Internal insulated plasterboard - plaster the wall under it?

MichaelR

We are looking to insulate a small back wall of a converted garage (cavity block) and are considering all options. While external appears to be workable it's highly expensive, so we're also looking at internal slabbing with Kingspan K18 insulation-back plasterboard.

The wall was previously (about 20 years ago, probably; we only own the house for 2 years) "insulated" with thin Aeroboard then plasterboard. We are removing this and here was lots under it - wallpaper in some places, remaining adhesive from past tiling, even an electric cable sandwiched between teh Aeroboard and the wall.

Upon inspection we found black spots that could have been mold. They are, however, only on or around the wallpaper. The wall does not seem damp.

If we just remove the wallpaper and then put on the internal insulated slabbing, I am worried about possible condensation and new mold in the little cavity under the new slabs. However, as there is also a layer of old plaster, striping down to bare block does not seem possible. And Ive been told that the bonding adhesive option, which would not leave a cavity, only works if the wall is stripped down to bare block; it does not sem to be able to fill an uneven space, which would result as the wall itself is uneven now with all the old plaster and adhesive and what not?

I am wondering if perhaps I should get a good plasterer (I know one) to plaster (skim) the wall without insulation. Then let the result dry for 2-3 weeks and then put on the insulated boards. Without a cavity there would be no place for the mold to grow? Would this work? And should I get some some special mold-resistant plaster mix and ask the plasterer to use it?

After the slabbing I am definitely asking him to skim over the slabs. one way or the other.

Calahonda52

some good questions at 2am!
First up, if you do even up the wall, which IMO is the better option, use sand cement [s/c] plaster and NOT, repeat Not gypsum skim.
Skim is 100% water vapour proof and will act for a substrate for mould growth.
s/c is water vapour permeable so it will draw any moisture that condenses on it into the block work.
Crucially, it will also make the blockwork airtight on the inside

The bonding adhesive must be vapour permeable as well, else it is a fool's paradise.

The cavity you speak of will not materially enhance mould growth so if the existing wall finish is s/c and not skim then,

Having said all that your choice of material is probably 100% vapour resistant anyway so its just what will escape through it via services etc ...
https://www.nsai.ie/S-R-54-2014-Code-of-Practice.aspx
may help
http://www.josephlittlearchitects.com/articles
is more advanced but a great reference site.
Keep well

MichaelR

There is existing skim finish on the wall in most places. Looks like there is no shortcut and I have to strip it to bare block to avoid condensation/mould growth? If so. what is the preferred method to do this? A belt sander would be like SUPER dusty.

BryanF

dry-lining: You need a vapour barrier internally tapped and sealed.

warm moist internal air, hitting the cold plaster (or blockwork) is causing the mould.

Calahonda52

Some folk reckon giving it a good scoring will do, followed by s/c render: its not Fab 24 spec.
The other option is that if you don't insulate too much, then the dew point will be in the wall before the skim but that needs a wu-fi calculation