Quote:
A draft is when air goes where you don't want it.
Ventilation is a good draft. Heat rises, this is why houses have ventilation in the attic, through the roof or gable ends, otherwise nasty things can happen like condensation at one end of the spectrum or overheating at the other.
You don't want drafts flowing onto the chickens so generally ventilation holes would be at a point higher than the birds' heads. It doesn't have to be a huge number of holes, and you can have a temporary covering for them in case of blizzard.
Leaving the entrance hole at the bottom open could be an invitation to predators and it does nothing to dissipate heat at the top. Plus, to have good ventilation you need you need 2 openings for air flow.
I live in northern MI, it can get to -20 at night. My coop is not insulated, it will have a heated water dish. The only time I will turn on the heat lamp is if the interior of the coop gets VERY cold, below 20 or perhaps 10 (this is my first winter so I'm experimenting). I'm getting a thermometer so I can tell when this happens.
PS - There's an article in a previous month of Backyard Poultry about using a slow cooker base for a water heater.