I have a typical Victorian semi where the rear of the properties narrow with the kitchen on the ground floor and a bedroom above it. The party wall is 9" on the ground floor and single brick from the top of the wall plate for the 1st floor joists which run sideways and serve both properties. Because of lack of/crumbling mortar in the joints on the bricks between the joists and the fact that the joists have shrunk a little over time, there are small "air gaps" in the wall between the two properties at joist level. The result is that we get food and cigarette smoke smells from next door penertrating through to the kitchen and bedroom. So, the question is, what is best way to seal this? My only idea at the moment is expanding foam. Steve