The only experience I've had with debri type shelter was during a year we decided to camp without benefit of a tent. We stacked dead leaves into a platform large enough to support our sleeping bags. Then we piled more dead leaves on top of the sleeping bag leaving only the opening to wriggle into. Even though the night time temp dropped below freezing we were very warm. I think even a canvas, or blanket bag would have kept us warm under the same circumstances. Oh I almost forgot the old fashion army "rubberized" poncho we pegged over the whole mess to waterproof and wind proof the whole thing.
In general I have found that 3 feet of insulation on top off the shelter is sufficient for above freezing weather wearing a T-shirt and shorts. In freezing weather the thickness will easily go to 5-6 feet. Another thing I will swear by it to stuff to inside to overflowing with leaves. This is the only way to provide enough internal insulation. A good door is essential also. I have gone overnight at 8 degress celcius in a rainstrom with heavy winds in 1 foot of insulation in only a t-shirt and shorts. The wind ripped through the shelter but I did not get wet.
I can usually find dry insulating material underneath logs or thick piles of leaves. I always try to place dry material inside of the hut and wet material on top. Even if wet material is used inside, I have found that body temperature will usually dry it out a little. Of course in an evergreen forest one will have trouble finding enough debris to build a shelter. Green boughs can be used, but you must double all thicknesses. In a pinch you can always stuff your clothes with leaves. This will usually help a little with insulation. I think the main aspect of the debris hut is not comfort but mainly to allow you to survive the night.