Sealing insulation board to granite foundations

[Lb1993]

We have acquired a listed cottage which has a section dating to Victorian age. The walls are brick onto granite foundations. Internally the walls have been insulated and plastered but we are stuck on how to seal the plaster down to the granite. Photos attached, ideas welcome please

 

[aockenden]

What sort-of appearance are you going for? Do you want the plaster to continue straight downwards until it intersects with the granite and then stop sharply? Or would you be ok with a kind-of "hump" / rounded or fillet intersection between the two surfaces?

Also what type of paint is that on the granite? It looks very shiny / smooth which may lack the key necessary to bond plaster to directly (so it may need to be scraped back to bare stone anywhere you're going to apply plaster, or you could use some Febond blue-grit type stuff).

 

[Lb1993]

Thank you - looking for a smooth (as can be!) join - perhaps could fill with expanding foam. Paint is masonry paint suitable for granite