If you do that, your loft would be warmer because the space would be better insulated.
However, you need to both provide a vapour barrier and address the issue of ventilation.
Vapour barrier:
Warm moist air rising up through the insulation will condense into water droplet in the insulation or on the cold surface on the other side of the insulation. Insulation works by keeping air stagnant, so that any water droplets that form in or on the wood up there will take forever to dry, and are as likely as not to cause water damage to your ceiling plaster and also be absorbed into the wood, causing the wood to get wet and wood rot to set in.
Normally, you minimize this by providing a plastic vapour barrier on the warm side of the insulation to keep the warm moist air from getting into the insulation in the first place.
Ventilation:
You also want to provide some way to eliminate the condensation from any warm moist air that does get through the vapour barrier. This is normally done by providing an air gap between the insulation and the underside of the roof. Also, a way of allowing the air in that space to get out of the roof and cold dry outdoor air to get in to replace it is needed.
Typically, what's done is that foam forms are pressed into place between the roof rafters and these shallow rectangular dish shaped forms provide for about a 1 inch air gap between the underside of the roof and the fiberglass insulation. The fiberglass batt insulation is pushed in place between the rafters over the forms. A plastic vapour barrier is then put up over the fiberglass insulation and stapled to the roof rafters. Finally the plasterboard is put on last.
ALSO, in order for there to be ventilation in that 1 inch air gap, you need to have the bottoms of those styrofoam forms open at the eves and also open at the ridge of the roof, and install a ridge vent along that ridge.
Then, cold outdoor air can enter into that airspace at the eves. As heat loss from the house warms that cold dry air, it's relative humidity decreases and it absorbs any moisture present like a sponge. Continued warming due to heat loss through the insulation causes that moister air to rise up to the ridge vent, where it escapes to the atmosphere carrying any condensation in your roof with it.
And, even a wind blowing will cause an air current through that air gap which will keep the underside of your roof dry, and therefore well protected from the wood rot fungus.
