Condensation, dew point and mould: the building science

Mould grows because condensation keeps surfaces damp. When warm, moist indoor air meets a cold surface its relative humidity climbs, and at the dew point it releases liquid water. Mould can start at roughly 80% surface humidity, before any droplets appear. Cold spots, winter temperature gaps and poor ventilation are what tip surfaces over the line.

Plenty of homes have that one window someone wipes every winter morning, with a grey patch that comes back in the same cold corner behind the wardrobe. Same spots, every year. That is not bad luck. It is building physics, and once you can see the physics the pattern stops being mysterious.

This page is the why. It works through dew point, surface humidity and thermal bridging in plain English, so you can read your own home and understand why the mould keeps choosing the same wall. The step by step for the two places it shows up most sits in two companion posts, linked further down. Here we stay with the physics.

What is dew point, and how does it cause condensation?

Air is a sponge for water vapour, and warm air holds more of it than cold air. Heat a room and the air can carry plenty of moisture with no sign of it. Chill that same air against a cold surface and its relative humidity climbs, because colder air holds less. Keep chilling it and you reach the dew point: the temperature at which the air is fully saturated, 100% relative humidity, and can hold no more. Past that point the vapour has nowhere to stay, so it condenses into liquid water on the surface.

The WHO Guidelines for Indoor Air Quality: Dampness and Mould put the mechanism plainly, noting that "allowing surfaces to become cooler than the surrounding air may result in unwanted condensation." The fog on a cold drink in summer is the same effect. So is the water running down a single-glazed window on a winter morning.

What humidity level does mould actually grow at?

Mould does not wait for visible droplets. It responds to the humidity right at the surface, and that can sit well above the humidity of the room. Germination can begin at roughly 80% surface relative humidity on the materials our homes are made of, gypsum plasterboard, paper linings and timber, so the first colonies often appear on a wall that still looks dry.

Building scientists set the threshold conservatively. In the reference paper Relative Humidity (RR-0203) by Joseph Lstiburek, the consensus figure is stated directly: "the critical relative humidity for adverse biological activity to occur on building envelope surfaces" is 70 percent. Sit a surface above that for long enough and mould, dust mites and decay all become possible.

The current engineering risk model draws the line a little higher and adds time. an explainer of ASHRAE Standard 160-2021 flags mould risk when the 30-day running-average surface relative humidity stays at or above 80% while the 30-day running-average surface temperature is between 5C and 40C. The detail worth taking from that: it is sustained surface humidity that matters, not one bad morning.

Because surface humidity is set by surface temperature, you can turn this into numbers. Take a normal heated room at 20C and 50% relative humidity. The moisture in that air does not change, but as any surface in the room gets colder, the humidity pressed against it rises.

So a surface only has to reach the mid-teens in Celsius before it is into mould territory, and it does not need to get anywhere near freezing to run with condensation. This is why the wall behind a wardrobe or the reveal beside a window grows mould while the middle of the room stays clean. The air is the same. The surface is not.

That surface is where mould gets going, from a landed spore to a visible patch. For the wider picture of how mould works across a home, that pillar walks through the biology; this page stays with the physics that feeds it.

Why does mould grow on cold walls and in the corners of rooms?

External walls, corners and the junctions where a wall meets a floor or ceiling run colder than the open middle of a wall, and cold is exactly what pulls a surface toward its dew point. Corners lose heat in two directions at once and are often starved of moving air. Put furniture against an external wall and you trap a pocket of still, cooling air behind it. That pocket is where the surface goes coldest and stays wettest.

What is thermal bridging?

A thermal bridge is a part of the building that conducts heat straight through to the outside faster than the material around it, so the inside face of that spot stays locally colder. Concrete slab edges, steel lintels over windows, uninsulated corners and window reveals are the usual suspects. Peer-reviewed building research is blunt about the consequence: a study of moisture in wall-to-floor thermal bridges found that moisture in the building envelope "not only affects the characteristics of heat transfer but also induces mould growth and the condensation of water vapour." The cold line maps the mould line. Trace where the black growth runs and you are usually tracing a thermal bridge.

Why are condensation and mould worse in winter?

Winter stacks the deck three ways. The gap between your warm indoor air and the cold outdoors is at its widest, so windows, external walls and every thermal bridge are colder than at any other time of year. At the same time the household is pushing more moisture into the air, from showers, cooking, drying clothes indoors and simply breathing. And the natural response to cold is to shut every window, which traps all of that vapour inside with nowhere to go.

The result is the seasonal pattern people know well: streaming windows, damp reveals, dark corners that arrive with the first cold snap and clear in spring. If cold walls are your problem, the room-by-room fixes are set out in winter condensation and mould on walls.

Windows and their surrounds are a case of their own, because the glass and frame are usually the coldest surfaces in the room and the first to run. The specific problem of wet sills and frames is worked through in mould on window sills and frames.

Does opening windows reduce condensation and mould?

Yes, and the reason is worth understanding. Ventilation swaps warm, moisture-loaded indoor air for outdoor air that, once it warms up inside, carries a lower relative humidity and a lower dew point. Lower the dew point and your cold surfaces have more margin before they start to condense. The WHO guidelines make control of moisture a matter of temperature and airflow together, stating verbatim that "management of moisture requires proper control of temperatures and ventilation to avoid excess humidity, condensation on surfaces and excess moisture in materials."

There is an Australian wrinkle here, and in 2026 it is only getting sharper. Newer homes are built far more airtight than the leaky older stock, which is good for energy bills and awkward for moisture, because the incidental draughts that used to dry a house out are gone. A 2015 scoping study for the Australian Building Codes Board found the scale of it: as reported by Siniat, up to 40% of new buildings in Australia showed signs of condensation and mould.

That finding is why the current code takes it seriously. Part F8 of the National Construction Code 2022 exists, in its own words, "to safeguard occupants from illness or loss of amenity as a result of excessive internal moisture," and it does the job with airflow: ducted exhaust to outside at 25 litres per second for a bathroom and 40 litres per second for a kitchen or laundry, plus roof-space ventilation in the cooler climate zones. Mechanical extraction where the moisture is made, because the building can no longer breathe on its own.

Why does mould keep coming back after I clean it?

Because cleaning treats the stain while the physics keeps running. If a surface keeps dropping below its dew point or sitting above that 70 to 80% surface-humidity band, the conditions that grew the mould are unchanged, and a landed spore will start again on the same spot. Wiping it off resets the visible clock. It does not touch the cause.

Lasting control comes from moving the surface out of the risk zone, and there are only two levers. Warm the surface so it stops reaching the dew point, by insulating, breaking the thermal bridge, or getting heat and air to the cold corner. Or dry the air so the dew point drops beneath the surface temperature, through ventilation and steadier heating. Treat the recurring patch as a reading, not a stain. It is telling you a surface in your home is repeatedly going too cold or too wet, and that is the thing to fix.

Read your home before you reach for a cloth. The mould is a symptom of a surface that goes too cold or a room that holds too much moisture, and both of those are fixable once you know which one you have. If you want the whole chain from airborne spore to visible patch, our guide to how mould takes hold in a home ties the biology to the building science on this page.

Condensation, dew point and mould: common questions

Does condensation have to be visible for mould to grow?

No. Mould responds to surface humidity, and germination can begin around 80% surface relative humidity, which a wall reaches before any liquid water shows. Visible condensation at the dew point is the extreme end of the same scale, not the starting line.

What surface humidity does mould need?

Building science puts the conservative threshold for biological activity at about 70% surface relative humidity (buildingscience.com), while the ASHRAE 160-2021 risk model uses a 30-day running average at or above 80%. Both point the same way: sustained high humidity at the surface, not a single damp morning.

Why does the mould always come back in the same corner?

That corner is almost certainly a cold spot or thermal bridge that repeatedly falls below its dew or mould temperature. Until the surface is warmed or the air is dried, the conditions reset every cold day and the growth returns.

Will a dehumidifier or better ventilation stop it?

Both work by lowering indoor humidity and the dew point, which gives cold surfaces more margin before they condense. In an airtight modern home that often means running mechanical extraction where moisture is made, the bathroom, kitchen and laundry, as the National Construction Code now requires for new builds.

Related guides

• Winter condensation and mould on walls in Australia
• How to stop mould in a humid Queensland home
• What actually stops that musty mould smell

By Ben, founder of Aurala Naturals. Last updated July 2026.