tanking and insulation

[rsk1]

The back wall of my kitchen is a retaining wall. I currently have it exposed brick / render and am about to insulate and plasterboard it, 50mm PIR / PUR. I'm thinking I should tank the wall: it feels pretty dry but there is soil / rubble / who knows what behind it up to 2m height and we have had quite a dry summer. However, cementitious tanking slurries need to have only breathable materials on top of them. Because i'm insulating i'll be having a vapour control layer on the inside, ie. non breathable.

insulation needs to be impermeable, tanking needs a bit of breathability. Does that mean tanking is not compatible with phenolic foam insulation?

 

[wessex101]

I would go with a cavity drain type membrane then dryline with insulated plasterboard.

 

[Leofric]

Where does the cavity drain to ?

 

[wessex101]

It isn't compulsory that you drain the cavity unless you have got very high moisture levels, as long as it is sealed along the bottom you should be fine.

 

[^woody^]

Cement render or slurry is not completely waterproof, so you could get some interstitial moisture if you then seal the wall with a non-breathable material, or dont have a drained/vented cavity to deal with any moisture.

 

[rsk1]

so what is the best way to tank AND insulate? If it's vented or drained , isn't that a way for moisture to get behind the insulation from the inside and form interstitial condensation?


I should add that the wall has had 15mm of extruded polystyrene insulation and plaster on it for 20 years: i've pulled it all off and it doesn't feel that damp underneath. It's just that i'm about to cover it back up with 50mm PUR and a brand new kitchen, so want to get it right...