That poncho liner can be good to mid 30's low 40's inside a vapor barrier (for me at least). If your poncho it's self is rubberized, try to find the straight nylon version they are much much lighter and packable. I sewed a zipper onto my poncho liner (onto two sides) and it makes a GREAT bivy and/ or 10-15 degree bolster to any sleeping bag.

Ironwood