soldering track joints vs leaving them loose

What is the consensus out there on track joins? My layout, under contruction, sits in the attic which experiences a wide range of temperatures from under 10C in winter to in excess of 28C in summer. At the moment I have left all joins (peco flexitrack) unsoldered as I was always led to believe that leaving a few gaps and letting the rails move would prevent buckling due to heat expansion. But is this correct? Would I be better off simply soldering the joins? And how much gap is needed if you leave them unsoldered? I am working on a millimetre or so every three or four lengths of track.