>>930628Hammocks require insulation under you. It's not optional in cold temps.
Do just a little searching on the topic and you'll see how big of an issue it is.
>>930619If your sleeping bag is truly rated for 30 degrees of comfort, then your top half will be warm. Many cheaper bags though, while they say "30 degrees", are really only comfortable at about 40 or 45. A high quality bag will be true to rating.
Your underside will be cold. My first ever hammock trip, before I knew much, had a sudden drop in temp overnight that was not expected. We went to bed at temps around 50, with an expected low of about 48. Temps dropped to about 35.
I had a 40 degree bag on top of me, and a 40 degree bag below me as a makeshift underquilt. I had on several layers, and a fleece blanket I laid onot. I froze my ass off. Took me an hour my the fire to get the chill out of me. Never again.
Fast forward several years and I have underquilts for 0 degrees, 20 degrees, 30 degrees, 40 degrees, and 50+ degrees. Insulation under you is key.
Get a cheap sleeping pad. It'll work in a pinch. Go to Walmart and buy a $9 blue camping pad. It's not pretty, but it'll keep you warm enough. When you're done with it you can cut it up for a sit pad or foot pad too.