Venting Plasterboard Internal Wall After Insulation Now Causing Damp Spots

Postby navyreg1970 » Fri Nov 01, 2013 12:54 pm

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?
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Simply Build It

Postby rap12 » Tue Nov 05, 2013 3:23 pm

You have an internal wall thats been dot & dabbed on both sides of the wall and is now showing "damp" spots at the dab locations?

Unfortunately, if there is damp in that wall then the moisture and salts will penetrate through the fixing dabs no matter how much ventilation is introduced behind the p/board.

Setting plumbed & plugged battens on narrow strips of dpc suggests itself as the answer.

There are remedial p/board adhesives available if you search google - they claim to deter damp and salts.

Some claim, that instead of using adhesive, that rapid foam will do the trick. I dont know, but i doubt it.

Why not try what you propose on one side of the wall only (simply vent - top & bottom) and then watch and wait? Warm air rises, by the way.

Was this an original interior wall, not a shippen wall?
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Postby navyreg1970 » Tue Nov 05, 2013 8:57 pm

it is an original wall.

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?
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Postby navyreg1970 » Wed Nov 06, 2013 8:07 am

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
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Postby navyreg1970 » Fri Aug 28, 2015 2:46 pm

Im a bit further along now (18 months) - I ended up cleaning up the efflorvescence spots and painted the wall with several coats of an oil based paint recommended for the task.

We then decorated in normal emulsion.

no problems for approx 1 year but I am now getting bubbling of the emulsion where the original spots were.

we had nothing all winter (but we had the log fire roaring for most of it so good airflow)

I have now also noticed similar above the skirting board behind the telly.

I still think this is condensation on the cold spots of dot and dab - and the bit behind the telly probably gets very little ventilation.
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