Which is better: bubble wrap or fleece, for greenhouse insulation?

No question - bubble wrap. Insulation of a greenhouse is the same as
insulation of a house. The best insulator is air. But it needs to be
trapped between two sheets of glass or plastic, as in double glazing.
Fleece does not trap air and would be useless. Fleece protects plants
outside because it creates an area of still air - and we all feel warmer
when we get out of the wind. Plain polythene would be almost as effective
as bubble wrap and could let more light in.