Wet windows: the free dehumidifier
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One morning the guest room windows had condensation 70 cm up the glass. The same night, in the same house, two other rooms had none at all — and the only difference was that those two had a window open.
My first thought was that the wet glass was the problem. It is the opposite. That water was in the room's air an hour earlier. The glass is simply the coldest thing available, so it is where the water goes first.
The measurements back it up. Overnight the guest room's moisture level fell, even with the two of us asleep in it and the door shut. Nothing in that room was removing water except the window.
The catch is what happens next. Leave it and it runs down to the frame and sits in the corners, and that is exactly where the black spots turned up.
We bought a RYOBI window vacuum at Bunnings for $119 — a battery-powered squeegee that sucks the water off the glass into a small tank as you draw it down. Unlike a cloth, it doesn't just move the water somewhere else. Tim Forcey, whose article describes a lot of this, recommends exactly that — a chamois for the everyday, or a window cleaner's electric vacuum when there is more of it. He also suggests opening internal doors to connect the musty rooms to the rest of the house, which is the same finding we stumbled into.
Then we found something that cost nothing at all. Friday night, guest room door shut: 70 cm of condensation. Saturday, door left half open: none. Sunday, open a quarter: none. Seven clean mornings since. Everything two sleepers breathe out over eight hours stays in a closed room; leave the door ajar and the rest of the house takes most of it.
We also wanted every mould spot and every bit of dirt off the glass and the frames, inside and out, which turned out to be a genuinely fiddly job — James and Zac from Clearwater Cleaning were here all day. About $900 for all the windows, $100 more for the fly screens, $300 more to detail-clean every track. Great local family company. Very thorough – they found specks at the bottom of the upstairs windows that we had not thought to look at. The same thing happening downstairs, caught years earlier, and they cleaned those frames while they were there.
That day gave us our highest moisture readings of the week, in all three rooms at once. Not their doing: washing every window puts water on every frame in the house, and the outside air happened to be as damp as the inside, so all the doors and windows they were working through bought us nothing.
So a window is two things, depending on what you do with it. Wiped dry each morning, it is a dehumidifier that costs nothing to run. Left alone, it is a mould collector.
Coming up
Did going all-electric cause our mould? What we found under the floor.
The sensors that had been recording all this in every room, for months.
Buying two dehumidifiers, and what desiccant actually means.
Related Posts
- Previous — flushing the wet air back out: Flushing wet air out of the house 4 Aug 2026
- Also in this series — was it the electrification?: Did going all-electric cause our mould? 6 Aug 2026
- Next — where the shower steam actually goes: Where does the shower steam actually go? 9 Aug 2026
Links
- Clearwater Cleaning, who did the windows, frames, fly screens and tracks: clearwatercleaning.com.au
- The window vacuum we bought, $119: Ryobi 18V ONE+ Window Vacuum RWV18 at Bunnings
- Tim Forcey on managing moisture in Australian homes, including the rainy side of the Dandenong Ranges: Moisture management and the death of an Aussie icon
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- Crack the windows twice a week during the day for fresh air
- leave bedroom door ajar at night
Expensive (but amazing!)
- replace windows with upvc frames (they are thermally broken so won’t get condensation) & double glazing
Free but impossible:
- stop breathing when asleep - this one might be impossible but it will stop you expelling 0.5mls per 5 mins (or something…I’m no scientist !!)
And if there is moisture in the air it will gravitate to the coldest surface in the room straight to the glass/aluminium frames.
Options! Lots of options & wiping down windows every morning for months is going to get bloody tedious after a while!
Ps - I’m biased as I sell upvc windows but not pitching for a sale (emeralds too far for us) BUT if you do have any questions I’m truly happy to help 🤓😊🙌🏻
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