Putting insulation onto fire door

[BracknellDIY]

Double garage with entry door from utility room. The door is definitely a point of heat loss and consequently cause of the utility room being particularly cold. I am assuming (since i expect it is meant to be) that this door is a fire door. Currently I have put a silvered 'bubble' type foil on the garage side of the door with intent of providing some level of conductive installation. However I'd like to put on something more effective but have been hesitant as concerned that I might undermine the efficacy of the fire barrier.

I have some spare Jablite Premium: EPS with Fire Reaction to class E (25mm thick in this case). Would I put nuts to put this directly onto the door (garage side) ? Part of me says the space is meant to cope with flaming car petrol tank, the other part says that backing polysteryne directly onto a door surface might not be smart.

ideas on other appropriate solutions ??