My kitchen floor joists are 100mmx75mm.I'm in process of putting 25mm ply down as it will be tiled over...Is it worth me putting insulation between joists as now would be easy to do? I was thinking rigid foil backed same depth size (100mm) with nails tapped below sideways into joist to keep them from dropping. Also. does it matter how tight fitting i fit the ply along the wall edge - is it best to leave a minimum gap?