Cold Weather Gear

Absolutely - the ThermalCore insulation makes it suitable for air/ground temperatures well below freezing. In critical situations, a closed-cell foam mat should always be used as a just-in-case insulating layer.

This depends on how well your sleeping bag retains warmth (not the same as its temperature rating), how well your mat insulates (not the same as its R-Value) and a lot of other factors. You should see a fairly solid boost if the liner is used with an appropriate sleeping bag and mat

The Spark Pro has a footbox zipper and a short right-side zipper (great ventilation, open-flat as a quilt, sit up with armes extended from the bag)