If it says 20 degrees it's good for 40 degrees at best. If you really want it to be comfortable for 20 degrees get a good sleeping bag that's rated for zero degrees. I have a Northface down bag rated to zero, i put it in side an LL Bean rectangular Hollowfill bag, it's good to 10 below .

Regards,

Chuck

"There's a saying in prize fighting, everyone's got a plan until they get hit"

Ghost And The Darkness