What a dehumidifier actually fixes, and what it only hides
It is an excellent machine for one of the three kinds of damp and close to useless against the other two. What it costs to run against the current capped rate, the room temperature that stops it working, and how to tell which situation you are in before you spend anything.
By The Damp GuyLast updated
The short version
A dehumidifier removes water from air. That makes it a genuinely good answer to condensation, and close to no answer at all to rising or penetrating damp, where water is arriving through the structure faster than a machine can take it out of the room. Running one costs roughly 5p to 9p an hour at the capped rate. Before you buy one, buy a hygrometer: if the room is already under 60% relative humidity and you still have a wet patch, the dehumidifier was never going to fix it.
One machine, three completely different answers
The searches split cleanly into rising, penetrating and condensation. So should the answer, and almost nowhere does.
| Kind | Does it help? | Why |
|---|---|---|
| Condensation | Yes | The water is in the air, which is the one place the machine can reach. It lowers room humidity without opening a window, so unlike ventilation it does not throw your heat out with the moisture. |
| Penetrating | Barely | Water is crossing the wall from outside at a specific place. The machine dries the room while the defect keeps resupplying the wall. It will make the room feel better and will not touch the cause. |
| Rising | No | Moisture moving up through the structure is not in the air until it has already come through the wall. Drying the room does not stop it arriving, and the tank filling up is not evidence that it is working on the cause. |
The reason this matters more than it sounds is that a dehumidifier produces very convincing evidence of its own usefulness. It fills a tank. You empty it in the morning and think, look how much water was in this room. But a full tank only ever proves the air was humid. It says nothing about where the moisture came from, and in a house with water coming in sideways it will fill just as convincingly while the wall stays exactly as wet as it was.
Which of the three you have is the whole question, and it is not answered by looking at the patch. It is answered by watching when it gets worse. Condensation tracks the cold. Penetrating damp tracks the rain. Rising damp tracks neither and sits at a height.
The temperature floor nobody mentions
It is the reason a machine bought for a cellar, a garage or a loft can sit there doing almost nothing.
There are two technologies and they behave completely differently in the cold. A compressor dehumidifier works by making a surface inside itself colder than the room, so water condenses onto it and drips into the tank. That is a heat pump doing what your window does, on purpose. The catch is in the mechanism: the colder the room, the harder it is for the machine to make anything colder still.
Performance therefore drops as the room cools, and a compressor unit can stop working around 5°C as its coils freeze. A desiccant unit does not work that way at all. It passes air through a wheel of material that chemically attracts water vapour, which it will do at any temperature, and that is the entire reason the type exists.
| Room temperature | What the manufacturer says to use |
|---|---|
| Below 10°C | Desiccant only. |
| 10°C to 15°C | Desiccant, or a large compressor unit in the 20 to 25 litre class. |
| Above 15°C | Either type. |
Read that against where people actually put these machines. An unheated cellar in February is not above 15°C. Nor is a garage, nor a loft, nor the spare room with the radiator turned off, which is very often the room with the problem precisely because it is the cold one. Buying the cheaper compressor unit for that room is the single most common way to spend money on a dehumidifier and get nothing back.
What it costs to run, with the arithmetic shown
Every page that ranks for this either skips the number or rounds it to a few pence an hour. Here is the actual sum.
Two inputs. The capped electricity unit rate is 26.32p per kWh for the period 1 October to 31 December 2026, for a Direct Debit customer in England, Scotland or Wales. And the manufacturer states its own power draw at 20°C and 60% humidity: 150 to 260 watts for a compressor depending on size, and around 330 watts for a desiccant. Multiply one by the other.
| Machine | Per hour | 4 hours | 8 hours a day, 30 days |
|---|---|---|---|
| Compressor, 200W | 5.3p | 21p | £12.64 |
| Compressor, 260W | 6.8p | 27p | £16.43 |
| Desiccant, 330W | 8.7p | 35p | £20.85 |
Those are our sums, not a manufacturer's claim, and they are deliberately simple so you can redo them. Take the wattage off the plate on the back of your own machine, divide by 1,000 to get kilowatts, multiply by hours, multiply by your own unit rate. Nothing here accounts for a humidistat switching the compressor off once the room reaches its target, which in a room that is genuinely drying out will pull the real figure below these.
The number worth holding on to is the monthly one. A desiccant running eight hours a day through a wet winter is a bill you would notice, and it is buying you a managed symptom. If the same money went towards finding out which kind of damp you actually have, it would be spent once.
What it is not, according to the government's own guidance
The bluntest statement of cause versus symptom in UK damp guidance is not on any dehumidifier page.
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 is unusually direct. It states that "it is totally unreasonable to blame damp and mould in the home on 'lifestyle choices'", and that "the fundamental cause of damp and mould will be due to building deficiencies, inadequate ventilation, inadequate heating and/or poor energy efficiency, not tenants' normal domestic activities".
It also says that "simply removing surface mould will not prevent the damp and mould from reappearing", and directs that the underlying causes be identified and tackled. The three interventions it names are ventilation, heating and the building fabric.
A dehumidifier is none of those three. That is not an argument against owning one. It is an argument against the machine being where the story ends, and it matters most for renters, because a dehumidifier is very often what gets offered instead of a repair. If you are a tenant being told to run one and keep the windows open, that guidance is the document to have read.
The independent expert view lands in the same place from a different direction. Research on mould and indoor air quality describes a dehumidifier used in place of proper extraction as "a band-aid for a deeper problem", and notes they are ineffective against flood damage, which needs professional drying and an assessment of structural damage instead.
If you are going to use one, use it properly
Everything below comes from the same sources as the rest of the page. None of it turns a dehumidifier into a repair.
- Aim at a number, not a feeling. Comfortable indoor humidity is 30 to 60% relative humidity, and below 65% is where microbial growth falls off notably. A hygrometer tells you whether you are there.
- Small room, door shut, air moving. The units work best in smaller targeted spaces with the door closed and good circulation, not parked in a hall trying to serve a house.
- Know what you are up against. Normal living puts six to 12 litres of moisture into the air per person per day. A mid-range machine removes 3 to 15 litres a day. A single unit against a full household is not a fair fight.
- Match the type to the temperature. Below 10°C it has to be a desiccant. Buying a compressor for a cold room is buying a machine that will underperform every winter, which is the season you bought it for.
- Treat a permanently full tank as a symptom. If it keeps filling and the patch never changes, the room is being resupplied from somewhere. That is information, and it is pointing away from the machine.
About this resource
The unit rate is Ofgem's published price cap figure of 26.32p per kWh with a 54.83p daily standing charge, for 1 October to 31 December 2026, Direct Debit, England, Scotland and Wales. The wattages are the manufacturer's own stated figures at 20°C and 60% relative humidity, as are the temperature bands. The running costs in the table are our multiplication of those two sourced numbers, shown as arithmetic so you can check it against your own machine and your own tariff rather than take it on trust.
The humidity bands, the extraction rates and the moisture-per-person figure come from Michael Taylor, an adjunct academic at Flinders University who researches mould and indoor air quality, writing in The Conversation. His examples are drawn from an Australian climate, so the regional humidity comparisons in that piece do not transfer to the UK and are not used here. The cause-versus-symptom wording is quoted from the government guidance for rented housing providers, updated 1 April 2026.
No product is named and no model is recommended, because nothing on this site earns a commission and no testing has been done. There is no drying-time figure for a wall either: it depends on the construction and on whether the source is still open, and no source we read gives a range worth repeating.
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
How much does it cost to run a dehumidifier for 4 hours?
Between about 21p and 35p, depending on which type you bought. A compressor unit draws 150 to 260 watts at 20°C and 60% humidity, and a desiccant draws around 330 watts. The capped electricity rate is 26.32p per kWh for the period 1 October to 31 December 2026. So four hours at 200W is 0.8 kWh, which is 21p. Four hours of a desiccant at 330W is 1.32 kWh, which is 35p. Check the plate on your own machine and your own tariff and the sum is the same one.
When should you not run a dehumidifier?
When the room is too cold for the type you own, and when the water is not coming from the air. A compressor unit loses performance as the room cools and can stop altogether around 5°C, because its coils freeze. That is the unheated cellar, the garage and the loft, which is exactly where people put them. And if water is crossing the wall from outside or coming up through it, the machine is drying air that is being resupplied from a source it cannot reach.
Where should you not use a dehumidifier?
Anywhere you have not found the source first, and any room colder than the machine is rated for. It also works best in a smaller space with the door shut and air moving around it, so putting one in a hallway to treat a whole house is the wrong shape of job. A house generates six to 12 litres of moisture per person per day, and a mid-range unit removes 3 to 15 litres a day, so a single machine covering an open-plan floor is fighting the whole household at once.
What is the downside of a dehumidifier?
That it is very good at what it does, which lets everybody stop looking. It removes moisture from air, it does it without letting your heat out of a window, and it will visibly fill a tank overnight. That tank is persuasive. It is also just proof that the room was humid, not proof that you have found the cause. Government guidance on damp and mould is blunt about the difference between clearing a symptom and dealing with a cause, and the same logic covers the air as covers the wall: take the symptom away and the deficiency behind it is untouched.
How many hours a day should you have a dehumidifier on?
Long enough to hold the room in the 30 to 60% relative humidity band, which is what the research on mould and indoor air quality points to, with under 65% being where microbial growth falls off notably. That is a target to measure against rather than a number of hours. Buy a hygrometer before you buy a dehumidifier: if the room already sits in that band and you still have a damp patch, a dehumidifier was never the answer and you have saved yourself the running cost.
Should you have the heating on when using a dehumidifier?
For a compressor unit in a cold room, yes, and not for the reason people assume. It is not that warm air holds more moisture in some vague way. It is that a compressor needs its own internal surface to be colder than the room in order to condense water out of it, so the colder the room, the harder it has to work. Manufacturer guidance puts compressors above 10°C, desiccants below 10°C, and either above 15°C. A desiccant does not care, which is the whole reason they exist.
Will a dehumidifier dry out damp walls?
It will dry the air next to the wall, and a wall that has no active water source will then give up moisture to that drier air. That is real and it is how professional drying after a leak works. What it cannot do is beat a supply. If rain is crossing the wall or ground water is moving up through it, you are removing water from the air while the wall refills, and the machine will run indefinitely with the tank filling and the patch staying put. That is not the dehumidifier failing. That is the dehumidifier telling you the source is still open.
How do I tell if my room needs a dehumidifier?
Measure it. A hygrometer costs a fraction of a dehumidifier and answers the question directly: if the room sits above 60% relative humidity and the damp is worse on cold mornings and better in warm dry spells, the moisture is in your air and a dehumidifier is a reasonable tool for it. If the damp tracks the rain, or stays at a fixed height up the wall regardless of weather, the number on the hygrometer will not explain it and neither will the machine.