Will a dehumidifier get rid of a musty smell?

It stops the smell getting worse. It does not remove the smell you already have. Those are two different jobs and almost nothing written on this subject separates them. A musty smell is a by-product of mould or mildew growing on damp material. Drop the humidity and the growth stops, so no new smell is produced. The material that has already been colonised keeps smelling until it is cleaned or thrown out.

Drying is not deodorising. If you buy a dehumidifier expecting the basement to smell fresh by the weekend, you will be disappointed by a machine that is working perfectly.

That characteristic damp-cellar, old-book, wet-towel smell is a family of microbial volatile organic compounds — MVOCs. They are metabolic by-products released by moulds and mildews as they grow. Different organisms on different substrates give slightly different smells, which is why a musty wardrobe and a musty crawl space do not smell identical, but the mechanism is the same one in every case.

Two consequences follow directly, and between them they are the whole page:

One: the smell is a growth indicator, not a moisture indicator. It tells you something is living somewhere damp. It does not tell you the air is damp right now — a colony that grew through a wet spring keeps sitting there in a dry June.

Two: the compounds are already in and on the material. A dehumidifier moves air across a cold coil and takes water out of it. It has no mechanism for removing a compound that has soaked into a joist, a carpet backing, a cardboard box or the paper face of a sheet of plasterboard. It is the wrong kind of machine for that, in the way a fridge is the wrong machine for washing up.

We are not a health authority and this page makes no claims about what any of this means for anyone’s health. It is about what a dehumidifier does and does not do. If you have health questions, ask someone qualified to answer them; if the affected area is large, that is a remediation contractor’s call, not ours.

Mould and mildew need moisture available at the surface they are growing on. In practice that translates to a rule of thumb worth memorising: hold indoor air below about 60% RH and you are outside the range where growth establishes on ordinary building materials. Below 50% you have margin for the humid week that will otherwise undo a month of running. Below 30% is drier than most people want to live in and there is nothing to gain from it.

The setpoint this site recommends is 45%. It sits far enough under the growth threshold to absorb a bad weekend, and it is not so low that the machine runs permanently to hold it.

The smell is strongest at the source. That sounds trivial and it is the most useful diagnostic you have, because MVOCs disperse — so a room that smells uniformly musty usually has a source that is either large or well-mixed by air movement, and a smell that strengthens as you move toward one corner has told you where to look. Do it with a cold nose, first thing, before the room is aired: your sense of smell fatigues fast. Open closed cupboards and sniff the air trapped inside them, not the room.

When your nose runs out, use the cold. Condensation forms on whatever surface in a room is coldest, so the coldest surface is where growth starts. That is predictable rather than mysterious — it needs one number, the dew point, which is the temperature at which the air you currently have becomes saturated. Any surface at or below the dew point is collecting liquid water out of the air, continuously, whatever the room hygrometer reads.

The pattern in that table is the useful part. At 65% the dew point is 55.8°F, so the 58°F wall is condensing and the smell has a permanent water supply. Bring the room to 50% and the dew point drops to 48.7°F — now the wall is above it and the pipe is not. That is why a dried basement can still smell faintly of the one cold spot it still has, and why insulating a cold-water line is sometimes worth more than another twenty pints a day of capacity.

Places worth checking with this in mind: the bottom foot of an exterior basement wall, behind and under any furniture pushed against one, under carpet or vinyl laid directly on a slab, inside boxed-in plumbing, the underside of a bath, the back of a wardrobe on an outside wall, and the joist bays over a crawl space. Also anything absorbent and forgotten — a bag of clothes, a rolled rug, a stack of cardboard.

It fixes: the supply. Air below the growth threshold means no new colonies establishing and existing ones going dormant rather than expanding, so no new MVOCs are being produced. It dries out the reservoir — the damp cardboard, the timber, the plaster — which is what stops the room re-humidifying itself every night. It stops condensation forming on surfaces once the dew point is below them. Over time it makes the space a place where the problem does not come back, which is the only durable outcome available here.

It does not fix: the compounds already in the material. It has no filter capable of removing an odour that is being released by a surface rather than floating past a coil. It cannot reach inside a wall cavity, under a floor covering or behind a fixed cabinet, because it can only treat the air it can circulate. It cannot out-run a leak. And it will not clean anything — that is a job for a scrub, a wet-vac, or a skip.

The honest sequence, if the smell has an identifiable source: dry the space, then clean or remove the colonised material, then keep the space dry so it does not recur. Doing step two first is how people end up doing it twice.

First 24–48 hours. The display drops. Almost nothing else changes. People often report the smell seeming stronger at this stage, which makes sense — you are moving a lot of air around a room you had been keeping shut.

First two weeks. The room stops climbing back up overnight as the contents give up their moisture. This is the stage that matters and the stage most people cut short. Judge whether a machine is sized correctly on where the room sits after a fortnight running continuously, not on day two.

Two to six weeks. If the source was diffuse — generally damp furnishings, cardboard, a slightly damp slab — the smell fades noticeably as those materials dry and stop releasing. This is the case where drying alone appears to “work”, and what actually happened is that the reservoir dried out.

Beyond six weeks. If the room is holding 50% or below and the smell has not changed, drying is not your remaining problem. Something colonised is still there, or water is still arriving. Buying a bigger dehumidifier at this point is the most common wasted purchase in this category.

A dehumidifier that cannot hold its setpoint is telling you something. If a correctly sized unit runs continuously and the room still sits above 60%, water is entering faster than it can be removed, and the question is where from — not which larger model to buy.

The usual sources, roughly in order of how often they turn out to be the answer: surface water finding its way in around the foundation, from a blocked gutter, a downpipe discharging at the wall or ground that slopes toward the house; a slow plumbing leak inside a wall or under a floor; a failed shower or bath seal letting water past on every use; bare soil in a crawl space with no vapour barrier, evaporating continuously into the floor above; and unvented dryers or unused bathroom fans putting the moisture back into the house on purpose.

Two tells worth knowing. A smell that is worse after rain points at water getting in from outside, and the fix is drainage, not dehumidification. A smell that is constant regardless of weather and strongest near plumbing points at a supply leak, which will not stop on its own and is damaging the structure while you dehumidify around it. In both cases the machine is the second step. Skipping the first means paying to evaporate water you are also paying for.

One thing to know first, because it is the most common sizing error in this category. In 2019 the US Department of Energy moved the dehumidifier test condition from 80°F to 65°F. A dehumidifier removes less moisture from colder air, so the same machine now earns a lower number. A modern “50 pint” unit removes roughly what a pre-2019 “70 pint” did. Every sizing chart written before 2019, and most of those circulating now because they were copied from those, is on the old scale. Follow one with a current machine and you undersize by roughly a third. We publish both numbers and always say which scale a figure is on.

And ignore the coverage claim on the carton. In our own product data three units all rated 155 pints/day advertise 7,500, 6,000 and 6,000 sq ft of coverage, and a 160-pint unit advertises 2,500. There is no standard behind the square-footage figure and it is not comparable between brands. Size on pints per day, which is tested.

Take out what is colonised. Cardboard, old carpet and underlay, chipboard that has been wet, damp foam, anything absorbent that has been sitting on a cold floor. Porous material that has grown mould does not reliably come clean; the decision is usually replace, not rescue. This single step does more for the smell than any machine, because it removes the emitter rather than the conditions.

Clean the hard surfaces. Sealed concrete, painted masonry, tile and glass can be washed and will stop smelling once they are clean and dry. Follow whatever product you use as directed and ventilate while you do it — and never mix cleaning products together.

Move things off the floor and off exterior walls. Both run colder than the room, which is exactly what the dew point table above is about. Two inches of air gap behind a wardrobe changes the surface temperature it sees.

Put a hygrometer where the smell is. A cheap sensor in the actual corner tells you more than an expensive one by the stairs, and it is the only way to know whether the machine is reaching the place that matters.

Drain it and leave it running. A tank that fills at 2am and shuts the machine off gives the room the rest of the night to climb back. Continuous drainage is most of the difference between a unit that holds a setpoint and one that averages it.

Deal with the outside water. Clear the gutters, extend the downpipes, and check the ground slopes away from the wall. It is free, it is dull, and it is the most frequently skipped step on this list.

We have never run these units — not in a basement, not anywhere. There is no test bench behind this page, no field trial, and no reviewer. What we do instead is publish the arithmetic: the dew points, both DOE rating scales, the sizing baseline and where it comes from. Every number above can be checked against a psychrometric chart and the DOE rule without taking our word for anything.

We also cannot tell you what is growing in your basement, how much of it there is, or whether it matters for anybody’s health — we are not a health authority and make no health claims. If the affected area is large, if it followed a flood, or if it is inside a wall you cannot open, a remediation contractor or your local building official will tell you more than any website, and neither of them pays us anything.

Product data and prices are a snapshot taken 21 August 2026 and will drift. Capacities are manufacturer figures on the modern DOE scale.