The old saying is "one underneath is worth two on top". You lose a lot (if not most) heat through the ground. Definately want to get a closed cell matt, insulated self inflating mat or just a thick duvet under you.Air is not a very good insulator. Have had the same when camping - lying on a bag of cold air on a thin groundsheet on the cold ground and it doesn't matter how many layers are over. Get some under, too. I use a quilted mattress topper on the air bed, then a duvet as a sheet. Duvet over the top and toasty warm.