Damp Questions Answered
Straight answers to the damp, condensation and timber questions we get asked most, from Simon Cooper (Damp Sam) and the All Dry team.
Condensation and mould
What Causes Black Mould on Walls and How Do I Get Rid of It for Good?
Black mould on walls is usually caused by condensation: warm, moist air from cooking, showers, drying clothes and breathing settles on cold surfaces and mould grows where it stays damp. To get rid of it for good you have to deal with the moisture and the cold surface, not just wipe the mould off.
How Do I Stop Condensation and Mould on Cold Solid Walls?
Solid walls have no cavity, so their inside face gets very cold in winter and condensation forms on it, leading to black mould. Better ventilation and steady heating help, and where the mould keeps returning, a thin internal thermal lining such as Isotherm can warm the wall surface so condensation is far less likely to form.
Why Do I Get Condensation on My Windows Every Morning?
Windows are usually the coldest surface in a room, so overnight the moisture you breathe out and the moisture from showers, cooking and washing condenses on the glass and frames. A bit on a cold morning is normal, but streaming windows every day means there's too much moisture in the house and not enough ventilation.
Why Is Condensation Worse in Winter?
In winter walls and windows get much colder, and we keep windows shut, dry washing indoors and run the heating in bursts. Cold surfaces plus moist indoor air means more of that moisture reaches its dew point and turns to water, which is why mould often appears from autumn onwards.
How Do I Stop Condensation and Mould in My Bathroom?
Bathrooms produce a lot of moisture in a short time, so mould grows wherever the steam settles and can't dry out. The fix is a properly working extractor fan ducted outside, running during and after every shower, plus some heat in the room and wiping surfaces down.
How Do I Stop Condensation in My Kitchen?
Cooking, kettles and dishwashers put a lot of moisture into the kitchen air, and it condenses on windows, cold walls and behind units. Use a cooker hood or extractor that vents to outside, put lids on pans, keep the kitchen door shut while cooking and give the room a short blast of fresh air afterwards.
Is It Bad to Dry Clothes Indoors?
Drying washing indoors puts all the water from the clothes into the air, and without ventilation it ends up as condensation and mould on cold surfaces. If you must dry clothes inside, do it in one room with the door shut and either a window open, an extractor fan or a dehumidifier running.
Why Is There Black Mould on My Ceiling and in the Corners of Rooms?
Corners and the junction where external walls meet the ceiling are often the coldest surfaces in a room, so moist air condenses there first and mould follows. Missing loft insulation at the eaves, cold bridging and poor air movement make it worse.
Ventilation
Do Dehumidifiers Stop Damp and Mould?
A dehumidifier can reduce condensation and mould by pulling moisture out of the air, but it treats the symptom rather than the cause. It won't fix rising damp, penetrating damp or a leak, and it works best alongside better ventilation and steady heating.
What Is a PIV Unit and Will It Stop Condensation?
A PIV (positive input ventilation) unit draws fresh, filtered air in, usually from the loft, and gently pushes it into the house, which displaces moist air out through the natural gaps in the building. In the right property it can make a real difference to condensation, but it won't fix leaks, rising damp or penetrating damp.
Do Extractor Fans Stop Mould?
A properly sized extractor fan, ducted outside and used every time you cook or shower, removes moisture at source and is one of the best defences against mould. Many fans fail because they're too weak, badly ducted, switched off or stop running too soon.
Should I Keep My Trickle Vents Open?
Yes, keep trickle vents open, day and night, all year round. They provide constant background ventilation that lets moist air out, and closing them is one of the most common reasons condensation and mould get worse after new windows are fitted.
Why Have I Got Damp and Mould Since Getting New Windows?
New windows make a house more airtight, so the moisture that used to escape through draughty old frames stays inside and condenses on the coldest surfaces. Damp can also come from installation faults such as poor sealing, damaged reveals or a missing cavity closer, so the cause needs checking.
Diagnosing damp and damp meters
How Can I Tell What Type of Damp I Have?
Look at where the damp is, when it gets worse and what it looks like. Black mould on cold surfaces that's worse in winter points to condensation, patches that grow after rain point to penetrating damp, and a tide mark with salts at the base of ground-floor walls may be rising damp, but a proper survey should confirm it.
Can You Trust a Damp Meter Reading?
Only if the person using it understands its limits. Most damp meters measure electrical conductivity and are calibrated for timber, so on plaster and masonry they give comparative readings, not true moisture contents, and salts, foil and condensation can make a dry wall read 'wet'.
What Are the White Salts on My Walls?
White powdery or fluffy deposits on walls are salts left behind as water evaporates from the masonry or plaster, known as efflorescence. On new brickwork it's usually harmless, but on internal walls it shows moisture is moving through the wall, and some salts attract moisture from the air and keep the wall damp.
Why Is My Wallpaper Peeling Off the Wall?
Wallpaper peels when the paste loses its grip, usually because the wall behind is damp, condensation is soaking the paper, salts are pushing through, or the wall wasn't prepared properly. Where and how it peels is a good clue to the cause.
Why Does My House Smell Musty?
A musty smell is usually caused by mould or fungal growth somewhere, often where you can't see it: behind furniture, inside wardrobes, under floors, in a cellar or in damp timber. It means there's moisture where there shouldn't be, so the source needs finding rather than covering up with air fresheners.
Rising damp, DPCs and replastering
Is Rising Damp a Myth?
Rising damp is real, but it's far less common than many reports and quotes suggest, and plenty of damp is wrongly blamed on it. True rising damp shows a recognisable pattern at the base of ground-floor walls, usually with ground salts in the plaster, and it should be confirmed by a proper diagnosis before anyone injects anything.
Do I Need a Damp Proof Course Injection?
Only if rising damp has been properly confirmed and the cause can't be fixed more simply. Before injecting, the surveyor should rule out bridging, high ground levels, leaks, penetrating damp and condensation, because a new DPC won't fix any of those.
What Is a Bridged Damp Proof Course?
A bridged damp proof course is one that's still there and working, but moisture is getting around it, through high ground, a raised path or patio, render carried down over it, or debris in the cavity. Removing the bridge is usually the fix, rather than injecting a new DPC.
How Far Below the Damp Proof Course Should the Ground Be?
Building guidance generally expects the damp proof course to be at least 150mm above the outside ground level. That gap stops rain splashing up above the DPC and stops soil or paving bridging it, so if your path, patio or flowerbed is higher than that, it can cause low-level damp inside.
What Plaster Should I Use After a Damp Proof Course Injection?
After a chemical DPC, the old salt-contaminated plaster normally needs removing and replacing with a salt-resistant replastering system, as set out in BS 6576. Ordinary gypsum plaster shouldn't go straight onto salt-contaminated masonry, because the salts will draw moisture and spoil it.
How Long Does It Take for a Damp Wall to Dry Out?
Longer than most people expect. Once the source is fixed, a thick solid wall can take many months to dry out fully, depending on its thickness, materials, how wet it was and the conditions. A commonly quoted rule of thumb is around 25mm of wall thickness per month in good drying conditions.
Penetrating damp, walls and windows
What Is Penetrating Damp and How Do I Stop It?
Penetrating damp is water getting into a building from outside through a defect, such as leaking gutters, failed pointing, cracked render, a poor chimney flashing or a leaking pipe. It usually shows as damp patches that get worse after rain, and you stop it by finding and fixing the defect, then letting the wall dry out.
Can Cavity Wall Insulation Cause Damp?
It can, but it isn't automatically the culprit. In walls exposed to wind-driven rain, or where the outer brickwork, pointing or render is defective, insulation can help water cross the cavity, and gaps in the fill can create cold spots for condensation. The cause needs proper investigation before anything is removed.
Can Cement Render Cause Damp in an Old House?
It can. On older solid stone or soft brick walls, a hard cement render can crack and let rain in behind it, then stop the wall drying out, so moisture builds up and shows as damp inside. Breathable lime render is often more suitable for those walls, but each wall should be assessed.
Should I Repoint My House with Lime or Cement Mortar?
The mortar should always be softer and more permeable than the bricks or stone it sits between. On older solid-walled houses built in lime, that usually means repointing in lime mortar, because hard cement pointing can trap moisture and damage the faces of soft bricks and stone.
Do Breathable Paints Help With Damp?
Breathable (vapour-permeable) paints let moisture escape from a wall rather than trapping it behind the finish, which matters on older solid walls, lime plaster and walls drying out after damp work. They won't cure a damp problem, but the wrong paint can make one worse.
Why Is My Chimney Breast Damp?
Chimney breast damp is usually caused by rain getting into the stack or flue, a capped chimney with no ventilation, condensation in the flue, or hygroscopic salts from years of burning fuel that pull moisture from the air. The fix depends on which one it is, so the stack and flue need inspecting as well as the plaster.
Is the Damp on My Ceiling a Roof Leak or Condensation?
Brown stains with defined edges that appear or grow after rain usually point to a roof leak or plumbing leak, while black spotty mould spread across a cold area of ceiling, often near external walls, points to condensation. Checking the loft above usually settles it.
Why Is My Bay Window Damp?
Bay windows are damp-prone because they stick out from the house, with their own roof, gutters and flashings, thinner or colder walls and lots of glass. Damp is usually rain getting in through the bay roof, a leaking gutter, failed sealant or a bridged DPC, or condensation on cold surfaces, and the pattern tells you which.
Floors, timber and rot
What Causes Damp Under Suspended Timber Floors?
Suspended timber floors sit over a void that needs a steady flow of air to stay dry. When air bricks are blocked, ground levels rise, extensions cut off the airflow or water gets into the void, the air under the floor becomes damp and the joists and boards can rot.
Should I Block My Air Bricks to Stop Draughts?
No. Air bricks ventilate the void under suspended timber floors, and blocking them lets moisture build up underneath, which can lead to wet rot, dry rot and woodworm. If draughts are a problem, seal and insulate the floor from inside instead.
What’s the Difference Between Wet Rot and Dry Rot?
Both are fungal decay of timber caused by moisture. Wet rot stays where the timber is wet and stops when it dries, while dry rot (Serpula lacrymans) can spread through masonry from a damp source to reach other timber, so it needs prompt, thorough treatment. In both cases, the source of moisture must be fixed.
How Do I Know If I Have Woodworm, and Does It Need Treating?
Woodworm is the larvae of wood-boring beetles, and the usual sign is small round exit holes in timber. Many infestations are old and no longer active; fresh, pale holes and fine bore dust (frass) suggest activity. Treatment is only needed for active infestations, and reducing moisture in the timber is a big part of control.
Cellars, basements and outbuildings
How Do I Fix a Damp Cellar? Does It Need Tanking?
It depends what you want to use the cellar for and how much water is getting in. A storage cellar that is just humid may only need better ventilation and dehumidification, but turning a cellar into a dry, usable room needs a properly designed waterproofing system, which may be tanking or a cavity drain membrane.
Can I Convert My Cellar or Basement Into a Habitable Room?
Usually, yes. Most cellars can be converted into dry, usable space with a properly designed waterproofing system, often a cavity drain membrane with drainage and a sump pump, alongside ventilation, insulation and the right approvals. The waterproofing design is the critical part.
What’s the Difference Between Tanking and a Cavity Drain Membrane?
Tanking is a barrier system: a waterproof coating bonded to the wall and floor that holds water back. A cavity drain membrane is a drained system: a studded membrane that lets water through the structure, collects it behind the membrane and drains it to a sump, so it's managed rather than held back.
How Do I Stop Damp in a Garage or Outbuilding?
Garages and outbuildings are often single-skin, uninsulated and unheated, sometimes without a DPC or damp proof membrane, so they get condensation and damp easily. For storage, ventilation, fixing leaks and keeping things off the floor and walls usually helps; for a habitable conversion, the walls, floor and roof need upgrading properly.
Buying, selling and renting
My Survey or Mortgage Report Says There’s Damp. Should I Worry?
Not necessarily. Many homebuyer surveys and mortgage valuations flag damp based on a quick meter reading and recommend a specialist inspection, without identifying the cause. Treat it as a prompt to get a proper damp survey from a qualified surveyor before you commit to costly work or renegotiate.
Who Is Responsible for Damp and Mould in a Rented Home: Landlord or Tenant?
Landlords are responsible for keeping the property in repair and fit to live in, which includes fixing leaks, structural damp and inadequate ventilation or heating. Tenants are responsible for living in the property reasonably, such as using fans and heating and reporting problems promptly. The fair answer comes from finding the actual cause.
Should I Buy a House With Damp?
Often, yes, as long as you know what's causing it and what it'll cost to fix before you commit. Many damp problems in older houses are straightforward, but some point to bigger issues, so get a specialist damp and timber survey before exchange.
Can I Negotiate the House Price Because of Damp?
Yes. If a specialist survey confirms damp that needs repairing, you can use the report and a written quotation to ask for a price reduction, ask the seller to carry out the work, or agree a retention. Negotiations are strongest when they're based on a clear diagnosis and real costs rather than a vague comment in a homebuyer survey.
Surveys, guarantees, new builds and DIY
Why Won’t My New Plaster Dry?
New plaster always takes time to dry, and it takes longer in cold, humid weather, on thick coats or on walls that were wet to begin with. If patches stay dark or keep coming back, the cause is usually moisture in the wall behind, salts in the background, condensation, or a finish that's trapping moisture.
Why Is My New Build House Damp?
New homes contain a lot of water from construction, which dries out over the first year or two and can cause condensation and mould. Damp can also come from defects such as missing cavity trays, bridged DPCs or poor sealing, so persistent or localised damp should be reported to the builder under the warranty.
How Much Does a Damp Survey Cost and What Does It Include?
The cost depends on the size and type of property, the areas to be inspected (such as cellars) and the type of report needed. A proper damp survey should include a full inspection inside and out, moisture testing at multiple points, an explanation of the cause and prioritised recommendations, not just a quote for treatment.
How Long Does Damp Proofing Last, and What Guarantee Should I Get?
Properly diagnosed and installed damp proofing should last for many years, but how long depends on the system, the installation and whether the original cause, such as bridging or leaks, was dealt with. Ask for a written guarantee, and ideally an insurance-backed guarantee that protects you if the company stops trading.
Do DIY Damp Proofing Kits Work?
Some DIY products can work for the right problem if they're used properly, such as DPC injection creams where rising damp has genuinely been confirmed. But most DIY damp failures come from treating the wrong problem, and products like damp-proof paints and sealers often hide moisture rather than fix it.
Health
Can Damp and Mould Affect Your Health?
Official UK health guidance says living with damp and mould can affect your health, particularly for people with respiratory conditions such as asthma, allergies, weakened immune systems, young children and older people. It shouldn't be lived with: find the cause, fix it, and speak to your GP or pharmacist if you're worried about symptoms.