wrote:

Looks like drilling into the window is not recommended, then....

Well.... We have the same problem. My brother out-law is a window fitter
and sent half a dozen trickle vents to us. Didn't say it would present
any problems, and the vents on our UPVC patio doors in the bedroom are
fitted to the top of the door part rather than the frame part.

This is where I am going to put ours, as and when I can get round to it.
Looking around other houses in the area, they are all fitted to the
"window" rather than the frame.

Though, I couldn't find any source of the vents anywhere. Perhaps if you
have a local UPVC company they would be able to sell you (or give you)
some. (You could try bluffing them with the fact you may be wanting to
change half a dozen windows later in the year to get a freebie)

:¬)

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