Using a dehumidifier is a smart step when you’re dealing with damp, condensation, or musty odours. But a common question many users ask is if a window should be kept open or closed while a dehumidifier is running.
Whether you’re drying out a bedroom, bathroom, or an entire property, this small detail can make a big difference in how efficiently your unit performs. Let’s explore the science behind it and give you practical tips based on real-world usage.
Why Does It Matter if the Windows Are Open or Closed?
A dehumidifier’s job is simple — to pull moisture from the air and collect it as water. It works by recirculating the same air within a room, gradually lowering humidity levels.
When a window is open, fresh moist air from outside continues to flow in, making your dehumidifier work harder — and often, less efficiently.
Keeping the windows closed helps the dehumidifier process the same air in a controlled space. That means faster drying, less energy use, and better long-term results.
When to Keep Windows Closed with a Dehumidifier
You should keep you windows closed when:
- Drying laundry indoors (common in winter)
- Treating damp walls or condensation
- Conducting post-leak water removal
- New plaster is drying during renovation projects
- Preventing mould in bathrooms or basements
Example:
One client drying out a kitchen after a leak was frustrated that the moisture levels weren’t dropping. They had a dehumidifier on full power but kept the back window cracked open “for ventilation”. Once they shut the window, moisture levels dropped by 40% in 24 hours, and the musty smell disappeared.
Are There Any Situations Where You Should Open the Windows?
There are a few exceptions where brief ventilation is helpful, such as:
- To release stale air temporarily (e.g., when there’s no extractor fan)
- On very hot, dry days, where outside air has less humidity
- After intense cleaning with chemicals, to air out fumes
- For 10–15 minutes before turning the unit back on, to clear condensation trapped in corners
But generally, keep the windows closed while the dehumidifier is in use.
Understanding How Dehumidifiers Work
To appreciate the difference, here’s a simplified breakdown:
- Air intake: Moist air is drawn into the machine.
- Moisture removal: Internal coils or desiccant materials remove the water vapour.
- Dry air release: The unit returns drier air back into the room.
- Continuous loop: The air gets drier with each cycle.
If a window is open, the cycle resets continuously — the unit never gets to dry the same air twice, making it less effective and more costly to run.
What If the Room Feels Too Stuffy?
A completely sealed room can feel airless after a few hours of drying. In this case:
- Open the window for 5–10 minutes every few hours
- Let fresh air in between dehumidifier cycles
- Use a small oscillating fan to keep air moving without introducing moisture
This balances comfort without compromising performance.
Conclusion:
To get the most out of your dehumidifier, keep your windows and doors closed while it’s operating. This ensures the unit can reduce humidity effectively, saving you time, energy, and frustration.
If you need occasional fresh air, do so in short bursts, ideally before or after running the machine. At Dehumidifier Rental, we help customers every day optimise their equipment use to achieve the fastest drying time — whether for flood damage, drying plaster, or long-term humidity control.
Need advice on choosing the right unit or how to set it up? Our team is here to help — from same-day hire to tailored humidity advice, across the UK.
FAQs:
Q: Should you close the window when using a dehumidifier?
A: Yes. Keeping windows closed ensures the dehumidifier can recycle and dry the same air efficiently.
Q: Can leaving a window open make the dehumidifier ineffective?
A: Yes. Open windows introduce fresh moisture, making it harder for the dehumidifier to lower the humidity level.
Q: Is it ever OK to open a window while using a dehumidifier?
A: Briefly, yes — for fresh air or ventilation. But close them while the unit runs to avoid drawing in more moisture.
Q: What if the room gets stuffy or hot while drying?
A: Open the window for 5–10 minutes between cycles, or use a fan to circulate air without bringing in more humidity.
Q: Will a dehumidifier work with a door open but windows shut?
A: It depends — if the door opens to a larger dry space, it’s fine. But if it connects to a damp area, it could affect efficiency.
