ventilation & cavity wall insulation

[beejayem]

I'm planning to have cavity wall insulation in my 1930s house. The bedrooms have ventilators near the ceiling - the sort typical in this age of house with a sort of louvred brick on the outside wall, and a hole on the inside, which at the moment has a plastic adjustable cover.

I've been told that if I seal these on the inside they will insulate the whole wall, if I leave them as they are they will fit a vent to keep the ventilator open.

My question is - do I need to keep this ventilation? It seems a bit crazy to insulate the walls and then have cold air blowing in. Even with the inside cover closed this still seems to happen. But - would I be creating problems for myself if I seal them? There is an extractor fan in the bathroom, so hopefully not too much damp air around.

Many thanks for any advice

[welsh brickie]

Blocking them up could cause condensation in the rooms.just keep them in place and shut if condensation starts unblock them