How the two differ in kind
Damp has three possible answers and they are not interchangeable: remove the source, remove the moisture by exchanging air, or remove the moisture by condensing it out. The first is nearly always the real fix, and the choice between the other two is decided by a number most people have never been shown.
That number is ABSOLUTE humidity — how much water a kilogram of air is actually carrying, in grams. Relative humidity, the figure on every hygrometer, is a percentage of what that air could hold at its current temperature, and warm air can hold far more than cold air. So the two readings tell you completely different things, and using the wrong one produces the wrong decision.
Winter is the favourable case for VENTILATION. Cold outdoor air, even at a high relative humidity, contains very little water in absolute terms. Bring it inside, warm it up, and its relative humidity collapses — it becomes thirsty air, capable of taking up moisture from the building and carrying it back out. This is why airing a house on a cold, damp-looking day genuinely dries it, and it is the mechanism behind winter ventilation strategies generally.
Summer inverts it. Warm outdoor air at a moderate relative humidity carries a great deal of water in absolute terms, and if it enters a space that is cooler — a basement, a cellar, a slab-on-grade room, anywhere below outdoor temperature — it is cooled, its relative humidity rises, and it may reach saturation and condense on the coolest surfaces it finds. Opening the windows of a cool basement on a warm humid day is one of the reliable ways to make it wetter. That is the case where a DEHUMIDIFIER is the correct tool: it removes water without bringing in outside air at all.
The factors that actually differ
Show
| Dehumidification | Ventilation | |
|---|---|---|
| What it removes | Water, condensed out of the air and drained away. Air stays inside. | Moist air, replaced with outdoor air — which works only if the outdoor air is drier in absolute terms. |
| When it works | Any time. It does not care what the weather is doing. | When outdoor absolute humidity is below indoor — typically winter, often not summer. |
| When it makes things worse | Rarely; it adds heat to the space, which in summer is an added cooling load. | Regularly, and invisibly: ventilating a cool space with warm humid air raises its moisture content and condenses on cool surfaces. |
| Energy | Runs continuously and consumes electricity the whole time. The waste heat is a benefit in winter and a penalty in summer. | Costs the heat in the air exchanged, unless heat recovery is fitted — a fan uses very little on its own. |
| Air quality | No effect. It recirculates the same air, so carbon dioxide, odours and pollutants remain. | This is what it is for. Dilution of everything, not only moisture. |
| Which measurement decides | Relative humidity is a sensible control setpoint, since it is what drives mould and comfort. | Absolute humidity, indoors and out. Comparing relative humidity readings between two temperatures tells you nothing useful. |
| Cold spaces | Works, but a refrigerant unit loses capacity as temperature falls and may frost. A desiccant type handles cold better. | The failure case in summer, and the right answer in winter. |
| Water disposal | A tank to empty, or a drain — and a tank that is never emptied is a dehumidifier that stopped working days ago. | None. The water leaves as vapour. |
| Against a live source | Loses. A unit removing water at a fixed rate cannot beat ground water arriving continuously. | Also loses, for the same reason. |
| What both are | Symptom management, valuable and legitimate — but not a cure for a source. | The same, with the additional benefit of air quality. |
Which one, and when
Choose dehumidification when…
- The space is cooler than outdoors — a basement, a cellar, a ground-floor room in summer.
- It is the humid season, when outdoor air carries more water than the indoor air does.
- A specific humidity level has to be held for stored contents, equipment or a drying-out period.
- The building is being dried after a leak or flood, where the removal rate needs to be high and controlled.
Choose ventilation when…
- It is the heating season, when outdoor air is cold and therefore dry in absolute terms.
- The moisture is generated indoors — cooking, washing, drying clothes, occupancy — and extracting at source is possible.
- Air quality, not only moisture, is part of the problem.
- The building is tight enough that the moisture has nowhere to go, which is a ventilation deficiency rather than a drying problem.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Why does opening the windows sometimes make damp worse?
- Because air brings its moisture with it, and warm air carries far more than cold air does. On a warm, humid day the outdoor air may hold well over twice the water per kilogram that the air in a cool basement holds, even if both hygrometers read a similar relative humidity — relative humidity is a percentage of capacity, and capacity rises steeply with temperature. Let that outdoor air into a cool space and it cools down, its relative humidity climbs, and if it reaches saturation it condenses on the coolest surfaces available: the floor slab, the walls, the cold water pipes. So the ventilation that dried the house in February wets the basement in July. The rule that works is to compare absolute humidity — grams of water per kilogram of air — indoors and out, and ventilate only when outdoors is genuinely lower.
- How do I know whether outdoor air is actually drier?
- By converting both readings to an absolute measure — grams per kilogram, or dew point — rather than comparing the percentages. Dew point is the most convenient because it is directly comparable: it is the temperature at which the air would become saturated, and air with a lower dew point genuinely holds less water regardless of its temperature or its relative humidity reading. So the test is simply whether the outdoor dew point is below the indoor dew point; if it is, ventilating dries the space, and if it is not, ventilating wets it. Outdoor dew point is published in most weather data, and a pair of inexpensive hygrometers reporting temperature and relative humidity gives you the indoor figure. This one comparison resolves most arguments about whether to open the windows.
- Should I find the source first?
- Yes, always, and the reason is arithmetic rather than principle. Both a dehumidifier and a ventilation system remove water at a finite rate, and if water is arriving faster than that rate the humidity never comes down no matter how long either runs. The sources worth eliminating before buying equipment are usually unglamorous: ground water against a wall or under a slab with no damp-proofing, surface water directed at the building by bad grading or a broken downpipe, a plumbing leak, an unvented tumble dryer, a bathroom or kitchen extracting into a roof space rather than outside, and a crawl space with exposed earth evaporating continuously into the building above. A dehumidifier running permanently in a room with one of those is an electricity bill treating a symptom, and it masks the damage continuing behind the plaster.
- What size dehumidifier do I need?
- It is sized on the rate at which moisture has to be removed, which depends on the volume of the space, how wet it currently is, how much moisture is entering it, and at what temperature it will be working. The last of those is the one most often missed: a refrigerant dehumidifier's rated extraction figure is quoted at a warm, humid test condition, and its actual output falls substantially in a cool space — which is exactly where basements are. Below a certain temperature the coil frosts and the unit spends part of its cycle defrosting instead of extracting. Desiccant units maintain output much better in the cold, at the cost of more electricity, which is why they are common in unheated garages and cellars. Check the extraction figure at the temperature you will actually run it, not the headline number on the box.
- Does a dehumidifier heat the room?
- Yes, noticeably, and whether that is good or bad depends on the season. A refrigerant dehumidifier moves heat around inside itself and adds both the energy it consumed and the latent heat released when the water vapour condenses, so all of it ends up in the room as sensible heat — the unit is a small heater with a useful side effect. In a cool space in winter, that is a genuine bonus and part of why the room feels better. In summer, or anywhere air conditioning is running, it is a penalty: the dehumidifier warms the space and the cooling system pays to remove the same heat again. Where summer humidity is the main problem, it is worth checking whether the air conditioning can be made to dehumidify better — longer, slower runs remove far more moisture than short bursts — before adding a separate machine.
- Is a crawl space or basement a special case?
- It is the classic case, and the modern answer for a crawl space usually runs against the traditional instruction to ventilate it. A crawl space with exposed earth is a continuous moisture source, evaporating ground water into the building above; venting it to outdoor air in a humid summer supplies warm moist air to the coolest space in the building, which is the condensation scenario in its purest form. The approach that works is to stop the source and close the space: a sealed vapour barrier over the ground and up the walls, air sealing, insulation at the perimeter, and conditioning the space — often with a small dehumidifier — as part of the building rather than as outdoor space. Local practice and code vary, so it is worth checking what your jurisdiction requires, but the physics of warm humid air meeting a cool surface does not.
- What humidity should I aim for?
- Somewhere in the middle, because both extremes cause problems. Too high and you get mould growth, dust mites, condensation on cold surfaces and musty smells; the commonly cited threshold for mould risk on a surface is a sustained high relative humidity AT that surface, which is why cold spots mould while the room average looks fine. Too low and you get shrinkage and splitting in timber, furniture and instruments, static, and uncomfortable respiratory dryness. A moderate band — comfortably below the mould threshold and comfortably above the shrinkage one — suits both the building and the people in it, and a dehumidifier set to hold it will cycle rather than run continuously. The figure that matters for mould is the one at the cold surface, not in the middle of the room, so insulating a cold spot can solve a problem that no amount of dehumidification reaches.
- Can ventilation with heat recovery do both jobs?
- It does the air-quality job always and the drying job seasonally, which is a genuine improvement over plain ventilation but not a replacement for a dehumidifier in a humid summer. A heat recovery unit exchanges heat between the two air streams so ventilating costs far less energy, and an energy recovery unit also transfers a share of the moisture — which in a humid climate limits how much outdoor moisture comes in, though it does not remove moisture already inside. The limit is the same as for any ventilation: when outdoor air holds more water than indoor air, exchanging air adds water, and no amount of recovery changes that direction. In climates with humid summers the usual arrangement is both — balanced ventilation for air quality year-round, and dehumidification during the months when ventilation alone would be adding to the problem.