Garage upstairs useful room

[zooankski]

am in the process of converting the attic area of my detached double garage in to a useful room.
I don't want it to be habitable but comfortable enough to perhaps stick a tv and small sofa up there.
The garage was originally built by he previous owners with the intention of being properly converted, cavity walls, single skinned bricked up window recess complete with cill etc that simply needs knocking out and a window fitting.
I don't want to go down the planning route and don't want a habitable bedroom, just a bit of chill out space and somewhere i can put my stuff without the kids touching it! Also it might also end up being used just for storage in the future.
I have fitted a staircase and now want to board the garage ceiling in fireline plasterboard although i am stuck on the insulation for between the joists.
The joists are 200's and i have a load of 150 insulation rolls. Question is do i need to fit this with it against the bottom of the upstairs floorboard or will it be ok to rest it on the plasterboard as i put them up?

 

[John D v2.0]

Is it the room you want to be insulated rather than the garage? You really need the insulation to touch your air barrier which should be on the warm side. Otherwise you're likely to get a draught blowing inside your insulating layer. Think of a loose coat on a windy day.
In your case it's a shame you already have the 150s, maybe you can get hold of some 100s and just squeeze it in. Should be fairly cheap.
The other option is staple netting to the joists, that's very common with insulation suspended ground floors.
Good luck, sounds like a nice job!

 

[zooankski]

That makes sense. I will try and have the insulation against the bottom of the floorboards then as it is the room that needs to stay warm. The rolls of 150 were free or i would have got 200 and as i dont want to class this as a habitable room, it needs to be kept as low cost as possible. Will be great to have a room i can just go and chill out in every now and again!

 

[Tigercubrider]

Is there insulation board on the roof?

It will be very hot or cold if you are in a glorified attic?

When my mate converted his garage to a recording studio he checked with planning and they were fine so long as it's not "habitable" but that was a while ago

 

[zooankski]

At present there is 100mm of foil backed expanded polystyrene board between the rafters, covered with a vapour barrier to the inside of the garage loft and also a 50mm air gap behind to the breathable roof membrane.
I would like to cover over this with PIR backed plasterboard just to boost the insulation and give a neat finish, Probably in 40mm.
Is there anything wrong with doing this? I assume simply screwing directly through it with long drywall screws and then tape and fill the joints?