quick math tells me that if we include urea (0.85M) and salt (0.71M) then freezing point of urine should be about -4.2C, in close to worst case scenario, which decreases roughly linearly as it is diluted. meaning that this is temperature where urine starts freezing, but below that point there is some amount of liquid where all urea and salt concentrates. just for water-salt mix some amount of liquid will remain down to -22C

i guess this gets more important at higher tempetatures