Condensation affecting attempts to 'dot and dab'

[dolland]

we have a 1930s house with an upstairs spare room extension (probably 50+ years old). This small room is single brick thickness and rendered externally on its three outside walls. I was advised that it could be made warmer if I added a plasterboard inner skin - dot and dabbed.

I am a reasonable DIYer who since 'had a go' myself. I covered one wall successfully late last year. Now that the Xmas/new year blur is over, I wanted to do the remaining walls. But on close inspection, I was alarmed to find a significant amount of damp/mould poking out from the 'cavity' behind the board in the corner. I hastily removed the board and discovered a large amount of condensation on the inner face of the external wall - after about one month. I know we have had terrible weather, with very low external temperatures for days on end recently, but clearly this accumulation is not good.

The damp has now dried out overnight on exposure to the room temperature, but I'm not sure what I should do now. Add vents, treat walls, just seal up and hope for the best?

Any suggestions would be appreciated please.

 

[jeds]

Water vapour will migrate through plasterboard very freely so if you have cold walls and a cavity behind then condensation is inevitable. Also dot and dab plasterboard has a very poor insulation value on its own so will do little to prevent heat loss.

My advice would be to use an insulated plasterboard instead. (see link below) This material is more or less vapour proof (apart from the joints) and will provide you much better insulation. It's fairly easy to fix with adhesive or mechanically with long stainless screws.

http://www.lafargeplasterboard.co.uk/products/concepts/thermal1.htm