I'm way more heartless than the others!
By the time mine were 5.5 weeks old it was them or me...and I was bigger! So on a halfway sunny day, April 1st in fact, out they went! I did put a heat lamp <shudder, did I just say that?>, and a wireless thermometer out there. That night I watched the temperature drop....25, then down to 18. I kept getting up to check on them. The little stinkers were nowhere near the heat lamp. They were snuggled beak to tail on the floor in front of the pop door. I don't even know how many times I got up, bundled up, and went out there that night. I went out in the morning dreading what I might see, and they were all bouncing around, eating, drinking, and wanting their pop door opened.
Next night, same story, except I only got up once. The third day I took out the heat lamp. If they weren't going to use it, I wasn't going to risk it - and that night it snowed. In fact, we got our last snowfall that year on June 6th. If I'd waited until it was "warm enough", they would been darn close laying eggs in the brooder! You've already started weaning yours off heat - I did the same thing that
@lazy gardener has suggested and opened a window in the small office room their brooder was in during the day. Huddle boxes are also a good idea, but you usually have to "teach" them to go into it.
If they've never been out there before, expect some behavior quirks. No chicken, old or young, likes change. They'll sit in a corner, huddled together and fuss like crazy until one or two get brave and break out of the pack to explore a bit. Huddling is done just as much for security as it is for warmth. And with no electricity out there, be prepared for a royal temper tantrum - x15 in your case - when the sun goes down. "Mom, there's stuff in here" "Mom, it's dark" "Mom, he's touching me!" "Mom, I want a drink of water"! Yeah, like that! You're already of the game since you've turned off the heat lamp....the dark won't scare them quite as badly but combine the dark with a new environment and there is bound to be some protest from them!
If I had to do it over again, I would have done a better job of acclimating them. In my case it worked out just fine. I had to harden myself to the notion that these little Divas who had taken over my hearth and home were chickens. And then I spent two weeks cleaning dust, thinking I was done, then finding more dust. <sigh> So glad I was attacked by the blinding light of common sense and brooded all subsequent chicks outside from the start!
You've got this!
Good luck 
