We moved into a very old house that currently has a single layer of insulation in between the joists. Also, at some point in the buildings history the ceiling has been raised into the roof space, above the natural 'A' frame of the joists, so we have about a 1mt length running down both sides of the house where the plasterboard below is screwed to the joists with a 100mm gap to the tiles. Lastly, there is no membrane or soffits. The longer term plan, in hopefully about 18 - 24 months is to get the roof taken off & re-done, at that point, besides getting proper ventilation put in, we'll also get PIR insulation boards added into the gap between plaster board & tiles. But in the short term, we are losing heat upstairs as it is considerably colder upstairs. My thoughts were to add the thickest loft insulation (200mm) over what we have and slide insulation foil down the gap between plaster board & roof tile. My concern is ventilation, everything I'm reading says I need to add it, but that's not taking into account the fact we have no membrane, so there's probably a lot of free air up there anyway. But would the best thing be to add a gable end air vent to either end in the short term?