An otherwise warm room suffers from condensation in one corner
because, I think, a 1m length is only half a brick thick where the
hallway on the other side ends and the open porch outside the front
door begins.

I want to insulate this wall from the porch side - is there any
material designed to be used externally, or should I use insulation
slabs of some kind with a covering of ply or similar? The area is only
about 1m wide x 2m high.
Will be grateful for any ideas.

Peter