My raised beds are 2 and a half feet deep at least. Chicken shed residue on the bottom which consists of shredded paper and guana. A really thick layer of that - about a foot or more. Then good compost on top (or not so good depending on what I want to grow). Beds are made of old pallet wood, so about an inch think. If you put a rod right to the bottom it comes out really warm. So I can't see a need for additional insulation.
Needs topping up every year, but plenty of chicken mature here, and an occasional top up of compst if equired. Rotation of raised beds of course - I have four one metre square and two which are one meter by two metres.