I have recently renovated a victorian single skin brick built farm house. Building control made me insulate the inside of all the external walls to current insulation standards as I had removed more than 50% of the old plaster. Now that the rooms are better insulated and the external walls have lost their ability to breath (i did try explaining this to building control) I am getting damp spots on the dot and dabbed internal wall between the kitchen and living room (this wall did not get insulated). My damp spots are from the dots and dabs - at approx 4 - 5 ft high. I am sure this is condensation on the thermal bridge between wall and board / hygroscopic salts drawn from the brickwork through the adhesive. Caused by the temperature difference between the internal wall behind the plaster board and the air in the room. Apart from ripping the plasterboard down and redoing in batten rather than dot and dab my other thought is to ventilate the gap between the internal wall brickwork and the plasterboard. Please feel free to knock me back if this does not sound like a fair plan. I want to make a row of holes along the base of the wall and cover with ventilated skirting, then do the same at the top of the wall and hide with ventilated coving. My hope is that the cold air behind the plasterboard will "drain" out of the bottom of the wall, causing warm air at ceiling height to be drawn in at the top - creating a continuos air flow that hopefully will raise the temperature behind the plasterboard. Does this sound feasible?
While you are correct on the fact that hot air rises, the opposite will happen with my proposed vent system.
The cold air is more dense therefore it will sit lower in the cavity. with the wall vented top and bottom the dense cold air will drain causing a pressure drop at the top of the cavity. In the warm room, the hot air has allready risen to ceiling level and is less dense, therefore it should draw into the top vent, hit the cold brick, cool, become dense and drop.
the proof of the pudding as they say.... wheres my drill?
I put two trial vents in - much to the dismay of my better half.
One at the top right of the living room wall, one at the bottom left of the wall.
A smouldering match at the top hole showed smoke being drawn in, a finger in the bottom hole showed a very slight cold draught coming out (had to wet my finger to feel it)
So there is now an air flow, just a case of waiting to see if it makes any difference (by the way, my finger could reach the brickwork behind the plasterboard at the ground level hole - it was bone dry