Damp inside: condensation, mould, ventilation and rising damp

A black bloom behind a wardrobe, a grey line along the bottom of a bathroom ceiling, a wall that feels cold and smells musty: damp inside a house is common, and it is easy to get wrong. It goes wrong in two directions. People scrub the mould and never find the water, so it comes back. Or they pay for a treatment aimed at a cause the house does not have; GOV.UK calls rising damp "often misdiagnosed".¹ The government's guidance on damp and mould in England was written after a death: "In December 2020, 2-year-old Awaab Ishak died from a severe respiratory infection caused by prolonged exposure to damp and mould in his home."¹ This lesson sorts damp into its kinds, explains where condensation comes from, shows how much mould is yours to clean and how, and sets out the biggest argument in this part of the subject, over rising damp, in the words of the people having it.

The one hands-on job in this lesson is cleaning a small patch of mould, and the lesson says exactly how small. Nothing here opens a fan, a window frame or a wall. Parts of it report what guidance says about building rules and about landlords in England. This is general information, not legal advice. The rules differ between England, Wales, Scotland and Northern Ireland, and between US states and cities, and they change.

Four kinds of damp

The UK guidance this lesson leans on most is a document on GOV.UK, developed by the Office for Health Improvement and Disparities, part of the Department of Health and Social Care, with the housing department and the UK Health Security Agency. It's written for landlords and housing professionals in England, and it was last updated on 1 April 2026. It sorts damp into four kinds:¹

• Condensation damp "happens when moisture generated inside the home cools and condenses onto colder parts of the buildings (for example window frames, corners and low points on walls behind sofas or wardrobes)." The guidance adds: "This is the most common form of damp."
• Penetrating damp is water getting in from outside through defects in walls, roofs, windows or floors. Lesson 17, "Keeping water out: roof, gutters, ground and walls", gave its definition and its checks, and this lesson does not repeat them.
• Rising damp "is moisture from the ground that rises up through parts of the buildings in contact with the ground (walls and floors); it is usually found in older properties and is often misdiagnosed." It's the last section of this lesson.
• Traumatic damp "can be caused by leaking water from waste and heating pipes, overflowing baths or sinks, burst pipes or defective water storage vessels inside the building." That is the water from lesson 2, "Water in: supply, leaks and frozen pipes", and lesson 4, "Water out: traps, vents and blocked drains", ending up in the fabric of the house.

They don't stay in separate boxes. As lesson 17 showed, GOV.UK says water from a leak or from outside can make condensation worse, so one house can have two kinds at once, and fixing one leaves the other.¹ And you don't always see it. "The smell of mould without visible evidence of mould may indicate that there is mould behind a surface", the guidance says, such as the back of wallpaper or panelling or the underside of a carpet.¹

Condensation: where the water comes from

The water in condensation comes from living in the house. GOV.UK: "Moisture from normal occupant activities, such as showering, drying laundry and cooking, builds up in the air around a home if it cannot be removed."¹ The US Environmental Protection Agency (EPA) puts it the same way round: "Condensation can be a sign of high humidity."²

Two things decide whether that moisture ends up on a wall. One is whether it can get out: "If a dwelling is inadequately ventilated, moisture in the air cannot escape the building and is therefore more likely to cause condensation."¹ The other is how cold the surfaces are: "Homes that are insufficiently insulated and heated are more prone to the build-up of moisture and condensation on the inside of external walls and windows."¹ Put those beside the definition's examples (window frames, corners, the low points of walls behind furniture) and you have the places to look first. The sources give the mechanism in those words and no further, so this lesson stops there too.

Humidity, and why the winter number is lower

The amount of moisture in the air is measured as relative humidity. The University of Minnesota Extension, a US university's advice service, defines it plainly: "Relative humidity is the amount of moisture in the air at a given temperature."⁴ You measure it with a humidity meter, or hygrometer; EPA calls it "a small, inexpensive ($10-$50) instrument available at many hardware stores" (a US price, on a page updated in February 2026).²

The targets the sources give are all American. The GOV.UK guidance read here lists "consistently high relative humidity" among its warning signs but gives no figure.¹

They look like they disagree, and on this course's reading mostly they do not. Minnesota gives the reason its winter figure is so low: "Generally, in cold winter conditions, the relative humidity levels need to be low to prevent condensation and potential damage to the house’s structure, as well as to discourage mold growth, which can cause allergies and respiratory problems."⁴ So on Minnesota's account, a figure that suits a summer can be too high for a cold January. This course reads the three as a range and a direction, not three rival answers.

Before any tip: whose fault it isn't

Every piece of advice for cutting indoor moisture risks sounding like an accusation, and a lot of damp is lived with by people who did not cause it and can't fix it. The GOV.UK guidance speaks to this, and this lesson puts it before any tip: "We are absolutely clear that it is totally unreasonable to blame damp and mould in the home on ‘lifestyle choices’." It goes on: "It is unavoidable that everyday tasks, such as cooking, bathing, washing and drying laundry will contribute to the production of indoor moisture." Lesson 17 gave the next sentence, which puts the fundamental cause in the building, its ventilation, heating and energy efficiency, "not tenants’ normal domestic activities". The guidance calls approaches that blame occupants "unjust and counterproductive".¹

It also says landlords can help tenants make small, reasonable changes, as long as that sits alongside fixing the building and not instead of it. Its ventilation checklist has landlords talk to tenants about "leaving a gap between furniture and external walls, using extractor fans in kitchens and bathrooms, closing internal doors when they cook or shower, covering pans when cooking, drying laundry outdoors (if possible) and opening windows periodically".¹ Every one of those is Yours, as far as the home allows. But the guidance's own limit comes with them: "Tenants cannot be expected to reduce moisture levels if their home does not enable them to do so."¹ A kitchen with no working extractor, or a flat that can't be kept warm, is a building problem.

Ventilation: build tight, ventilate right

This is general information, not legal advice.

It's easy to assume a draughty house airs itself. The US Department of Energy's Energy Saver pages, which have since been taken down and are read here as archived, disagree: "Relying on air leakage for natural ventilation is not recommended."⁵ They give the reason: "During cold or windy weather, too much air may enter the house. When it's warmer and less windy, not enough air may enter, which can result in poor indoor air quality."⁵ The recommendation is "to reduce air leakage as much as possible and to provide controlled ventilation as needed", and the same pages say energy-efficient homes "require mechanical ventilation to maintain indoor air quality".⁵⁶

On the British side, GOV.UK's guidance says of windows: "Under building regulations, all replacement windows are required to have trickle vents".¹ That is GOV.UK's statement in guidance for England; this course didn't read the building regulations on ventilation themselves. Trickle vents are what the Energy Saving Trust describes as "small vents above windows that let fresh air trickle in".⁷ The GOV.UK guidance adds that trickle vents "should be controllable by the tenant, rather than permanently open, as this may encourage tenants to block them", and that air bricks, which it calls "a cost-effective and easy means of addressing inadequate under-floor ventilation", "should be kept clear and free of debris and obstruction".¹

The Energy Saving Trust, the UK's government-funded energy advice body, says the same from the draught-proofing side. When you draught-proof, it says, don't block extractor fans, underfloor airbricks, wall vents or trickle vents, and it adds: "Good ventilation is still essential, so you shouldn’t block vents, chimneys or airflow needed for safety and air quality."⁷ Some of those openings are there for a fuel-burning appliance, not for damp. Lesson 11, "Carbon monoxide and the alarms that warn you", gave the Trust's warning about the fixed vents a gas heater needs, and it holds here without exception.

The US pages add a warning about whole-house systems. They list the drawbacks of each kind, and say exhaust-only systems "Can cause backdrafting in combustion appliances", while supply systems "Can cause moisture problems in cold climates".⁶ Of the energy-recovery kind, the pages say only that "Only some HVAC contractors have enough technical expertise and experience to install them"; they do not say whether a householder may fit any of them.⁶ This course reads the backdrafting warning as a reason to leave whole-house ventilation in a home with a boiler, furnace or fire to a professional.

Here's how the jobs sort. Keeping trickle vents open, air bricks and wall vents clear, and running the extractor fans you have is Yours, in a rented home too. Replacing windows is Call someone: in England, GOV.UK calls installing windows "controlled building work" that "should be undertaken by someone competent to do so".¹ Fans and mechanical ventilation systems, the guidance says, "should be regularly inspected and maintained by qualified professionals".¹ So a fan that doesn't run is, on this course's label, Call someone, reported or booked and not opened up. Lesson 5, "Electricity: what trips, and why", set out where a householder's hands stop with electricity: at the front of the consumer unit or panel.

Mould: moisture first, then a small patch

EPA's guide on mould, A Brief Guide to Mold, Moisture and Your Home, opens with its whole argument in one sentence: "The key to mold control is moisture control."² It explains why you can't win the other way: "It is impossible to get rid of all mold and mold spores indoors", and spores don't grow without moisture.² So cleaning is half the job, and the other half is finding the water. EPA: "If you clean up the mold, but don't fix the water problem, then, most likely, the mold problem will come back."² GOV.UK says the same: "Simply removing surface mould will not prevent the damp and mould from reappearing."¹ After a leak or flood, EPA's rule is to "dry water-damaged areas and items within 24-48 hours to prevent mold growth", and CDC gives the same window after a flood.²³

Why bother at all? GOV.UK's key message is that "Damp and mould primarily affect the airways and lungs, but they can also affect the eyes and skin", and that in the most severe cases the respiratory effects can cause death.¹ The US Centers for Disease Control and Prevention (CDC) reports that the Institute of Medicine found in 2004 "sufficient evidence" linking indoor mould with upper respiratory symptoms, cough and wheeze, asthma symptoms in people with asthma, and hypersensitivity pneumonitis, an immune-mediated condition, in people susceptible to it; this course read CDC's summary, not the report.³ CDC also says exposure "may cause a variety of health effects, or none at all", and EPA says "Research on mold and health effects is ongoing."²³ Symptoms and treatment are for a doctor, not this course.

How much is yours

This is where the two countries' guidance is worded differently.

• In the US, EPA gives a size: if the mouldy area is "less than about 10 square feet (less than roughly a 3 ft. by 3 ft. patch), in most cases, you can handle the job yourself".² Ten square feet is a little under a square metre (0.93 m², this course's conversion). Over that, or after a lot of water damage, EPA sends you to its longer remediation guide, and says any contractor you hire should have experience cleaning up mould. If the water was sewage or other contaminated water, it says to "call in a professional".²
• In the UK, the GOV.UK guidance gives no measurement. It says "most tenants could reasonably be expected to remove condensation and very small amounts of mould using an appropriate mould and mildew cleaner, larger areas of mould should only be addressed by qualified professionals."¹

So a small patch on a hard surface is Yours, on conditions: small enough (under about 10 square feet on EPA's figure, and "very small amounts" on GOV.UK's), on a surface you can clean without pulling anything apart, and with the moisture found. If you rent, it's yours too, and reported to the landlord: GOV.UK expects most tenants to deal with very small amounts. Anything larger, anything soaked with sewage, and anything hidden behind wallpaper, panelling or a carpet is Call someone. The hidden kind is on that list for a reason both countries give: GOV.UK warns that "removal of carpets or wallpaper can lead to a significant release of mould spores", and EPA that "removal of wallpaper can lead to a massive release of spores if there is mold growing on the underside of the paper".¹² EPA also says: "If you have health concerns, consult a health professional before starting cleanup."²

Cleaning a small patch

The sources' steps, put together:

1. Record it. GOV.UK: "it is advisable for tenants or the landlord to photograph the area affected by mould and (if possible) draw around it to preserve a record".¹ If the patch comes back, the pencil line tells you whether it's growing.
2. Protect yourself. GOV.UK: "Regardless of the extent of mould, the person removing the mould should wear protective equipment, such as a mask, gloves and goggles".¹ EPA is more specific: "Gloves, goggles, N-95 respirator", long gloves to mid-forearm, and "Goggles that do not have ventilation holes are recommended"; it says you "may want to" wear the respirator.² Lesson 13, "Tools, dust and fumes: a small kit used safely", covered how a disposable respirator has to fit to work, and what a beard does to that.
3. Air the room the right way. GOV.UK: "Doors should be shut to help prevent mould spores from being spread to other areas of the house, but windows should be left open during and after the clean up activity."¹
4. Clean and dry. EPA: "Scrub mold off hard surfaces with detergent and water, and dry completely." GOV.UK says to use an appropriate cleaning product, follow the product's guidance, and leave the surface "to dry completely".¹²
5. Throw out what can't be cleaned. Both say absorbent things such as carpets, soft furnishings and ceiling tiles may have to go.¹²
6. Don't cover it up. EPA: "Do not paint or caulk moldy surfaces."²
7. Find the water. EPA won't call the job finished until the moisture problem is "completely fixed".²

Those steps assume a small patch, on a hard surface, where nobody has already put a product on it. If something has been sprayed on it already, read that product's label before you add anything.

Bleach: three bodies, three answers

This is a real disagreement between official bodies, if a small one, and each says it differently.

In metric, CDC's limit is about 240 ml of bleach in 3.8 litres of water, roughly one part bleach to sixteen parts water (this course's conversion from US measures). EPA adds a reason killing isn't enough: "Dead mold may still cause allergic reactions in some people, so it is not enough to simply kill the mold, it must also be removed."²

Where they agree matters more. All three put fixing the moisture first. And the two that discuss mixing say never mix bleach with ammonia: CDC, "Never mix bleach with ammonia or other cleaners", and EPA, because "toxic fumes could be produced".²³ Lesson 4 gave Poison Control's wider rule, never mix household cleaning products at all.

A flat on Wearside, in January

A woman renting a first-floor flat on Wearside moves her wardrobe to vacuum behind it and finds black mould on the wall, about 50 cm by 30 cm, at skirting (baseboard) height. It is an outside wall. The wall above the wardrobe line is clean.

Which kind? It's behind furniture, low on a cold outside wall, in winter, which is almost word for word GOV.UK's description of where condensation damp forms.¹ Before settling on that, she looks at the obvious alternatives. Is there a pipe in or near that wall (traumatic damp)? Is there a gutter overflowing or a crack in the render on the outside of that wall (penetrating damp, lesson 17)? If not, condensation is the lead suspect.

Whose job? At about 0.15 m² (1.6 square feet, on this course's conversion), it is well under EPA's figure and fits GOV.UK's "very small amounts". It is on painted plaster. So cleaning it is hers, on conditions: she photographs it and draws round it, and because she rents, she reports it to her landlord as well. GOV.UK says tenant cleaning "should not be a substitute for assessing and addressing the underlying issue, which should always be the priority".¹

The clean, by the steps above: mask, gloves and goggles, door shut, window open, a mould and mildew product by its label (GOV.UK's preference over bleach), dried completely.

The water. She leaves a gap between the wardrobe and the wall, which is the first item on GOV.UK's list, uses the extractor when she cooks and showers, and starts the condensation diary from the practice section. If the patch comes back past its pencil line, the diary goes to the landlord with the photographs.

Rising damp: what's agreed, and what isn't

Apart from bleach and the exact humidity figures, the sources so far have agreed with each other. On rising damp they don't. How often it happens, and whether the usual treatment works, are disputed among surveyors, damp-proofing firms and people who look after old buildings, and the dispute has gone on for years. This is a contested empirical question, so the lesson gives each side in its own words and does not decide it. Logic and Argument lesson 7, the strongest version first, is the rule for reading it.

The official line in England

GOV.UK's full paragraph is the place to start, because it holds both halves. Rising damp "is usually found in older properties and is often misdiagnosed. It can be identified through visual inspection; however chemical testing is the most appropriate way of confirming it. Often it is due to defective damp proof courses and membranes."¹ A damp-proof course (DPC), in the Property Care Association's description, is a waterproof barrier "installed roughly at about 6 inches from the ground" (about 15 cm) to stop water climbing the wall.⁸ So the guidance treats rising damp as real, says it's often misdiagnosed, and says how to confirm it.

The case that it's real and treatable

The Property Care Association (PCA) describes itself as "the trade body representing specialists across the UK who can be trusted to help resolve problems affecting buildings such as rising damp".⁸ Its account: "Rising damp is simply due to the process of external ground water being drawn up through brickwork by what is known as capillary action." The signs it gives are moisture at the base of a wall that reduces with height, often a "‘tide-mark’", and salts: "Hygroscopic salts (particularly chlorides/nitrates) are almost invariably present in groundwater".⁸ It explains what normally stops it: "Most types of masonry used in the walls of buildings will allow some water movement by capillary action; however, this is usually controlled by a physical barrier or damp proof course. If this physical barrier is absent, has broken down or is damaged then it is often possible to install a remedial damp proof course (DPC) to control water rising from the ground."⁸

Its first remedy isn't a treatment. Where the DPC has been bridged, by raised ground levels, a patio or planting area, bridging render or debris in the cavity, it says "removing the ‘bridge’ should hopefully be sufficient to control the capillary rise of moisture in the wall." Only where the DPC "has failed in some way" does it suggest a PCA specialist and a survey, and it says: "Without a proper investigation (ideally with a qualified damp specialist) it is impossible to advise what the exact cause of your issue is."⁸ Where salts have got into the plaster, it says, the plaster may need replacing with salt-resistant plaster.⁸

The thesis comes to this course through the journal of the Royal Institution of Chartered Surveyors (RICS). Two chartered surveyors, Michael Cooper and Andrew Thompson, wrote there in 2020 that rising damp "could benefit from greater academic investigation", and summarised a 2017 University of Salford doctoral thesis by Leslie Sellers as concluding: "Rising damp is a real phenomenon that warrants treatment and the contemporary method of damp-proofing, installed as it would be on a real construction site, does provide effective control."⁹ This course read the article's summary, not the thesis.

The case that it's rare, and the treatment wasted

The sceptics' words here come from one report in The Architects' Journal, from June 2009, eight years before that thesis. Stephen Boniface, former chairman of the construction arm of RICS, said that "‘true rising damp’ is a myth" and that chemically injected damp-proof courses are "‘a complete waste of money’". His account of what's really going on: "‘The most likely causes of damp are moisture penetration and, most commonly, condensation’".¹⁰ Elaine Blackett-Ord, then chair of the Register of Architects Accredited in Building Conservation, said of injected DPCs that "they are ineffective and grossly expensive", and, of what she called "damaging impermeable cement based internal renders", that they "serve simply to conceal the problem in the wall behind. For most historic buildings this is extremely damaging and irreversible."¹⁰ Jeff Howell, a qualified bricklayer and the author of The Rising Damp Myth (2008), said, in the article's words, that "trials in the laboratory confirm the falsehood": "If you build a brick pillar and stand it in a tray of water, the bricks in the water will get wet, but the water doesn’t rise by capillary action". And he gave his reason: "Cement-based and most lime-based mortars will not allow water to go through."¹⁰ This course read the article, not his book.

The article itself set a reply beside them; the choice of reply is the article's. Terry Brown, of GMW Architects, said: "It’s right to question the diagnostic skills of commercial damp proofing firms, but to state categorically there is no such thing as rising damp undermines a whole litany of rules of brickwork detailing I’ve adhered to all my professional life." And: "Of course there is no reason why inherited conventional wisdom shouldn’t be challenged." But "the challenge has to be scientific and not anecdotal."¹⁰

Two things to hold while you weigh this. The sceptics' quotations are from 2009, and this course did not read any later statement of their case. The thesis is known here only through the RICS authors' summary, and this course did not read any assessment of it by the other side. The next document is one that the trade body and the conservation bodies both put their names to.

The common ground: the 2022 joint statement

In September 2022, RICS, Historic England and the PCA wrote and produced a joint position statement, Investigation of moisture and its effects on traditional buildings. Cadw, Historic Environment Scotland, Northern Ireland's Historic Environment Division, the Institute of Historic Building Conservation (IHBC) and the Society for the Protection of Ancient Buildings adopted it too.¹¹ So the trade body, the surveyors' institution and the conservation bodies are behind the same text. It is written for surveyors, "aimed at those providing consultancy advice or surveys to owners and buyers of a building", and it never uses the words "rising damp".¹¹ It sets out how any damp in an old building should be investigated.

By "traditional" it means "buildings with solid walls built from permeable materials such as brick, stone, earth, timber and lime-based mortars, plasters and renders. Traditional construction absorbs moisture but allows it to evaporate when conditions become drier. This is in contrast to modern construction, which relies on impermeable barriers to prevent moisture entering the fabric."¹¹ On this course's reading, that difference is what the conservation voices above were defending when they objected to impermeable renders. What the statement asks of a surveyor:

• Recognise that in old solid-walled buildings "moisture presence in such buildings may not be down to a construction failure."¹¹
• Understand salts and "how this might influence a resistance meter". The statement lists resistance meters among many measuring methods, alongside "gravimetric moisture analysis" and "gas-carbide meters".¹¹ It does not say how any of them works, and nor does this course.
• Deal with damp in stages: "Recognise how to deal with the obvious defects first (including, for example, matters such as ventilation, ground levels and lifestyle). Then, allow a period for monitoring and natural evaporation, and reassess before moving on to other treatments."¹¹ The statement does not define "lifestyle". Elsewhere its list for surveyors includes "Understand that occupation patterns can influence the moisture within the building."¹¹ It says nothing about blame, and GOV.UK's guidance, earlier in this lesson, calls blaming damp and mould on lifestyle choices "totally unreasonable".¹
• Make recommendations that "must be proportionate to the defect and the significance of the building."¹¹
• Where a surveyor or contractor also quotes for the work, "report objectively on the facts of the problem, only suggest remedial works that are proportionate to any defects discovered", and not specify "a product, treatment or process that will benefit the surveyor or their firm financially without declaring this to the client."¹¹

That's the standard you can hold any damp survey to, whichever side of the argument the surveyor is on.

What would settle it

For one wall, GOV.UK's answer is testing: chemical testing is, in its words, "the most appropriate way of confirming it".¹ For the general question, how common rising damp is and how well injected DPCs work, the sources read here disagree about what the evidence already shows. The RICS authors take Sellers' thesis to show that rising damp is real and that the treatment works, and also say the subject "could benefit from greater academic investigation".⁹ Jeff Howell, on the other side, pointed to laboratory trials that he said already settle it.¹⁰ This course did not find a study that both sides accept. Until then, a householder does not need to pick a side to act well: the cheap outside causes first, a diagnosis that does not rest on a meter alone, and a remedy in proportion.

An 1890s terrace in the Cynon Valley

A man has bought an 1890s terraced house in the Cynon Valley, in south Wales, with solid walls. A damp company's surveyor walks round with a resistance meter, gets high readings along the bottom of the front room's walls, points to a stained band on the plaster, and quotes for an injected DPC and replastering with salt-resistant plaster.

What the sources let him check before he decides.

• Outside first. The PCA's own first remedy is removing a bridge, such as raised ground or a path against the wall.⁸ The joint statement's first stage is the obvious defects, ground levels and ventilation among them.¹¹ So the gutters, downpipes and ground level along that front wall, lesson 17's walk-round, come before anything is injected. Cadw, in Wales, is one of the bodies that adopted the statement.¹¹
• The meter. The joint statement says salts can influence a resistance meter, and the PCA says salts are almost always present where groundwater has risen.⁸¹¹ Put together, which is this course's reading of the two, a high reading at the foot of the wall does not by itself tell him what put the moisture there. GOV.UK's guidance for England names chemical testing as the most appropriate way to confirm rising damp; none of the sources this course read describes how that testing is done, so ask the surveyor what they would test and how.¹
• The quote. The surveyor's firm would also do the work, which the statement expects and allows, on its conditions: objective reporting, proportionate work, and any financial interest in the product or treatment declared.¹¹

Getting the survey and any treatment is Call someone. What's yours is the outside walk-round, the three questions, and, if the answers don't satisfy you, a second opinion (this course's suggestion; the statement itself allows a surveyor who also quotes).

If you rent

This is general information, not legal advice.

In England, GOV.UK's guidance, written for landlords, says: "Irrespective of the type of damp (condensation, penetrating, rising or traumatic), landlords are legally responsible for addressing damp and mould".¹ That's GOV.UK's summary; this course did not read the laws it lists, and doesn't teach landlord and tenant law, including how quickly a landlord has to act. In the US, EPA tells renters to report leaks and moisture problems to the building owner or manager straight away, as lesson 17 quoted, and adds: "In cases where persistent water problems are not addressed, you may want to contact local, state, or federal health or housing authorities."²

For a renter in either country, the practical part is the same: photograph and mark the patch (GOV.UK), report it (EPA), and keep your own dated notes of what you reported and when, which is this course's suggestion rather than either source's. Cleaning a very small patch is yours on GOV.UK's account, and reported. Anything that changes the building, from a new vent to a new window, is something to agree with the landlord first, on this course's reading.

What people get wrong

"A damp meter reading means rising damp." GOV.UK's guidance for England says rising damp is often misdiagnosed and names chemical testing as the way to confirm it, and the joint statement says salts can influence a resistance meter.¹¹¹

"Mould at skirting height behind furniture means rising damp." GOV.UK's definition of condensation names "low points on walls behind sofas or wardrobes".¹

"Bleach kills it, so bleach fixes it." EPA says dead mould can still cause allergic reactions and has to be removed, and that the job is not finished until the moisture is fixed; GOV.UK prefers mould and mildew products to bleach.¹²

"Clean it well enough and it's gone for good." EPA: "It is impossible to get rid of all mold and mold spores indoors."² What stops regrowth is keeping things dry.

"An old draughty house ventilates itself." The Energy Saver pages: relying on leakage isn't recommended, because it lets in too much air in cold, windy weather and too little when it's warmer and calmer.⁵

"If it's condensation, the people living there caused it." GOV.UK calls blaming damp on lifestyle choices "totally unreasonable".¹

"Draught-proof everything and the damp will go." The Energy Saving Trust says not to block extractor fans, airbricks, wall vents or trickle vents, and GOV.UK warns that energy efficiency work can reduce ventilation.¹⁷

Practice

For your plan

This lesson adds to the map: every extractor fan, trickle vent, wall vent and air brick, and whether each works. It adds to the calendar: a look behind furniture on outside walls each winter, and the vents checked clear each autumn (this course's suggestion; no source gives an interval). It adds to the stop list: mould over about 10 square feet, mould behind wallpaper or panelling, any water from sewage, bleach mixed with anything, any fixed vent near a fuel-burning appliance blocked, and a damp treatment quoted on a meter reading alone.

Connections

Lesson 17, "Keeping water out: roof, gutters, ground and walls", is the outside half of this lesson, and the first stop for any damp wall. Lessons 2 and 4 gave the pipes that cause traumatic damp. Lesson 11, "Carbon monoxide and the alarms that warn you", set the rule this lesson kept: never block the air a fuel-burning appliance needs. Logic and Argument lesson 7 gave the method used on rising damp. Next, lesson 19, "Asbestos, lead paint and radon: what not to disturb", takes the materials an old wall may hide, and lesson 21, "Pests: find the cause, then control it", comes back to damp where it takes up woodworm.

Go deeper

• GOV.UK, "Understanding and addressing the health risks of damp and mould in the home", updated April 2026, free. Read here in full. Written for landlords in England: the kinds of damp, what causes them and what to check.
• US EPA, A Brief Guide to Mold, Moisture and Your Home, free. Read here in full. The size rule, what to wear and how to tell when a clean-up is finished, on one page.
• RICS, Historic England and the PCA, Investigation of moisture and its effects on traditional buildings, 2022, free, 15 pages. Read here in full. Written for surveyors, and the standard to hold a damp survey of an old house to.

Sources

1. Department of Health and Social Care (Office for Health Improvement and Disparities), Ministry of Housing, Communities and Local Government and UK Health Security Agency, "Understanding and addressing the health risks of damp and mould in the home", first published 7 September 2023, updated 1 April 2026. Guidance for landlords and housing professionals in England. Read: full body text; re-read 26 September 2026.
2. US Environmental Protection Agency, A Brief Guide to Mold, Moisture and Your Home, last updated 18 February 2026. Read: full text; re-read 26 September 2026.
3. US Centers for Disease Control and Prevention, "Mold" (cdc.gov/mold-health), dated 26 September 2024 on the page, read in full from Internet Archive capture 20260923211319 because the live page blocked automated reading. The 2004 Institute of Medicine report is known only through this page.
4. D. H. Corrin, R. Stone and P. Huelman, "Controlling moisture problems in your home", University of Minnesota Extension, reviewed 2023. Read: full text; re-read 26 September 2026.
5. US Department of Energy, Energy Saver, "Air Sealing Your Home", as archived: Internet Archive capture 20241231180120 (the page no longer exists on energy.gov). Read: full text.
6. US Department of Energy, Energy Saver, "Whole-House Ventilation", as archived: Internet Archive capture 20250102092834 (the page no longer exists on energy.gov). Read: full text.
7. Energy Saving Trust, "Draught proofing", read from Internet Archive capture 20260916151311 (the live site refuses automated readers). Read: full text. Its savings figures are not used.
8. Property Care Association, "Rising Damp Advice". The trade body for damp-proofing and related specialists, in its own words. Read: full text; re-read 26 September 2026.
9. M. Cooper and A. Thompson, "Rising damp needs continuing research", RICS Property Journal, 23 November 2020. Read: full text; re-read 26 September 2026. Leslie Sellers' 2017 thesis is known only through this article.
10. The Architects' Journal, "Rising damp is a myth, says former RICS chief", 26 June 2009. Read: full text; re-read 26 September 2026. Jeff Howell's book is known only through this article.
11. RICS, Historic England and the Property Care Association, Investigation of moisture and its effects on traditional buildings: Principles and competencies, joint position statement, first edition, September 2022, from the IHBC's copy. Read: full text; re-read 26 September 2026.