Look for missing or crumbling pointing and cracked render, especially around windows.
• Check that window and door frames are properly sealed.
• Keep airbricks clear of soil, plants and debris so the space under suspended timber floors stays ventilated.
• Make sure garden soil, paths and patios sit at least 150mm below the damp proof course so they do not bridge it.

8. Measure your humidity

A digital hygrometer costs very little and takes the guesswork out of the problem. For most homes, indoor relative humidity of around 40 to 60 percent is a comfortable target. Readings that sit above 60 percent for long periods, particularly in bedrooms in the morning, are a clear sign you need more ventilation, steadier heating or both. In rooms that stay stubbornly humid, a dehumidifier can make a real difference while you tackle the cause.

9. A note for tenants and landlords

If you rent, report damp and mould to your landlord or letting agent in writing as soon as you notice it, with photos and dates. Landlords have a duty to keep homes free from serious hazards, and in England Awaab's Law now requires social landlords to investigate and fix dangerous damp and mould within set timescales. For landlords, sorting out ventilation and heating early is almost always cheaper than repairing damaged plaster and redecorating later.

How Precise Supplies can help

Our team in Wimbledon helps homeowners, landlords and tradespeople tackle damp every winter, from choosing the right extractor fan for a windowless bathroom to finding the ducting, vents and valves for a full ventilation upgrade. We stock extractor fans with timers and humidistats, ducting and fittings, thermostatic radiator valves, sealants and the plumbing and electrical supplies you need to keep your home warm and dry.

Visit our trade counter at Unit 17, Windsor Park Industrial Estate, Wimbledon, London SW19 2TJ. We are open Monday to Friday from 7am to 5pm and Saturday from 8am to 1pm. You can also order online for click and collect, or take advantage of free delivery on orders over £150. Not sure where to start? Call us on 0208 143 4192 and we will be happy to give you some expert advice.