What's This Ceiling Made Of?

Hi.  This is my first post - I hope I'm putting it in the right place.

I am removing the ceiling of my 1970 Utopia.  The material is approx 4.5 mm thick.  The top surface facing the insulation has a metallic colored surface layer.  The bottom surface facing inside is a sort of plastic-like film glued to the surface.  The material itself is brown and seems to be a composite type board - it reminds me of masonite (?).


Anyone know what this is and what would be a good material to replace it with?  Thank you!