Need advice. Quebec Canada attic insulation, air sealing from inside?

Ok 1930’s home, it’s always had huge icicles hanging from eves in the winter. When I go on the attic, which is incredibly difficult as it has a bead board ceiling with tiny access and it’s at its highest 4’, it has multiple sections so I’m assuming roof had a different configuration at one point so it’s compartmentalized so I have two roof vents. I had someone come and cut vents at the soffit and also had new roof put on, full membrane and shingles. It has “some” insulation but I suppose no one added much because of the difficulty of getting in there and moving around. I was wondering if I could air seal from the inside by adding 2” styrofoam over the bead board, caulking edges and adding plasterboard on top.