How to fill in old vent

[DreamThrum]

I have a hole which was originally a vent in an external wall, but an extension was added and so the external wall is now an internal wall. The previous owners didn't bother doing anything about the vent, just left it. I want to fill it in and prepare the wall for painting. Wondering if anyone can advise how best to fill it?

I’d use a piece of insulation board (or even polystyrene, as it’s an internal wall and doesn’t need any thermal properties) of a depth which leaves an indent of a couple of millimetres, and then plaster over, sand and paint in the usual way.

Thank you, yes thermal insulation is not a concern. I wonder if I need to think about sound insulation? As there is only plasterboard (on the extension side) and then whatever I add now, between the two rooms.

we had a vent that we filled with expanding foam then a piece of board and some filler.
We had another small vent on the side of the chimney that was filled with spare bricks and then filled in.

I would fill it as it will stop what ever you use to cover from sagging inwards and stop anything using the hole as a nest! eg bugs etc.

Can you cover it with new plasterboard cut to fit the hole? You'll need to make a small wood frame within the hole to fasten the plasterboard to. Then just skim or fill any gaps and paint.