There can be issues with what I'm about to recommend, but it has worked extremely well for me. I have used a 3oz Climashield top quilt along with a 2-layer IX topquilt shell (IX over the Climashield) into the 20's and was plenty warm. I tend to sleep cold. The IX traps your warmth in the climashield very effectively. But there is a caveat - IX is also somewhat of a vapor barrier, so if it's not extremely cold, or you're a heavy sweater, you can end up with a bit of a soggy climashield bag in the morning. Good news is, synthetics still work when wet, though with slightly diminished effectiveness. The worst I've ever had with this setup is a tiny bit of moisture in the foot end of my climashield quilt, which dried out in 30min hanging over my hammock in the morning. Again - this involves a bit of a vapor barrier, so that must be taken into account when working out your sleep system as it could cause significant dampness for some people.