I carry my filter in an insulated bottle holder. I originally did this for padding in case of a significant fall. However, good insulation will prevent freezing.

I have kept a filter in the bottle holder attached to the outside of the pack for quick access during the summer, but when <40, I'll put it inside the pack. I'll put it deeper inside the pack surrounded by clothes and sleeping bag if much colder.

Think about the differences between a bottle of water left outside the tent all night, one kept inside the tent at night, and one kept in the sleeping bag during the night.

I would like to have some detailed temperature ranges, but all I have is varied experience. Maybe I haven't experienced serious cold weather conditions like some on here, but I have spent nights out with windchill of approximately -25 (I believe it may have actually been -17 F). That's as cold as I've been experienced, but I'm sure somebody here can tell you about -40 before windchill.

Anyway, warm the filter and then insulate it from the cold.

Another cold weather thing I like to do is make the hot chocolate, etc. at night, put it in LEAKPROOF containers, and put it in the sleeping bag with me at night. It helps me stay warm, and is warm when I wake up.