Dehumidifier: Window Open or Closed?
Closed. A dehumidifier works on the air in the room, and an open window keeps replacing that air with more from outside. There are a few moments when opening up is still the right thing to do.
The short answer
Run a dehumidifier with windows closed. Keep internal doors open if you want it to dry several rooms, or closed if you want it to dry one room fast, for example when drying washing. Open the windows for a short burst when the machine is off and there is steam or smells to clear.
Why closed works better
A dehumidifier lowers the relative humidity of a fixed volume of air. With a window open, damp outdoor air keeps flowing in, the machine never reaches its target, and the compressor never rests. You pay for more hours and collect water that came from the garden. In a British autumn or winter the outside air is often at 80 to 90% relative humidity. It holds less water than warm indoor air, but it still needs warming and drying once it is inside.
Window open or closed, case by case
The rule changes slightly with what you are doing.
| Situation | Window | Internal door |
|---|---|---|
| General damp control | Closed | Open, so air circulates |
| Drying washing | Closed | Closed, one room only |
| Bedroom overnight | Closed | Either |
| Just after a shower or cooking | Open for 10-15 minutes, machine off or extractor on | Closed, to keep steam in that room |
| Trickle vents | Leave them open | - |
| Warm, dry, breezy day | Open, and switch the machine off | Open |
Opening windows instead of a dehumidifier
Ventilation is free and it does work: ten minutes of windows wide open clears steam and stale air. Its limits show in winter. You lose heat, and on a wet day the air you let in is damp too. A dehumidifier removes moisture while keeping the warmth in, which is why the two are used together rather than as alternatives: ventilate in short bursts, dehumidify the rest of the time. More on this in how to stop condensation on windows.
Doors: the part people get wrong
To cover a whole flat or floor with one machine, leave internal doors open and stand it on the landing or in the hallway. Shut it in a hallway with every door closed and it dries the hallway. For laundry, do the opposite: one room, door shut, laundry mode on. See where to put a dehumidifier.
What humidity to aim for
Set the humidistat to 50 to 55%. Below that you are paying to over-dry the air; above 60% for long periods, mould and dust mites do well. A dehumidifier with a target mode will switch its compressor off when it gets there, which only happens if the room is closed. A cheap hygrometer lets you check the reading: see how to measure humidity in a room.
Summer and winter
In winter, keep windows shut while the machine runs and air the house in short bursts. In summer, look at the weather. On a warm, dry day with a breeze, open windows will dry the house for free and the dehumidifier can stay off. On a close, humid day, or in a basement room that stays cool, closed windows and the dehumidifier work better.
Bathrooms and kitchens
Steam is best removed at source. Run the extractor fan during a shower and for 15 to 20 minutes afterwards, or open the window with the door shut. Bringing a dehumidifier into the bathroom afterwards finishes the job, provided the model is suitable for that room and kept away from splashes: see our bathroom dehumidifier picks.
Do not seal the house completely
Leave trickle vents open and keep using kitchen and bathroom extractor fans. A home needs some fresh air for the people in it, and for any gas appliance. A dehumidifier manages moisture; it does not replace ventilation.
Meaco MeacoDry Arete One 12L
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Meaco MeacoDry Arete Two 20L
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Devola DV12L Low Energy Dehumidifier
12 litres/day · 4.6/5
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Frequently asked questions
Should windows be open or closed when using a dehumidifier?
Closed. With a window open, the dehumidifier keeps drying air that is replaced from outside and never reaches its target humidity.
Should I close the door when using a dehumidifier?
Close it to dry one room quickly, for example when drying washing. Leave internal doors open if one unit is meant to cover several rooms.
Is it better to open windows or use a dehumidifier?
Use both: open windows for a few minutes to clear steam, then close them and let the dehumidifier hold humidity at 50 to 55% without losing heat.
Should trickle vents be open with a dehumidifier?
Yes. Trickle vents give the background ventilation a home needs and let in too little air to stop the dehumidifier working.