Replacing floorboards with mdf in Victorian flat

They are in a bad state, after being lifted one time too many. Plus exposed floorboards are very draughty, and anti-social for the flat below. I plan to cover them up with carpet (living room) and laminate/vinyl (in the hall), so it's better to have level, draught free base.

Ian

Update: I ended up using 25mm ply. Three sections, cut to size by the timber yard. Used plastic spacers to get the surface level. I then sheeted the whole lot with 6mm ply, and sealed edges and under skirting with mastic.

I borrowed a router to route the underside of the ply in places where the old conduit was sitting higher than the joists.

Ian