Installing insulation in roof

[SteveKay]

Good afternoon. I'm new to the forum so apologies if this has been covered. I've looked through the threads and couldn't find anything similar. I've recently bought a property and the top floor bathroom (the house is over 3 stories) is extremely cold. The bathroom seems to have been an add-on to the original house. I have a large double panel radiator installed in there but it makes little difference to the temperature. It's constantly freezing in there. I can only assume that there is no insulation between the roof of the building and the ceiling of the bathroom. So really, my question is as follows: what are my options with regards to insulating the bathroom? Would I be better off removing all the roof tiles and laying insulation underneath or ripping out the ceiling and installing it that way? The removal of the roof tiles seems are far less messy and costly job. Any thoughts or suggestions would be appreciated. I've added 2 photo's of the bathroom for reference. Thanks in advance.

Steve

 

[Alastairreid]

Lovely big bathroom Steve.

Insulated plasterboard Here.

 

[SteveKay]

Thanks Alastair. I'm assuming I'd have to either knock the old ceiling out and put the plasterboard in its place or put the plasterboard over the current ceiling and just drilling holes for the light fittings.

 

[catlad]

No need to pull the old ceiling down just ram a sharp object through the ceiling to work out were your joists are then mark the walls.

 

[SteveKay]

Thanks catlad. Apologies for the basic questions but how would I go about attaching the insulated plasterboard to the current ceiling? I'm assuming it would somehow be secured to the joists

 

[freddiemercurystwin]

Screwed to the joists, that's why catlad said how to find 'em.