Hollowcore V Timber Joicing in Attic

Advice please! I am building a split level house; one part of the house is a storey and a half (or a dormer) and I have put hollowcore in this section as it is soundproof and fireproof and my sleeping area will be upstairs in this dormer section. In the single storey part of the house, I also have an attic and when drawing up plans, was just going to use this as an empty attic/i.e. storing area and have a stira access to it. I thought no more about it (being a novice at housebuilding) until I saw the house walls going up and could then see in reality how the house is looking. I then decided to scrap the stira idea and put in an actual wooden stairs to this attic area instead so that I have definite access to it if ever I have a few quests staying and as I realised I had quite a bit of space up there. So, that was fine, builder agreed and he has now left a space for the stairs to go into the attic. Builder went ahead with 9x6 joicing in this attic area over the last two weeks and timbers for roof went on Friday and felt is to go on this week. However, when going over to look at house yesterday, I suddenly realised that I should have put hollowcore in this attic area also to future proof the house if I ever want to put rooms up there and for soundproofing etc. I am sick at the thought that the time is to late now to change from the joicing and am consumed with the thought of 'HOLLOWCORE IN ATTIC' at the moment!! I understand it would just have cost me 3k extra! Can someone please stop me going mad and tell me that 9x2 joicing is perfect and stable and that I can easily put beds up in the attic on this floor and walk around up there in the future (without floor wobbling and without much noise in the rooms below!) if I want and to stop worrying!!! Arrrrrrrrrgh. (ps: or is there anything I can do at this stage?!).

9x2 seems fine, but depends on the span obviously.

Hi cattorcake,

Are you sure that it would only cost 3k to take off the roof, place the hollowcore slabs and pour the screed & then re-do the roof.
I think it would be over the top to do all this work.Talk to the builder and ensure that all the bridging between the joists is put in.A good idea would also be to sheet out the top of the joists with 18mm wbp plywood and then put your floor finishes on top of this.I would be confident that this would significantly help with the sound.Just be careful that you don't end up with a step in level from your concrete floor to the area that is joisted - if both areas are connected.
You could also use a simple underlay on top of the plywood similar to what is used under laminate flooring.All these combined should really help your noise issue.

carrotcake,

You could also insulate between the joists with rockwool.
Do you know the make-up of your ceiling at the underside of joist level.Is it slabs nailed to the joists.You could talk to your builder about foriming a suspended plaster ceiling which would give a small cavity between bottom of joist and top of ceiling.This would really help but it could be getting too expensive.

Thank you Wexford Dude and Lollix for your helpful replies.