Warm air is moist and if it meets cold air it condenses. Condensation can be a problem in a loft if you insulate the floor. No heat will escape into the loft but moisture will pass upwards and condense in the insulation material, which becomes ineffective if it is damp.
Foil-faced plasterboard is impervious to moisture so if the ceilings have been constructed of this material you do not need to take additional protective measures. If not, make sure that insulation material is protected by some form of vapour barrier below it so that the risk of condensation is reduced.
Sometimes blanket is supplied with a plastic or foil backing which moist air cannot pass through. But if you are using paper-backed or unbacked blanket or loose-fill insulation, you need to install a vapour barrier first.