Does it get below -20 F in your part of Connecticut? That's kind of the cut-off where you need to start doing things because of the cold.1. Do I need to insulate the coop? There will be no electricity/heat source (although an extension cord will be used to keep water from freezing in winter).
Also, they will eat insulation so you need to cover it. If you cover it you have just created a safe place for Mommy Mouse to raise a family.
The water issues can be real but there are ways to get around that. My concern is more about being able to see inside before I blindly stick my hand in there. I'm not that worried about a hen being in there, I'm thinking more about how your heart really gets pumping when you grab a snake that's in there eating an egg.2. Top opening or front opening nest box door? There will be an external nest box on the left side of the coop. I was going to do a top door but am worried about water issues. My main concern about a front opening door is the hens will get out when my daughter collects the eggs.
Good question. My guess is probably but as Rosemarythyme said, cut a hole on the side under the run roof where rain and snow won't come in. Cover it with hardware cloth for predator protection. That will definitely be enough. This will let in a lot of natural light too so you can probably skip one window. My 8' x 12' coop only has one window but the ventilation openings let in a lot of light.3. Will the openings between the header above the door and roof (the spaces between the rafters) provide the correct amount of ventilation? The same openings are on the back (lower) side of the coop. I was planning on putting hardware mesh over them. Additional windows will be cut in but the question pertains more to winter when the windows will be shut.
The way I determine roost height is to figure the top of the floor, including bedding. Then install the nests. Then make the roosts noticeably higher than the nests or any other place you don't want them to sleep. If the roosts are directly over the nests 6" should be enough. If they are a bit away, figure at least 12".How high off the floor would you say for the roost bars?
I have two other considerations. As mentioned, the higher they are the more landing space they need so they can fly down without banging into things like nests, walls, or feeders. Also, speaking of ventilation, you do not want a cold wind hitting them on the roosts in winter so I want them to be low enough that they are below any vents open up high. The way you have yours they will be at the top of the opening for the nest, well above the bottom, so they should be OK. Just don't cut the ventilation opening under the run roof too low.
Personally I like mine higher than 26". I integrate a lot of chicks every year since I hatch and raise them to eat. When I integrate my younger chicks they try to avoid the older ones. One way they do that is when I go down there in the morning to let them out, I usually find the adults on the coop floor while the juveniles are on my 5' high roosts. They are high enough above the adults that the adults cannot peck them. It is a safe place for them.