This topic is going to get even more popular real soon and you’ll get all kinds of advice. There is no one right answer for everyone because or different conditions and how we have raised them.
The rule is they can go out when they have feathered out. What does that mean? Probably different things to different people and different times with different chicks. How fast a chick feathers out depends some on diet. The higher protein feed they eat, the faster they feather out. Another factor is how well they are acclimated. Chicks exposed to colder conditions feather out faster and are better able to face the cold that chicks that have been raised in tropical conditions. Most people say that is 4 to 5 weeks and for most people that’s probably pretty close. But it may be a bit early for some people.
Part of it too is what kind of conditions are they going to? If a direct breeze hits them, they can suffer. If they have decent draft protection they can handle quite a bit. It doesn’t hurt to have bedding to snuggle down in either. It also helps to have enough that they can keep each other warm if they need to.
In the heat of summer I’ve seen a broody hen take her chicks to the roost at two weeks. She could not cover all of them so some were exposed to overnight lows around the mid 70’s. They did great.
In our serious heat wave a couple of years back, I turned the daytime heat off at 2 days and the overnight heat off at 5 days. Overnight lows were probably in the lower 80’s but I didn’t measure that. The chicks’ actions told me they did not need the heat. They stayed as far from the heat source as they could.
In colder conditions I’ve put chicks in an unheated grow-out coop at 5 weeks. It had good draft protection and there were probably around 20 of them. It has a wire bottom so they had no bedding to snuggle in. The overnight lows were regularly in the mid 40’s. Before they hit 6 weeks we had an overnight low in the mid 20’s. They were fine.
My brooder is a 3’ x 6’ in the coop. It has great ventilation up high but draft protection down low. I heat one end and let the rest cool off as it will. Sometimes that is really cool. They play in that cool end a lot and go back to the heat when they need to warm up. In really cold weather like now I do block more of the ventilation. I’ve had chicks less than a week old in that brooder when the overnight lows were around 4 degrees Fahrenheit. Ventilation was pretty well blocked then.
At 8 weeks I can’t imagine your chicks not being feathered out enough for your overnight low of 30 to be any kind of problem in the coop you describe. You might want to move them in there during the day when it’s warmer a time or two to see how they react, but that’s more to boost your confidence than because of the chicks.
Good luck with it. It’s a good question and will be addressed a lot in the coming months. That’s fine. If people only asked questions that have never been asked before this site would dry up and die.