Insulating floor - breather membrane

[Idrisdraig]

I'm installing a raised floor (aprox 5m^2) over a concrete slab. There's enough heat loss through the slab, to make it worth insulating between the joists.
Some websites I've seen mention adding a breather membrane under the insulation. Some say it's not necessary if using someting like foil coated Celotex.

1) Opinions on whether the membrane is needed?
2) If it is needed, is Option B a bad idea, or is A the only way to lay the membrane?
3) Are the breather membranes sold for roofing suitable?

https://1drv.ms/u/s!AiSdxKZH7MXJgTW9h7XV-y29Z0CZ

 

[Mr Chibs]

Breather membrane won't make any difference if you use celotex, as the foil face is not breathable.

Personally I wouldn't bother with an air gap either, if you are laying a DPM on top of the concrete, then joists on top, there isn't any point to an air gap, unless there is some ventilation (air bricks) available at the ends.
I wouldn't lay joists directly on the concrete (pic A) as any damp could be transferred through the joists.

I'd do option B without air gap.