I've used a space blanket as a floor and it does help separating you from the cold ground. Wearing some warm clothes to bed, such as a down jacket, warm hat, and wool socks or down booties does well to extend the range. Make sure your pad will insulate from the cold ground. Lots of air mattresses will leave you freezing! Check their r-value. You can also get sleeping bag liners that weigh in the neighborhood of a pound, but they seem to not extend the range of a bag as far as they claim. I've used one, and it seems to maybe help extend the range 5 degrees, maybe 10 at the very most. I live in a cold climate, so I test out most sleep systems before I use them on the trail. Early last May, my wife and I hiked into a shelter on the AT in Vt. that's on a very good brook trout pond. I tried to go as light as possible, as I hauled in a float tube, waders, fins and my fly fishing gear. We slept on a platform with bags rated to 30 degrees, which in the past had worked to 20 for me. The temps got below 30 and it snowed several inches, but the worst problem was my Thermarest neo-air trekker pad let the cold wind underneath the platform come right through and I had a bad night. I have an older, insulated self inflating Thermarest, or I could have brought a closed cell foam pad to put beneath the Trekker, but I was opting for light. My wife's new neo-air that has an r-value of 3.9 worked for her. I did have a great hike and caught some nice brookies, so it was well worth it. Your pad will have much to do with warmth. I have a MSR Whisperlite liquid fuel to use in the winter when it's below 15 degrees F, but my Micro-Rocket has worked well to that with the warm-up and a piece of heavy foil wrapped around as a windbreak about 2/3rds of the way. As suggested, don't close it completely!!!!!! It helps to construct a rock windbreak, or use a natural feature of the terrain, besides the foil. It's better to make sure the windbreak will work and is sturdy before you light the stove. Many years ago, I dumped my dinner in the snow when it was well below zero, and had nothing to do, except crawl in the bag hungry and cold.
A lousy night could have been prevented had I not been in such a rush to get dinner cooked.