Damp on interior walls - any help appreciated!

[nme234]

Hi

I'm new on these forums, so be gentle if this is a really silly question! I bought my first home last year and am seeing damp patches all over the house - one in the living room, one in the kitchen and two upstairs in the bedrooms.

The house was built in the sixties and has cavity walls - there is insulation in there, I know that condensation is a problem in a small house but we have reduced the causes of this and we are still seeing the damp - the only common factor with all the damp patches is that on the outside of the walls there are vents in each spot - the vents are built into the walls.

Will putting a plastic cover on the vents help as it seems like moisture is getting in at the interior walls through these - has anyone heard of this type of problem before? e are getting black mould and loose plaster which really is not nice!

Thanks for reading this!

Nigel