Please see diagram below:
This is basically what is happening in my house - I have a brick lintel which is both inside and outside the house - so it is exposed to the cold air outside and when you touch the brick inside the house it is also freezing.
The annoying thing is - whenever it is warmer in the kitchen - or you cook something, the vapour in the air condenses straight away on this lintel because it is so cold - and it gets really wet - even dripping, this is staining the paintwork around it and making an unsightly mess.
Bear in mind this is Yorkstone - very old house - what is the best way to stop water condensing on the lintel? Are the some repelling treatments available? Should I insulate it with polystyrene - and plasterboard - and box it in and plaster over it? Or will I still get problems? Any experts out there who can help - reallly, really, really appreciated - as I have no idea what I can do to stop it!