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Casement window leak?

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15 Oct 2025
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Hi there, this is a long shot, but could anyone tell me if this likely to be a leak or condensation?

2 new uPVC casement windows, one upstairs and the other downstairs directly below it. Each one of them have water beads right down the corner (where the side and the front of the casement window join together). This happens on both windows. Window installer says its nothing to do with them :-)

I've had the roof checked and sealed around any areas that could be letting water in. The next step is to get a leak detection specialist in, and the quote I have received is very expensive to have this done. I thought I'd post here on the off chance anyone has seen this before.

I've attached images, if you zoom in you can see the little drops of water on the frame corner crease.

Thanks a lot!
 

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Very likely condensation because within that square corner post there will a square aluminium box section for reinforcement and it's causing a cold spot so any moisture in the air inside the property it will condense on the cold spots, are you creating excess moisture like drying clothes on radiators, not opening trickle vents, not opening the window when taking a shower, cooking without an extractor fan ect?
 
Very likely condensation because within that square corner post there will a square aluminium box section for reinforcement and it's causing a cold spot so any moisture in the air inside the property it will condense on the cold spots, are you creating excess moisture like drying clothes on radiators, not opening trickle vents, not opening the window when taking a shower, cooking without an extractor fan ect?
Many thanks for your reply and the information, that could explain it with the aluminium reinforcement at each corner. The casement window is a bedroom window (upstairs) and a living room window (downstairs). The bedroom window is directly above the living room window. They are not near bathroom or kitchen, and there are no clothes drying. We've tried leaving trickle vents open, and the top window panels open ajar, but it does not seems to make any difference. Its random also, sometimes there is no moisture on the frame.

At first I was convinced it is a leak, but when I took a very close up photo and saw tiny clear droplets, I thought condensation. Also, the drops are totally confined to the corner joins, there is no wetness up above at the ceiling or anywhere else, just in the join where the uPVC panels meet. Then they sometimes result in a little puddle at the bottom of the join on the wood. This happens on both windows.
 
I know water is water but usually you can tell the difference between dripping water and water droplets which is what you have.

Further though excess moisture in the property won't necessarily be confined to one room, it will float under doors, gaps around doors, through keyholes until it finds that cold surface, it's not my area of expertise so I may be wrong but it's as though the moisture is drawn to cold surfaces, keeping rooms warm helps keep surfaces above the dew point, that's the line between water remaining in vapour form OR condensing back to a liquid

Try a dehumidifier in that room and see how much water it suck out the air, quite alarming sometimes
 
A leak would appear / get worse when it rained.

You could also dry it off, then open the top window and see if it reappears. If it doesn't, it was more likely to be condensation in my opinion, as the open window would decrease room humidity.
 
I know water is water but usually you can tell the difference between dripping water and water droplets which is what you have.

Further though excess moisture in the property won't necessarily be confined to one room, it will float under doors, gaps around doors, through keyholes until it finds that cold surface, it's not my area of expertise so I may be wrong but it's as though the moisture is drawn to cold surfaces, keeping rooms warm helps keep surfaces above the dew point, that's the line between water remaining in vapour form OR condensing back to a liquid

Try a dehumidifier in that room and see how much water it suck out the air, quite alarming sometimes
Thanks, yes I think it's a good idea to try a dehumidifier and see if it makes a difference, even if only to rule out condensation as a cause. I will see if I can get hold of one.
 
A leak would appear / get worse when it rained.

You could also dry it off, then open the top window and see if it reappears. If it doesn't, it was more likely to be condensation in my opinion, as the open window would decrease room humidity.
This is a great idea I will try it, thanks.
 

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