Dehumidifier or Open Windows: Which One Actually Works?
Your hygrometer reads 68%, the window corners are misting up, and the internet offers you two camps: “buy a dehumidifier” and “just air out more”. Both are right — on different days. Which one wins in your home comes down to a single number: how much water the outdoor air is carrying compared to yours.
Do I need a dehumidifier, or is opening windows enough?
Most homes don’t need a dehumidifier most of the year: opening windows when the outdoor air is drier in absolute terms removes litres of water in minutes, for free. The trap hides in “absolute terms”. Your weather app shows relative humidity, and a cold morning at 90% often carries far less actual water than your living room at 65% — that paradox is unpacked in relative vs absolute humidity.
Put numbers on it. A living room at 20 °C and 70% holds about 12 g of water per m³. Winter air at 5 °C and 90% holds about 6 g/m³. Swap the air of a 200 m³ flat once and you’ve carried out more than a litre of liquid water — a decent cross-draught does that in five to ten minutes. A compressor dehumidifier needs roughly two hours of electricity for the same litre.
When is airing the cheapest way to lower humidity?
Airing wins whenever the outdoor air holds fewer grams of water per cubic metre than your indoor air — and that’s most of the year. Typical windows of opportunity:
- Winter, almost any time of day — cold air is poor in water, even at high RH.
- Summer nights and early mornings, once outside has cooled below your rooms.
- Right after a cold front or a storm has passed and drier air moves in.
Your household breathes, cooks and showers 5 to 10 litres of water into the air every day (how much moisture a home produces). Well-timed airing gets rid of it at exactly zero cost — and keeps you inside the healthy 40–60% band (ideal indoor humidity).
When is a dehumidifier actually worth it?
A dehumidifier earns its keep when the outdoor air stays wetter than yours for days on end — or when the room has no window at all. Opening then simply imports water, and the machine becomes your only lever. Realistic cases:
- Muggy, stormy summer weeks when even the nights stay wet outside.
- Windowless bathrooms and interior rooms with no airflow path.
- Cellars and semi-buried rooms: cool walls plus damp air, all summer long.
- Intensive indoor laundry drying — see drying laundry indoors without mold.
Order of magnitude: a domestic compressor unit removes 10–20 litres a day in real conditions (spec sheets are measured in tropical test chambers) and draws roughly 200–400 W — count on a few kWh per day if it runs long hours.
What should you use in your situation?
A quick mental table — the tool follows the situation, not the other way round:
| Situation | Best tool |
|---|---|
| Winter condensation on windows | Airing, short and several times a day |
| Summer night after a hot day | Airing, everything wide open |
| Muggy or stormy spell | Dehumidifier, windows shut |
| Cellar, windowless bathroom | Dehumidifier |
| Laundry drying indoors | Airing if outside is drier; dehumidifier if not |
The winning strategy is hybrid: air out during the dry slots, keep the dehumidifier as targeted backup for the blind spots — the windowless bathroom, the cellar, the drying rack on a soggy week.
Which mistakes waste the most money?
The most expensive mistake is running a dehumidifier with the windows open: the machine is now trying to dry the whole street, running nonstop for nothing. Shut the room while it works — a dehumidifier and an open window cancel each other out.
The second mistake is quieter: switching the machine on at 4 PM when a dry airing slot arrives three hours later. That’s paying to remove water the night would have carried out for free. Check what the outdoor air is about to do before you press the button.
FAQ
Can I run a dehumidifier with the windows open? No — the outdoor air replaces the moisture as fast as the machine extracts it. Close the room, let it finish, then air out later during a dry slot.
How many litres does a dehumidifier really remove per day? A typical domestic compressor unit removes 10–20 litres per day in real homes. The headline figure on the box comes from much hotter, wetter test conditions.
What’s the cheapest way to lower indoor humidity? Opening the windows when the outdoor air is drier in absolute terms (g/m³). It removes litres of water in minutes and costs nothing.
How do you know when airing is enough?
That’s the whole question — and the answer changes hour by hour. Open/Shut compares the absolute humidity inside your home with the air outside and tells you precisely when opening the windows does the job for free — and when to keep them shut and let the machine run instead. Before you buy anything or switch anything on: check whether the sky is about to hand you dry air. Free, private, no account.