How do you fill in cavity gap around window?

19 Jun 2020
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United Kingdom
Hi, need some help, so bought an ex housing association house, this is our first home so didn't know much but wanted to do a bit of work ourselves, All of our windows have horrible UPVC boards around the whole window, they look ugly as well as some being broken so plan was always to remove them, have one board at the bottom only and to paint up to the window.
Currently doing the nursery first while its empty, I have pulled one of the side boards off and what i have found is what you can see in the photo. It seems instead of closing them off the HA just chucked these boards on to hide the cavity gap.

Whats the best option to close these off? I want to be able to paint up to the window to have a neat finish.

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Cavity closers.
can i put those in after the window is already in, it seems to suggest it needs to go in before the window.
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You might still be able to fit them, or ring the council and ask them to come and finish the job properly.

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