In my bedroom, condensation accumulates only in the lower corners of the plastic window, while the rest of the surface stays dry. The window is new - we had it installed two years ago. Could this be an installation problem or an issue with the sealant?
Most likely, those are cold bridges in the corners - the glass cools down the most there, and moisture condenses first. Check if there are any gaps in the sealant or places where the slopes have pulled away from the frame, especially at the bottom; even hairline cracks let in cold air that chills the corners. If the sealant is intact, try improving ventilation in the bedroom - air it out more often or crack the window just slightly for micro-ventilation, that'll bring the humidity down and the condensation will disappear. If the fogging doesn't stop and the sealant is clearly damaged, you should call the installers - they should redo the sealing under warranty if the window is still under guarantee.
It's a disruption in air circulation at the corners - humid air gets trapped there and cools down fastest. Check if the ventilation openings in the frame and window reveals aren't blocked, and make sure the radiator under the window isn't covered by a curtain or furniture - the heat needs to rise to the glass. If the corners are freezing badly, it could indicate poor insulation right there, but often the problem is solved by just airing out the room properly and sealing any drafts 😊
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