Heating the Coop: When is a layer chick fully feathered and able to stand cold?

Chicks are fully feathered at 6 weeks of age.
If you've been heating them up, then you'll want to wean them off gradually since you've probably hindered them from growing nice thick down feathers to keep themselves warm. (If they're warm, they have no need to grow the extra feathers.)
Do not use the heat during the day and keep the heat on the cooler side at night for about a week gradually making it cooler.
Allow them to develop naturally and they'll be fine.
You could also just withdraw the heat altogether and watch their behavior at night.
Give them a nice thick deep bedding to snuggle in.
Either way, they'll be fine.
In the future do not provide heat past 6 weeks of age and begin weaning them off heat at 4.5-5 weeks old.
Good luck!

 

Yours are probably ready. I keep my 3' x 6' brooder in the coop. It has good draft protection but is well ventilated and I only heat one small area. The rest cools off as it will. They play in the colder areas and go back to the heat when they want to so they can warm up. We talk about them finding their own comfort zone but mine find their comfort zone all over that brooder.

Last fall I stopped the heat at 5 weeks and put them out in the unheated grow-out coop. It has good draft protection but no heat of any kind and is also well-ventilated up high. The night-time lows were hitting the mid 40's Fahrenheit when I put them out. A few days later, when they were 5-1/2 weeks old, the night-time lows hit the mid 20's.

I had 21 chicks so there were enough to keep each other warm. They had been used to cold temperatures. I think that helps them feather out. They had good draft protection and good ventilation, both of which I think are important.

From what you have described, I think yours are ready.