Last Updated on October 24, 2026 by Umar Farooq
How to reduce humidity in a cellar: ventilate only when the air outside holds less moisture than the air inside, keep rainwater away from the foundation, and run a dehumidifier for whatever is left. The window-opening instinct is the one most likely to backfire in July.
Most cellar advice online jumps straight to products. The sequence matters more, so this is the order I would work through it, cheapest first.
Why a cellar runs damp in summer, even when the walls are dry
A cellar is cold because the ground around it is cold, and warm summer air that wanders in meets surfaces cooler than its dew point. The water has to go somewhere, and it goes onto the stone.
Building Science Corporation describes the summer problem for below-ground walls as warm moist air coming into contact with cold surfaces below the dew point of the interior air. No leak is involved. A cellar with perfectly sound walls can still read 75% humidity in August, which is the part that surprises people who have already checked the walls for damp.
Two things are worth separating, because the fixes differ. Condensation from humid air is an air problem. Water arriving through the walls or floor from wet ground is a water problem. If the wall is visibly wet after rain and dry after a fortnight of sun, you have the second one, and no amount of ventilating will fix it. For the summer-specific version of this, I cover it in why a basement is humid in summer.
Ventilate by comparing dew points, not by how the day feels
Open the cellar only when the outside dew point is lower than the dew point of the cellar air. Temperature alone is a poor guide: a warm dry day can be fine, and a mild muggy one can be a disaster.
Dew point is the temperature at which air can hold no more water. Compare outside with inside and you know which way the water will move. The numbers below are my own calculations using the Magnus formula, rounded, not measurements from a particular cellar.
In practice that means summer ventilation happens on cool, dry nights and early mornings, and stops by mid-morning. Winter is usually kinder. If you cannot be bothered with arithmetic, a weather app that shows dew point and a hygrometer in the cellar give you the comparison in ten seconds. Mind the other end too: if you store anything that suffers from frost, close up when the air outside gets properly cold.
Keep roof and surface water away from the wall you cannot see
You can often cut cellar humidity by changing what happens to rainwater at the surface, without touching the cellar. None of this involves a contractor.
- Keep gutters clear. The EPA brief guide says to clean and repair roof gutters regularly.
- Check the ground slopes away from the building. The same guide says to make sure the ground slopes away from the foundation.
- Extend downspouts so they discharge well clear of the wall. Energy Star lists extending downspouts, clearing gutters and sloping soil away, and notes that better drainage around the foundation may reduce basement humidity.
- Do not dry washing, vent a tumble dryer or leave a hose connection dripping into the cellar.
If the cellar still takes in water after all that, you are past what a humidity guide can fix. That is a building question, and a surveyor, not a sales call, is the right first step.
Give the air somewhere to move
Stale air in the gap behind stored boxes is where cellar mould tends to start, because cold wall plus still air plus cardboard is a good combination for it. Keep shelving and boxes roughly 20-30 cm (8-12 inches) off the walls where you can, and raise anything cardboard off a stone floor onto a pallet or a plastic crate. A small fan on low, running while you are working down there, moves air across the wall surface for free.
Stored wine and root vegetables are a special case: they are deliberately kept more humid than the rest of a house, so use whatever target suits the thing you are storing rather than the numbers below.
Measure it, then pick the number to aim for
A hygrometer is the only way to know whether any of this is working. They cost little, and a week of readings tells you more than a month of guessing. Put it at head height, away from the damp wall, and check it at the same time each day.
The EPA’s advice, in the guide linked above, is to keep indoor humidity below 60%, ideally 30-50%. For a general-purpose cellar, I would treat 60% as the line you do not want to live above and 50% as a sensible target in summer. What humidity should a basement be goes into the reasoning and the winter figures. A visible mould patch changes the job entirely, and mould in a basement covers that.
Where a dehumidifier fits when the air itself is the problem
A dehumidifier is the right tool when the moisture is in the air rather than coming in as water, and when ventilating cannot help because the outside air is wetter. That describes most of July.
It is not the first step and not a substitute for fixing the gutters, but once the water is dealt with it is the thing that holds the number through the humid months. Size it to the cellar, not to the house, and put it on a drain if you can, because a full tank stops the machine. Buying a dehumidifier for a basement walks through the decision, and what size dehumidifier you need covers capacity. Cold is the catch: compressor models lose efficiency in a cold space, and a desiccant model suits a genuinely cold cellar, so check the operating temperature in the manual before you buy.
How to reduce humidity in a cellar: air first, water second, machine third
The order that works is the cheap one. Stop letting humid air in, stop letting rainwater pile up against the wall, measure the result, and only then buy a machine for the gap that remains. If a hygrometer shows the number falling through the first steps, you may never need the last.
FAQ
Should you ventilate a damp cellar?
Only when the outside air is drier than the cellar air, judged by dew point rather than temperature. Warm humid air brought into a cold cellar condenses on the walls and can leave it damper than before. Cool, dry nights are the usual opening.
What is the fastest way to lower humidity in a cellar?
A dehumidifier, running on a drain, with the door shut and windows closed. Ventilation is quicker only when the outside air is clearly drier, and that is rarely true in a humid summer.
What humidity should a cellar be?
For a general-purpose cellar, below 60% at the very most and ideally nearer 50%, in line with the EPA’s guidance of below 60% and ideally 30-50% for indoor humidity. Stored wine or vegetables want a different, higher figure.
Why is my cellar so humid in summer?
Warm, moist air meets walls kept cool by the surrounding ground and condenses. It happens even with sound walls, which is why a cellar can feel damp without a leak.
Do moisture absorbers work in a cellar?
Small tubs can cope with a cupboard-sized space, but a whole cellar produces far more moisture than a tub holds. I would use them only as a stopgap, not as the plan.
