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I should consider an alternative
Being an external wall, an insulated tile backing board would keep the tiles warmer than being onto plaster.
It would mean stripping the wall, so more mess and its going to be more expensive upfront. Though less condensation on the wall and reduced heat loss in the future
user69121-
I'd probably be doing this - it looks like lime plaster & mortar, suggesting a solid wall - Condensation / cold bridging is going to be a problem if you are sealing with impermeable materials.
Which could be why the lime plaster crumbled - it looks like it has been skimmed with gypsum at some point - water gets in through holes / condensation permeates the soft brick & mortar, but then can't escape anywhere, and the mortar & plaster suffer.
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This was my other thought. Take it back to brick and stick insulated plasterboard up to tile over. The tile backer board looks like a great shout. Having never used it before would that just be a case of sticking that to the brick with some adhesive?
NotThamesWater
Pmccee
Removing tiles as part of bathroom renovation. Plaster under the window crumbled away. Was going to pva solution the brickwork and fire a load of bonding plaster onto it. Plan is to retile this area. Just sounding out if a sensible plan or if I should consider an alternative. First foray into plastering potentially.