Heating and damp, and the mechanism nearly everyone gets backwards
It is not that warm air holds more moisture. What decides whether mould grows is the temperature of your coldest surface, and once you see it that way the cheapest fix is often not the boiler at all. Also: where the 18°C figure actually came from.
By The Damp GuyLast updated
The short version
Heating helps against condensation only, and it helps by warming the surface, not by drying the air. Mould risk is governed by the relative humidity at the coldest surface in the room, with 80% as the critical value, so a warmer wall sits further from the line. That matters because there is a second way to warm a surface, and it is usually cheaper: insulation. Against water crossing the wall from outside or rising through it, heating does nothing at all.
Warm air holding moisture is the wrong mechanism
It is not quite wrong as physics. It is wrong as an explanation of why your wall has mould on it, and it sends people to the wrong fix.
The version you will read almost everywhere goes: warm air holds more moisture, so heating the room takes the moisture out of it. The trouble is that it describes the air in the middle of the room, and nothing grows mould in the middle of the room. Mould grows on the coldest surface, and what governs it there is the relative humidity at that surface.
BS EN ISO 13788 puts the risk of mould growth at monthly mean surface relative humidities above a critical value taken as 0.8. That is why mould can appear on a wall that is warmer than the dew point and never visibly runs with water: it does not need a wet surface, only a humid one.
Now the useful part. Surface humidity depends on two things: how much moisture is in the room air, and how cold that surface is. There are therefore two levers, not one. Heating pulls the second. So does insulation, permanently, without a running cost, and that is the option missing from nearly every page that answers this question.
Two levers, four tools, and which one your problem needs
Every remedy people suggest for damp is doing one of these two jobs. Knowing which is how you stop buying the wrong one.
| Tool | Which lever | The catch |
|---|---|---|
| Heating | Warms the surface | Works, and you pay for it every day you use it. It also cannot out-run moisture: heat a sealed house with washing drying in it and the humidity climbs anyway. |
| Insulation | Warms the surface | Same lever, paid once. A neutral charity says cavity wall insulation is "much more likely to solve problems of damp caused by condensation because it makes your walls less cold". Upfront cost, and not every wall can take it. |
| Ventilation | Removes moisture from air | Cheap and effective, and it throws your heat out with the moisture, which is exactly why people stop doing it in the month they need it most. |
| Dehumidifier | Removes moisture from air | Same lever as a window, without losing the heat. Roughly 5p to 9p an hour to run, and it does nothing about a cold surface. |
Read down that table and the honest answer to "should I put the heating on?" becomes a different question: which lever is your problem actually on? A warm house with one freezing north wall has a surface problem, and more heat is an expensive way to solve it when the wall is the thing that needs changing. A cool house with five people, a tumble dryer and no extractor has an air problem, and heating it will not fix that either.
Where the 18°C figure actually came from
It is quoted as settled science in almost every article on cold homes. Its own issuing body is more careful than that.
The World Health Organization recommended 18°C for the general population, and 20 to 21°C for elderly residents. That guidance dates from 1987.
When the WHO came back to it for the Housing and Health Guidelines, its own systematic review recorded something that almost nobody repeats: those guidelines "were based on older recommendations, the evidential basis for which can no longer be traced". The review was commissioned partly to find out whether the threshold held up.
What it found was moderate certainty evidence supporting benefits of warming homes to at least 18°C for respiratory health and blood pressure, and "no clear temperature threshold for these benefits". So 18°C is a reasonable target supported by real evidence about warmth generally. It is not a cliff edge, and the confident way it gets quoted is not how its authors describe it.
This site makes that point about numbers a lot, because damp is an industry built on them. The reading on a damp meter gets quoted with the same confidence and turns out to describe wood. A number repeated often enough starts to sound like a measurement.
If you are a tenant being told to put the heating on
This is the part the council leaflets telling you to heat and ventilate leave out.
The guidance published for rented housing providers by the Ministry of Housing, Communities and Local Government, the Department of Health and Social Care and the UK Health Security Agency names inadequate heating as one of the fundamental causes of damp and mould. It names inadequate heating, not inadequate use of the heating, and it puts it in the same list as building deficiencies, inadequate ventilation and poor energy efficiency.
It goes further and says that "it is totally unreasonable to blame damp and mould in the home on 'lifestyle choices'", and that the fundamental cause will be those building factors "not tenants' normal domestic activities".
That reframes the standard advice considerably. If a home cannot be heated to a reasonable temperature at a reasonable cost, that is a property with a heating or an efficiency problem, and the guidance treats it as one. Being told to run the heating harder in a poorly insulated flat is being asked to pay, monthly, for a building deficiency.
What to do, in the order that costs least
Nothing here is a treatment. It is the sequence that stops you buying the expensive answer to the cheap problem.
- Measure before you spend. A hygrometer settles which lever you are on. Aim at 40 to 60% relative humidity, the range UK professional and public-health guidance lands on. Above it, you have an air problem. Inside it with damp still present, you have a surface problem or you are not looking at condensation at all.
- Find out which kind of damp it is first. Heating is the wrong tool for two of the three, and the way to tell them apart is when the patch gets worse, not what it looks like.
- Heat on a timer, not permanently. On cost, the charity view is blunt: do not pay for heat you are not using, and set the timer for about 30 minutes before you get home or wake up.
- Price insulation against a winter of gas. It pulls the same lever as heating and you pay for it once. On a cold external wall that keeps growing mould in the same corner, it is the intervention most likely to actually end the problem.
- Do not put bowls of water near heating vents. It is offered as a comfort tip and the same charity warns it can create problems with damp caused by condensation. You are adding moisture to solve a moisture problem.
About this resource
The 18°C material is from the WHO Housing and Health Guidelines systematic review on the effect of indoor cold on health, including its own statement about the traceability of the 1987 recommendation and its finding of no clear temperature threshold. The constant-heat and insulation positions are from the Centre for Sustainable Energy, a registered charity, chosen deliberately because it sells neither heating nor insulation. The cause-versus-symptom wording is quoted from the government guidance for rented housing providers, updated 1 April 2026.
The 80% critical surface humidity criterion is from BS EN ISO 13788 and the 40 to 60% range is guidance from CIBSE, the WHO and the HSE rather than anything in BS 5250, both already sourced for the condensation pillar on this site. The dehumidifier running costs are carried across from that page, where the wattages and the capped unit rate are sourced in full.
There is no cost per hour for heating on this page. It depends on the heat loss of the building, the fuel, the boiler and the flow temperature, and no source we read gives a general figure that would survive contact with a real house. There is no recommended thermostat setting either: what WHO recommended is reported, and what to do with that is not ours to prescribe.
This site does not survey, treat or quote for work, and it cannot tell you what is happening in your wall. If the damp tracks neither the cold nor the rain, that is the point to get an inspection from someone with no interest in selling you the treatment.
Common questions
Does putting the heating on reduce damp?
It helps with one of the three kinds and does nothing for the other two. For condensation, yes: warming a cold wall raises that surface's temperature, and the relative humidity at the surface falls as a result. Mould risk is governed by surface humidity above 80%, so a warmer wall sits further from the line. For water crossing the wall from outside, or moving up through it, heating changes nothing about the supply. It will dry the face of the plaster and the patch will come back.
Will an unheated house get damp?
It is much more likely to get the condensation kind, and that is not a comment on the occupants. An unheated house has colder surfaces, and colder surfaces reach the mould threshold at a lower room humidity. Government guidance is explicit that inadequate heating is one of the fundamental causes of damp and mould, in the same list as building deficiencies, inadequate ventilation and poor energy efficiency, and that these are the cause rather than normal domestic activity.
Is damp worse in winter or summer?
Condensation damp is a winter problem, because it depends on the gap between how humid the air is and how cold your surfaces are, and winter widens that gap at both ends: you ventilate less and the walls are colder. Penetrating damp follows the rain, so it is worse in any wet spell whatever the season. Rising damp is the odd one out, because it tracks neither, which is one of the more useful things to know about it.
Is it cheaper to run a dehumidifier or the heating?
They are not doing the same job, so the comparison misleads. A dehumidifier changes the air, and its cost is easy to pin down: a compressor unit draws 150 to 260 watts and a desiccant around 330 watts, which at the capped 26.32p per kWh is roughly 5p to 9p an hour. Heating changes the surface, and its cost depends entirely on the heat loss of your building, so no honest general figure exists. The right question is which variable your problem actually turns on.
Is it cheaper to leave the heating on low all day?
The Centre for Sustainable Energy, a charity with nothing to sell, puts it bluntly: "Don't pay for heat that you're not using! If you are out during the day (or tucked up in bed at night), you don't need the heating on." They suggest setting the timer to come on about 30 minutes before you get home or wake up. The argument that constant low heat prevents damp is real in mechanism, because surfaces stay warmer, but it is being made mostly by people who sell heating.
What stops damp coming through walls?
Nothing you do indoors, if it is genuinely coming through. Water crossing a wall from outside is a building fault with a location, and it is fixed at that location: the pointing, the render, the gutter, the sill, the cavity. Heating the room and running a machine will change how the inside face feels and will not touch the reason water is arriving. That is the distinction worth getting right before spending anything, because the two situations have no fix in common.
How do I make my house less damp?
Work out which of the two variables you can move, then move the cheaper one. The variables are how much moisture goes into the air and how cold your coldest surfaces are. Ventilation and a dehumidifier act on the first. Heating and insulation act on the second, and insulation is often the better buy: the same charity notes that cavity wall insulation "is much more likely to solve problems of damp caused by condensation because it makes your walls less cold". Aim for 40 to 60% relative humidity and measure rather than guess.
Should I heat my home to 18°C?
That is the number everyone quotes and it is worth knowing where it came from. The WHO recommended 18°C for the general population and 20 to 21°C for elderly residents, in 1987. Its own later systematic review says those guidelines "were based on older recommendations, the evidential basis for which can no longer be traced", and that while there is moderate certainty evidence for warming homes to at least 18°C, the review found "no clear temperature threshold for these benefits". So it is a reasonable target with a weaker pedigree than its confident repetition suggests.