New Oak Floor - what underneath?

I've got a nice new stack of oak floor aclimatizing for a couple of weeks, and have conflicting advice on what to do with it. It's 20mm thick and the final floor level needs to come up 70mm total, so obviously I've got to put 50mm of something underneath.

Suggestions have been: float it; nail it to battens; fill the space with insulation; don't put any insulation; put some but leave a gap; glue it; don't glue it; put a membrane under it; don't put a membrane under it; make sure there's some ventilation underneath; don't bother - you get the idea, every possible combination. I'm guessing that probably most of these will work fine, but is there anything I should really be avoiding? The floor is new concrete with underfloor insulation and membrane; and pretty much dead flat and level.

Thanks in advance,
Neal

I would construct it as if it was a new timber floor, ie

New 'joists' and noggings across floor, nail new boards but allow a gap around the floor.

Fit new skirting over the exapnsion gap and hey presto.

Unless of course you are in a flat with another flat beneath you, then i would include some insulation, more for the benefit of your neighbours to try and limit the noise.

Hi Neal

I'm not in favour of installing new joist on new concrete (feels like a shame to drill many holes in the screed, plus it will costs you more in money and time).
Place DPM on the concrete to prevent any residue moist going into whatever material you use for increasing the floor level 50mm.
You can use chipboard, plywood etc.
Then, install a minimum of 2mm (foam) insulation. Not only for reducing sound (footfall) downwards, but also for reducing the sound in the room itself.

You can install the floorboards floating (if the width of the room is not more than 5 - 6 meter).
Leave a minimum 15mm expansion gap and if you are installing new skirtingboards, make sure they are a minimum of 20 - 22 mm tick.

Thanks for the replies - and apologies for the delay in getting back.

I'm leaning towards joists and secret nailing - mainly because that would mean I'm only doing things I've done successfully before and it's too expensive to mess up.

Would it really be cheaper to pack out a 50mm gap with ply or chipboard? (I'll concede it would definitely be quicker

It's a ground floor living room, so noise downwards is not an issue, but I guess noise in the room is (a bit). However, I guess I'd have to more-or-less completely fill the underfloor cavity to really make much difference and I'm not sure I want to do that. (Am still a bit tempted to put down, say, 20mm of Kingspan to add to the 50mm under the slab though.)

Oh, and when I said the concrete floor was "new" I should have said "been sitting drying out for a year"

Regards,
Neal