I have a split level home (1950's) that after upgrade made the list for energy efficienty in the 70's Thereafter I had central air conditioning installed, with the air return in the headspace over the stairs to the upper level.

Now I have a wicked cold draft on the stairs, flowing down to the living room level. I have cut and plugged the filter space on the air return with a 3" slab of polystyrene insulation. But it either does not work, or I am missing where the draft is coming from.

Can you please help?

Ablediver