We use a rectangular sleeping bag under the bottom sheet to insulate us from the air bed. Right now we are using some very old sleeping bags (1 under the bottom sheet and 2 on top of the top sheet) in freezing weather. The current sleeping bags are ok for down to about 26F, but I am looking to replace the 3 bags with 2 new 0 degree bags for sub-freezing weather. We just used this setup over thanksgiving in the Yosemite area (outside the park and down at about 1400 feet) and it was freezing all 3 nights we were there, but we stayed warm. I'm glad we did not try to get a campsite in the park, as it was down to 10F there! It also helps to have a small propane heater to warm up the space before bed and in the morning.