Insulating a floor of a bedroom which is joined to next doors bedroom at first floor level

[Pke]

The bedroom is over the driveway. From below the bedroom, looking up from the driveway are just white boards nailed to the joists. From above, there is 180mm gap from the top of the rafters to the top of the white boards. Fitted on top is a 3x3M 20mm thick board. Above we have underlay and a carpet. The room is very cold and the heat in this room from a radiator is being wasted. I was thinking of insulating with 100mm celetex boards. Is this a good choice and should I lay them at the bottom or to try to fix them up tight against the top board. Either way what do I fo with the 80mm space depth left between the rafters. Any help would be appreciated