Bob Minchin wrote:

Can't do any harm. I'd use 25mm polystyrene sheet.

Might not do much good though. What's above the existing ceiling? If
it's a loft space and there's already 6 in. or more of insulation
therein, adding another inch of EPS or PIR foam is going to make the
square root of not very much difference. OTOH if the space above is
cold and poorly insulated this could constitute a significant improvement.

NB lowering a bathroom ceiling might have electrical implications. A
ceiling light or other fitting that was previously outside the zones
might be brought into zone 1 or 2 (if brought below 2.25 m above floor
level). If this applies the light must be suitable (at least IPx4, and
approved by the manufacturer for use in zone 1, if relevant). Also the
earth of the lighting circuit will need to be connected to any
supplementary bonding required in the bathroom (unless exempt by dint of
meeting the relevant 17th edition conditions).

--
Andy