My current sleep system is an EMS Mountain Light bag and a BA Insulated Air Core pad. Both are rated to 15-20 Fahrenheit but I find them uncomfortable below about 35° if there's wind. Typically I'll feel cold against the pad, though my top side is warm.
Last week I was about to head out for a 2-night trip. Forecast was blustery with low 30s at night, so I needed more insulation.
I could have picked up a blue CCF pad at Wal-Mart, but I had a roll of 18" wide Reflectix left over from making a pot cozy. I cut off 6 feet of the stuff to make a very packable 6.3 ounce pad.
Unrolled inside the sleeping bag, the Reflectix pad makes a significant difference. The first night out, it got down to 28°. Against the Reflectix, I was still toasty, even though my top side was cold. Throughout the night, I slowly rotisseried myself to keep warm enough. This pad turned what would've been a poor night of sleep into an acceptable one.
The second night, it was nearly as cold, but I decided to experiment with a trash compactor bag (2.3 oz.), which I use as my sleeping bag stuff sack, as a vapor barrier liner for my feet and legs. Again, this made a significant difference. My lower half was super toasty, albeit a bit clammy. I slept soundly and woke up warm.
For under $10 of materials and an additional 6.3 ounces carried, I probably added 10 degrees of comfort to my sleep system. I encourage anyone else to try the same!