metal joists and cold bridges
 

I'm about to complete the ceiling with plasterboards but there is
going to be an air gap between the boards and the insulation above
(about 40-50mm) and I was thinking of completing the insulation shell
from the walls (which hasn't yet been installed) to the loft (so to
limit cold bridges) - this means that the ceiling boards don't touch
the walls but are about 50mm from it...so the wall insulation can be
connected with the loft insulation.
This should reduce cold bridges, but there are metal joists on 2 sides
of the house which can't be insulated- is it still worth bothering
trying to complete the insulating shell as much as possible and leave
the metal joists as they are? I was thinking of wrapping them in
insulating aluminum foil (bubble wrap), but it is rather awkward right
now and to achieve that I'd have to remove the ceiling boards.

The property is an end terrace house (so 3 external walls) with walls
with air gap of about 30mm....the insulation on the loft is typical
glasswool 200mm and the one to be installed on the walls is celotex
50mm.

Any help?