Damp in 1940s semi

[hughsykes]

Can anyone help?
My semi has an effective damp proof course in the walls but not on the floor. I have fitted a platen isolation membrane on the floor which is designed to channel any damp to the edge of the room where it dissipates. This seems to work as the laminate on top of it is fine.

The walls have been re plastered to above the floor by a reputable damp proofing company. The brickwork and render is dry but the plaster skim has a water-tide mark around 0.5m above the floor on the inside surface of the outside walls. The damp proof company have given up saying that it must be due to condensation on a cold wall - why only above the floor then?!

Is the skim absorbing moisture from the floor even though it is not in contact? I don't seem to be able to cure the damp. Can I decorate over this? Can I apply a waterproof skim? Help?

[welsh brickie]

the tide mark could be salt in the render (effloresance).Use a pva solution over the wall before decoration,this will seal the wall.

[hughsykes]

Thanks - there is no sign on crystals and the skim is definitely damp - the underlying render and brickwork is no however.