Seeings you are in New England, I'll suggest that you do not button up to snug as our birds have MG, aka to folks, a cold, and lack of air exchange will make them sick. Build up of humidity and amonia can cause us damage...
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To be honest, I have a 4x8 coop for 5 hens, a storm window that is not covered and the only insulation we placed on it was tar paper on the inside of the shed and shingles on the exterior wall. I also allow my girls out in the pen on 'warmer' days. If the sun is shinning, they get to go out around noon to 2, then place them inside to re-warm their surroundings. The only insulation is the cardboard on the ceiling to keep their warms down.
As long as your perches are wide enough so they can keep that middle toe warm and under your feathers all should be well. In winter we do use a 40 watt bulb in the evening to gain a bit of egg production and a heated waterer.
Personally I feel that the folks that feel they need heaters in their coops set their birds up for failure when the power goes out. It seems to do that a lot where I am, if I heated the coop, then they would not be physically prepared, and can die.
So use care when your deciding to do the extras for your girls. Not that you shouldn't, but being cautious in Maine winters is how we keep our critters alive!
The one thing I do, is use fat & finish, or higher protein/fat content food, give my 5 a couple cups of this in the mornings with 'hot' water and they eat it up like oatmeal! It seems to keep them insulated, er, energy enough to give them warmth.
I do not believe that corn as many say makes them any more warm then regular food. Perhaps its takes more energy to grind it down, but until I hear old timers say that, mine still get corn summer/winter, spring/fall!
Good luck and build your coop like you want!