thought i'd add my 2 cents ...
i *try* to be logical about my chickens, i only have 6 girls but i'm fortunate enough to have a large coop for them; 8x8x10.
i only have the roof area insulated, not the walls. i figured most of the heat would be lost through the roof, and when it came to retro-outfitting the coop insulating the roof was more than enough work to make me NOT want to do the walls.
it's 99% draft free, and the 1% that has a draft provides decent ventilation. i've NEVER had moisture problems, even in the heat of august. bone dry in thar.
it's a peaked roof. the roost is set up so it's above the insulation line.
i have a heater i bought for $30 at
tractor supply suspended from the ceiling. it's set to turn on & off around 25-35 degree's.
do the chickens need heat? no.
do i feel better knowing they're not freezing their tailfeathers off? yes
does my electric bill go up? a little bit
last year the heater rarely turned on. the insulated area held the 30 degree temps nicely, and my chickens remained *gasp!* healthy.
heating to a balmy 70degree's is impractical.
heating to keep the deep freeze at bay is sensible. keeps the water & eggs from freezing instantly and all my girls still have their combs. and i personally don't see anything wrong with that.
do what you feel is right for your situation.