Mould in homes
Mould is a symptom. The building is the cause.
Mould in a home is not bad luck, and it is not poor housekeeping. Mould grows wherever three things meet: moisture, a surface to live on and still air. Every one of those is a decision the building makes for you — which means mould is almost always a building question long before it is a cleaning one.
The mechanism
How mould actually works.
Mould spores are everywhere — outdoors, indoors, in every breath. That is normal, and it is not the problem. The problem begins when a spore lands somewhere that gives it what it needs: persistent moisture, an organic surface such as paint, plasterboard, timber or dust, and time. A water event gives it the fastest start; a bathroom, laundry or kitchen produces moisture all on its own.
Given a damp surface, many common moulds begin growing within twenty-four to forty-eight hours. But the more common path is slower and quieter: a surface that sits just below the point of visible wetness — a wall at a steady temperature below the air’s dew point, a wardrobe door that keeps the space behind it cooler than the room — stays damp enough, often enough, for months. Relative humidity held above roughly seventy percent at a surface is, broadly, how long it takes before growth becomes visible.
That is why wiping mould away rarely solves it. You removed the fungus; the moisture that invited it is still arriving on schedule.
Where it hides
The confined spots mould prefers.
Behind furniture on external walls
A wardrobe or bed pushed against a cool wall traps still air against a cold surface. The space behind it becomes the most likely mould site in many bedrooms.
Bathrooms and laundries
The wettest rooms in the house. Steam that has nowhere to go settles on ceilings, around showers and inside vanity cabinets — especially where extraction stops the moment the light does.
Window reveals and corners
Corners and reveals are coldest and least ventilated. Condensation forms there first, night after night, long before anything is visible.
Under sinks and behind appliances
Slow leaks, condensation on pipes and the warmth behind fridges and washing machines create small, hidden climates that mould exploits for years.
Roof voids and ceilings
Warm, moist air rising from living spaces meets a cold roof deck above the insulation. Without a sealed and ventilated ceiling plane, mould grows where nobody looks.
Slab edges and wall bases
Ground moisture moving through a slab or rising a wall finds the junction between floor and wall. Skirtings that stain seasonally are usually telling you about this.
Condensation
Condensation: the everyday mechanic behind most mould.
Warm air holds more moisture than cold air. When warm, humid air touches a surface colder than its dew point — the temperature where the air can no longer carry its water — the moisture condenses there. This happens in two ways, and homes get both.
Surface condensation is the visible kind: fogged windows in the morning, beads on a cold window frame, dampness on an outside wall behind a bed. It arrives nightly in winter, in the coldest corners, wherever ventilation is weakest — and single glazing and uninsulated wall edges give it thousands of extra stages.
Interstitial condensation is the kind you never see, forming inside the wall or roof assembly itself where warm indoor air reaches a cold layer within the structure. It is the more serious of the two, because it can grow mould and decay inside a closed wall for years before anything shows. Its prevention is invisible and unglamorous: continuous insulation, vapour management, airtightness, and detailing every thermal bridge — the framing and fixings that carry cold through an otherwise warm wall — out of the design.
The missing fresh air
Stale air: the ingredient nobody schedules.
The third ingredient in mould is stillness. A family of four produces litres of water vapour a day simply by breathing, cooking, showering and drying laundry. In older, leaky homes that moisture drifted out through gaps in the building fabric. In a well-sealed modern home — which is a good thing for comfort and energy — it has nowhere to go unless the building plans a path for it.
This is why bedrooms are often the worst-affected rooms. A sealed bedroom overnight pushes both carbon dioxide and humidity upward at exactly the time the windows are closed and the room is coldest. The result is a window that fogs by morning and a wall behind the bed that is slightly damp for months. Health-wise, a bedroom that cannot shed its own moisture is also a bedroom with stale air — the same cause, two symptoms.
The fix is not a dehumidifier, any more than a bucket under a leak is a roof. The fix is ventilation that is designed, measured and balanced — supply and extraction sized for the home and how it is actually used — paired with surfaces warm enough that the moisture that does move through the air never finds a cold stage to settle on.
The signs
What a mould problem looks like before it looks like mould.
By the time mould is visible, the building has usually been trying to tell you for a year or more. The signs that come first are quieter: a musty smell that returns no matter how often you air the room; windows that fog every cold morning; clothing in a wardrobe smelling stale between wears; a corner that stains seasonally; a bathroom ceiling that keeps a grey tint after cleaning.
People notice the health signals too, even when they do not connect them: congestion that clears when you leave the house, asthma that is worse at home, a child who sleeps badly in one particular room. These are not diagnoses — but they are reasons a building biology assessment looks at moisture and air together rather than room by room.
It is worth being plain about health: common indoor moulds are triggers for allergic reactions, asthma and irritation in sensitive people, and people with weakened immune systems are advised to take mould exposure seriously. The honest position is that the healthiest response is to remove the conditions, not to debate the species on your wall.
Which is the entire prevention argument: a home designed against condensation and for fresh air does not spend its life having these conversations. That is how our consultancy treats it — as a design decision, made once, early.
Designing it out
How a home is designed so mould never starts.
Every measure below is cheap at the drawing stage and expensive afterwards. Ventilation comes first: balanced, continuous air exchange — not an extractor fan switched on by a timer, but a system that moves air through the home on purpose, bedroom to living to outside, with warmth recovered along the way in colder climates.
Then thermal design: insulation laid continuously rather than interrupted, junctions and edges detailed so no framing or slab edge carries cold through the building fabric, warm interior surfaces, and glazing that keeps its inner pane above the dew point. A surface that stays warm cannot collect the condensation that feeds mould — so the coldest corners of the house are eliminated before they are ever cold.
Then the water side: roof and plumbing details that keep water outside and under control, drainage that moves stormwater away from the slab rather than toward it, and materials chosen for what they do under a wet decade as much as for how they look on day one — a principle the Homey Standard holds throughout its materials pillar.
None of this shows in a photo. It shows across the years — in a bathroom that dries, a bedroom that breathes, and a building that never needs the certification conversation to explain itself, because the record already says how it was built.
If it is already there
When a mould inspection makes sense.
If you can smell mould but not see it, see it repeatedly in the same place, or your home has had a leak, flood or long-term damp, an inspection is the efficient next step. A proper one is a building diagnosis, not a wipe test: moisture mapping through walls and floors, thermal imaging to find the cold surfaces and thermal bridges, humidity and ventilation readings, and laboratory sampling where a sample actually answers a question — rather than as a routine ritual.
Expect the findings to read like a cause chain: the surface that is cold, the reason it is cold, and the moisture that keeps arriving there. Removal of the growth without addressing the cause is repainting a symptom. Our building-biology work always ends with the cause on the page — and for a new home, it never has to start with the discovery.