In both cases, this should be done around the entire house. It may in fact be easier to clean the cavity from inside the house. Take out the skirting board and wall linings, and remove the insulation material. If that sounds a lot of work, remember that everything is probably wet and needs replacing anyway. Plaster sheet is very weak when wet but may recover its strength when dry.
Bulk insulation can act like a sponge, soaking up water to the full height of the walls.
The inside wall linings on the outside walls may therefore have to be completely removed. Apart from having lost most of its insulation value, wet insulating material may hold moisture for months, causing dampness and mould to appear after redecorating has finished. If water entered the roof, take out the wet ceiling insulating material as soon as possible. The extra weight of wet insulation may damage the ceiling. Water can also be trapped in the internal wall cavities, behind the wall lining.