How to cover this wall ventilation?
Are you sure these aren't part of your HVAC? If I had to guess, these are natural draft returns to the boiler room. If your heat comes through a grate in the floor, you need these.



Ok, you have radiators, so these probably are just fresh air intakes. Covering them or not is your prerogative.

Temporary solution: tape some card stock over the opening.

Semi-permanently: use a can of spray foam to totally fill the duct. You may need to get the grate open to stuff something in there, so the foam doesn't just drop down into the duct.

Permanently: remove the grates, and if necessary, remove or bash the remaining duct work into the wall with a hammer so that you can plaster over it, after having stuffed a piece of batt insulation into the duct.



Magnetic duct covers are another option.