Condensation, and why a dry wall still grows mould
The number that decides whether mould grows is not the dew point. It is not the reading on your hygrometer either. What condensation is, the humidity to aim at, and how to tell it apart from the two kinds of damp people pay to have treated.
By The Damp GuyLast updated
The short version
Mould needs a humid surface, not a wet one. BS EN ISO 13788 uses a critical surface relative humidity of 80%, and a surface held above it for long enough is where many mould spores germinate. A surface can pass it while still being warmer than the dew point. So your wall feels dry and grows mould anyway. That is the point most people decide they must have real damp, and go looking for a damp company. Aim for 40 to 60% relative humidity in the room, the range UK professional and health guidance settles on. Keep the air humid for long enough and your cold surfaces cross that 80% line.
Room humidity is not surface humidity
This is the gap that makes your hygrometer reading reassuring and wrong.
A cheap hygrometer sits in the middle of the room and reads the air around it. Mould grows on the coldest surface in that room. The humidity there runs higher, because the air touching it is colder. In most rooms that surface is the glass in the window, which is why it wets before anything else.
The link is real, but it is not a fixed number. Under the BS EN ISO 13788 method, sustained high indoor humidity drives a cold, poorly insulated external wall into the mould-risk band. How fast depends on how cold that surface runs against the room. So your room can read in the seventies and feel unremarkable while the outside wall behind the wardrobe is already over the line, and smelling faintly of it before anything shows. A better-insulated wall in the same room is not.
The number to aim at is lower than most people expect. The Chartered Institution of Building Services Engineers (CIBSE) recommends 40 to 60% relative humidity for homes. Now you have something to measure against, instead of a vague instruction to open a window.
| Who says it | The range | What kind of guidance |
|---|---|---|
| CIBSE | 40 to 60% | building services guidance, for comfort as much as for mould |
| World Health Organization | 40 to 60% | public health guidance, arrived at independently of the building trade |
| HSE | 40 to 70% | workplace guidance, and the most permissive of the three at the top end |
Three bodies, and they do not quite agree at the top end. They are listed separately rather than merged into one range, because a single averaged figure would read as a settled standard and none of this is one. It is guidance, and it is not from BS 5250.
Find the line in your own room
Two numbers off a hygrometer give you the temperature a surface has to drop to before it is in the mould band, and the lower one at which it starts running wet. The gap between them is the thing nobody publishes.
Mould risk starts at
15.4°C
Any surface in that room colder than 15.4°C is above the 80% surface humidity BS EN ISO 13788 treats as the mould risk threshold. It does not run wet until 12.0°C.
- Mould risk threshold
- 15.4°C
- Dew point, where it runs wet
- 12.0°C
- The gap between them
- 3.4 degrees
That gap is the answer to why your wall feels dry and grows mould anyway. Both figures come from one formula in WMO-No. 8, solved for 80% and for 100%. The pressure correction it also gives is not applied, because it moves a dew point by under a tenth of a degree and cancels out of the humidity ratio entirely.
This locates a threshold on a thermometer. It does not know your wall, and it is not a forecast that anything will grow. ISO 13788 states its criterion as a monthly mean, and whether mould actually takes hold depends on how long conditions hold, what the surface is made of and what is in the air. Use it for one thing: seeing how much colder than the room a surface has to be before it is in trouble.
The same thing, at a glance
For a room held at 20°C. Read across from whatever your hygrometer says and you have the two temperatures for your house.
| Hygrometer reads | Mould risk below (80% at the surface) | Runs wet below (dew point) | Gap |
|---|---|---|---|
| 40% | 9.3°C | 6.0°C | 3.3 degrees |
| 50% | 12.6°C | 9.3°C | 3.4 degrees |
| 60% | 15.4°C | 12.0°C | 3.4 degrees |
| 70% | 17.9°C | 14.4°C | 3.5 degrees |
| 80% | 20.0°C | 16.4°C | 3.6 degrees |
Look at the 20°C room reading 70%. Every surface below 17.9°C is over the line, which in a heated room is very nearly everything touching an outside wall. It will not run wet until several degrees colder than that, so nothing in the room looks wrong.
Computed from the saturation vapour pressure formula in WMO-No. 8, Annex 4.B, solved for 80% and again for 100%. The 80% criterion is BS EN ISO 13788's, stated there as a monthly mean. How cold a particular wall actually runs against the room depends on how it is built, which no table can tell you.
Watch when it changes, not what it looks like
Appearance is the least reliable signal you have. All three can put the same dark patch on the same wall.
| Kind | When it gets worse | What it is |
|---|---|---|
| Condensation | Cold still weather, windows shut, room in use | Moist air meeting a cold surface |
| Penetrating damp | During and after rain | Water crossing the structure through a defect |
| Rising damp | Neither, in particular | Moisture moving up by capillary action |
Timing beats appearance, and it costs you nothing. A fortnight of paying attention, not a survey. One thing sits outside the timing test entirely: water sealed between the panes of a double glazed unit ignores the weather and the room alike, because it is a failed component rather than damp.
What actually shifts it
Nobody wants a lecture on vapour. You want the black spots round the window gone by spring.
Three things decide whether they go: how much moisture goes into the air, how much of it leaves, and how cold the surfaces are that it can land on. So you have three levers. Which one moves your problem depends on your house, not on a general rule.
- Make less of it. Drying washing indoors, cooking uncovered and long showers all pump water into the air of a sealed house.
- Take it out where you make it. Extracting at the source beats spreading it through the whole house and diluting it later. Ventilation in dwellings is covered by Approved Document F. Moisture more broadly sits in Approved Document C.
- Warm the cold surfaces. The mould picked the coldest spot for a reason. Warm that surface and the humidity at it drops, and that is the number deciding whether anything grows.
About this resource
The thresholds here come from published standards. BS 5250:2011 was the code of practice for the control of condensation in buildings. BS 5250:2021 has superseded it, retitled "Management of moisture in buildings". The 80% surface criterion is from BS EN ISO 13788, which puts the risk of mould growth at monthly mean surface humidities above a critical value taken as 0.8. The 40 to 60% range is CIBSE's recommendation for dwellings, from its TM40:2020 guidance, not anything in BS 5250. That distinction is worth making, because plenty of pages quote it as though it came from the standard.
No ventilation rates in litres per second, and no costs. Health is a medical question this site cannot answer, so the page on whether black mould is dangerous reports what the government's own guidance says about damp, mould and health rather than judging it here.
This site does not survey, treat or quote for work. It cannot tell you what is happening in your building. If your pattern tracks neither the cold nor the rain, it is neither of the two kinds this page separates, and the next step is an inspection by somebody with nothing to sell you.
Common questions
Does opening the windows reduce damp?
It moves one of the three levers, and only one. Opening a window takes moist air out, which lowers the humidity in the room. It does nothing about how much moisture you put into the air, and nothing about how cold your coldest surfaces are. It also does nothing at all if your damp tracks the rain rather than the cold, because then the water is arriving through the wall and no amount of fresh air stops that.
Does having the heating on help with damp and mould?
Yes, by the surface route rather than the air one. Warming a cold wall raises its temperature, and the humidity at that surface falls as a result. Mould risk rises once the humidity at a surface stays above 80%, so a warmer wall sits further below the line. What heating cannot do is out-run moisture. Heat a sealed house that has washing drying in it and you can end up back where you started.
Why are my new windows running with condensation every morning?
New glazing usually makes a house tighter, so less air leaks out overnight and the moisture from cooking, washing and breathing stays in. The room humidity rises. If the glass itself is still the coldest surface in the room you see it on the glass. If the new units are warmer than the walls, the water goes to the walls instead, and that version is worse because it lands somewhere you cannot wipe. Aim for 40 to 60% relative humidity in the room and the problem moves either way.
Will condensation clear up on its own?
The water will. The conditions will not. A window dries off by lunchtime and the wall behind the wardrobe dries a little in a warm spell, then both go back to doing the same thing the next cold night, because nothing about the room has changed. Mould does not need a wet surface to keep going either. It needs a humid one, which is why walls grow it without ever visibly running.
Do dehumidifiers get rid of damp?
A dehumidifier pulls moisture out of the air, so it works on the same lever as opening a window, and it does it without letting the heat out. That is genuinely useful for a condensation problem. It does not warm the cold surface the mould picked, and it does nothing whatever for water crossing the wall from outside. No unit is recommended on this website, because nothing here earns a commission and no meaningful testing has been done.
Does condensation mean my house is too cold?
Not on its own. Condensation is about the gap between how humid the air is and how cold the surfaces are, so it turns up in warm humid houses with one cold wall as readily as in cold ones. The useful number is the humidity, not the thermostat. The Chartered Institution of Building Services Engineers (CIBSE) recommends 40 to 60% relative humidity for homes. Measure that first, then decide whether the answer is heat, ventilation or insulation.
Sources (6)
- marley.co.uk: bs5250 2021 new guidance on managing moisture risk in buildings, read 30 August 2026
- tradecalculator.co.uk: condensation risk calculator, read 30 August 2026
- condair.co.uk: industry guidelines and regulations on indoor humidity, read 30 August 2026
- landingpage.bsigroup.com: Standard, read 30 August 2026
- standards.iteh.ai: iso 13788 2012, read 30 August 2026
- WMO, *Guide to Meteorological Instruments and Methods of Observation* (CIMO Guide), WMO-No. 8, seventh edition 2008, Annex 4.B. Read as the published PDF at, read 4 September 2026