Damp and Retrospective Ventilation to Dry Living Room and Under Floorboards

[slyzenoui]

Hello,

At the end of my tether!

I have rising damp in a chimney breast and after checking under the floorboards, saw standing water. I have eliminated a leak and the chimney flue was dry. We are on heavy clay so I suspect it is ground water. After removing the water, it has not returned (no recent rain) but under the floorboards it is still damp-ish. My front room is bone-dry (under the floorboards) but my living room is still damp - I had an rear extension 4 years ago but there appears to be no cross-through ventilation from front to back. They stuck in a couple of air vents in the extension but there is no evidence that they go to the rest of the house. What is the best way to improve ventilation to dry out my living room under the floorboards? Can I retrospective fit vents under the extension?

Any help would be greatly appreciated.