Suspended Timber floor in new build?

Hi

I'm about to start on a selfbuild. Would like to use a suspended timber floor, in order to reduce the thermal mass of the house.

I would be grateful if anyone has any advice on how to insulate the floor inorder to minimise thermal bridging. I will be insulating between the joists, but am worried about heat transfer through them.

Also, how would you make such a floor airtight, should I lay the airtight membrane underneath the floorcovering and on top of the joists? or is it enough to tape the joints of the ply-floor covering?

Thanks in advance All help appreciated!

Porridge