Insulation
Situation
Bunkies are not meant to be full-time living quarters, but they can be! Some folks even live in them way up north in the winter. If you wanted to, you could add insulation and siding to the outside (since your probably don't want to take up valuable indoor space). But usually you just want enough insulation to have effective climate control for occasional or seasonal use. You should consider your own situation. For most people, it doesn't make sense to spend thousands that will take years to decades to recoup.
But for almost everyone, we recommend doing the following....
Solution
Use 2x4 furring strips to attach your roofing (not provided). Put 1.5" insulation between the 2x4s (yes, standard 2x4s are actually 1.5 x 3.5 inches). You can also use insulation tape and/or construction wrap in accordance with best practices.
Then you'll do almost the same thing below the floor. Put the same insulation between the pressure treated floor sleepers (provided in kit). Now your bunkie has a warm hat and warm socks.
Caulk it Like it's Hot! (or Cold)
If outside air is blowing through your sweet bunkie, it really won't matter how much insulation you have. So take some time to caulk on the INSIDE of the bunkie around the windows, door, where the roof hits the walls, and the edges of the floors. If you caulk on the outside, DO NOT caulk the bottom of the windows or door! These are made to shed water.