Insulated plasterboard - condensation

[ntl142]

I have some damp patches on the internal surface of the living room wall. The wall is also cold (exposed SW facing in Scotland). It is an external cavity wall, rendered. The cavity is clear of debris. The damp is 100% condensation, and the 3 patches match the location of airbricks on the outside. Humidity is already reduced in the house as much as possible (trickle vents, dehumidifier, extractor fans, lids on pots, bathroom door closed, etc etc).
The problem is the suspended floor is lower than the external pavement level, and the airbricks are not ducted/periscoped to the underfloor void. Instead, the airbricks flow cold air directly onto the inner skin of the living room wall, leading to (very) cold spots and condensation. Previous owner to blame I think. One airbrick does actually have a periscope, but I can feel that it's hanging loose in the cavity.
As the wall is cold anyway I have been considering fitting insulated plasterboard to improve the warmth in the living room. It will also help reduce the condensation on the wall by keeping the cold brickwork separate from the warm room. I know the best thing is to fit the ducting/periscope to the airbricks, but this requires removal of quite a few bricks in multiple locations. As I was considering insulating board anyway, I'm hoping it will do both things at once.

1. Is that a daft idea?
2. Do I need a vapour barrier between insulated board and the existing wall?
3. Can I fit the insualted board straight onto the existing brick/plaster wall, and then skim/paint the new board?
4. How do I 'blend' between the added plasterboard and the existing window surrounds etc?
5. I'll need to extend a wall socket out from the original wall through the new boarding. If the wires are too short, how do I proceed? Some of these? https://www.screwfix.com/p/ideal-in-sure-2-port-push-in-wire-connectors-pack-of-100/50427

Thanks.

 

[JP_]

1. Not if it means a warmer, drier house, with lower heating bills.

2. If you use separate insulation boards and plasterboard sheets you can use kingspan (or similar) and these come with vapour barrier each side.
Easy to work with too. But maybe no need to create vapour barrier with a cavity wall - I have solid walls so didn't want interstitial condensation.

3. You can stick boards onto brick / block with drywall foam.

4. Tricky! I was lucky as I was putting in new windows too.

5. Could you expose the cable and move forward to the internal edge of the insulation? I would have thought that would be better, especially as that socket will be a cold spot and attract condensation.

 

[ntl142]

Hi, thanks for the comments, much appreciated.

2. If you use separate insulation boards and plasterboard sheets you can use kingspan (or similar) and these come with vapour barrier each side.
Easy to work with too. But maybe no need to create vapour barrier with a cavity wall - I have solid walls so didn't want interstitial condensation.

Interesting. I wonder if sheets of insualtion and p/board separate would be much cheaper. Maybe more hassle than it's worth though. I think I'll plan for the all-in-one approach of insulated p/board.

3. You can stick boards onto brick / block with drywall foam.

I plan on sticking them straight onto the existing plastered and painted wall. If the coatings are sound then I'd hope this would be ok.

4. Tricky! I was lucky as I was putting in new windows too.

I don't like the rubbish white plastic window surrounds so this might be a good excuse to get rid of them...

5. Could you expose the cable and move forward to the internal edge of the insulation? I would have thought that would be better, especially as that socket will be a cold spot and attract condensation.

Yes that's a good idea. Thankfully the socket isn't anywhere near the cold spots so should be ok, but yes I'll try to do as you suggest. Then there's the radiator, although thankfully that's microbore so I should be able to get enough flex to move it out to accommodate the insulated board.