One of my backpacking partner uses a Mountain Hardwear down bag rated for 15 degrees and I used the synthetic version by the same company. In the same 19 degrees night at the Linville Gorge, we both stayed plenty of warm. He did get his bag a little wet from condensation because he was using a single wall tent, but it didn't get wet enough to do affect him. The difference between him and I on that night, his down bag packed a lot smaller and lighter. He probably saved a pound and I want to say somewhere around 10 liters in volume in his pack.