will I need to take the existing render off the building before putting the pads on first?

Hi there They will have a mechanical fixing so you should not have to remove the render but if the render is damp then you will be trapping it inside I think it would be best to have a survey carried out to see what type of damp ness you have whether its penetrating or condensation caused from within the house which forms on the exterior walls where they are cold so maybe an internal thermal board might be more suited,or you might have a problem with the cavity ties but only a survey would determine this. Regards Joseph

To remove existing render off back to bricks it's really depends if its blown or very damp and cracked,polystyrene boards will be fixed with long plastic plate plugs and steel nails fitted through so not necessary need to hack off becouse whole system hold on the wall but would recommend to insulate with 70mm externally and 20mm from in.