A suprising number of screenwashes will only stay liquid to -6 degrees and we've been seeing up to -20 in Scotland this year.
It may be smelly, but you can add Ammonia which on it's own freezes at -80, so you can dilute it down to about 1 part ammonia to 4 parts water and it'll be good for -20. Add some distilled white vinager and some lemon juice and you'll get sparkly clean windscreen which does not freeze as it hits the screen or stits around the nozzles. The alternative is Isopropyl alcohol, again, very low freezing point. Think about how you can store Vodka in the freezer without it freezing.