Advice needed on building up ground floor level

[graemer57]

Am building a 6m x 7m two storey extension on back of house (living room downstairs, bedroom upstairs).

Originally we were planning to put wet underfloor heating in the living room but have now decided against this on grounds of cost and practicality - doesnt fit our busy lifestyle, so have now opted for radiators and woodburning stove for more instant heat without having to run all the time... please no arguements in favour of UFH, already have the stove :-)

As a consequence we now want to make up the approx 150mm floor depth that would have been taken up by the insulation and screed. We thought of putting 125mm Jabfloor, vapour barrier, 22mm T&G chipboard over the concrete slab and having a floating floor but there is a chance we might move a very heavy piano into this room later and am concerned that the floor will 'flex', Now have decided to fix battens to slab at 600mm centres (cut down to right height from 50mm x 150 treated timber) with full depth insulation tight between and 22mm glued T&G chipboard over and screwed to battens.

Need advice in two areas

- is it ok to put vapour layer under the battens then build away from there? Some advice seems to suggest it should be immediately below the T&G but then it would be pierced by screws

- any suggestions on how to fix battens to slab, or will they be held tight enough by the 125mm Jabfloor wedged between them. I've seen various batten supports that sit on the slab and can be adjusted with shims to take out any inconsistencies in slab levels but they seem either v expensive or hard to get a price on

Any advice gratefully received! Graeme

[TheDoctor4]

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[the specialist]

Hi

I think I would probably put kingspan insulation and screed over. The floor will be solid and still warm. I'm assuming you have a dpm under the slab?

[graemer57]

Yes have DPM under the slab Was trying to avoid screeding due to cost and fact that have a lot of the materials already (Jabfloor 70, chipboard and battens). Was only me getting 'cold feet' about the possibility of floor sagging a bit under the point weight of the 3 legs on a grand piano that as stopped me putting a plain insulation/vapour/ chipboard floating floor down and considered supporting the floor on 50 x 125mm battens with insulation between