how cold is too cold

Hi. My Cornish cross and production red chicks are 4 1/2 weeks old. I have had them in a pen in my basement and I have to say, my house STINKS. I setup a pen in my barn but I am concerned that it is too cold for them out there, even with the red heat lamps.
The overnight lows are in the 30's-40's right now(in Connecticut) with a couple days having lows in the upper 20's.

My barn is not 100% airtight, but it gets very little if any drafts through it. I do have a wood stove out there, but it would never run all night long.
At what stage would these chicks be able to go into my non drafty barn under heat lamps?

Ween from heat and do something to stage down heat over 4 days or more to acclimate them to colder ambient temp. Basement is a good place as usually they are around 55F. If you've not turned off the heat lamp at all then start to turn it off for few hours and steadily increase time off. Once you get the birds to acclimated to a cooler area and no heat you can easily put them outside. You just don't want to take from heat lamp to outside all at once. Heck if you've electricity in barn and can really secure the lamp so no fire you can ween them from it there.

I'd be more concerned about predator protection out in the barn.

I don’t raise Cornish Cross, just dual purpose chickens, but mine go straight to the brooder in the coop straight out of the incubator or from the post office. I don’t know how many you have so I don’t know how big an area you would need to heat, but as long as they have a warm area to go to so they can warm up when they need to, the rest of the area can cool off as much as it wants. Most years I have chicks out there with outside temperatures below freezing. I’ve seen ice on the far end of my brooder some mornings, but the end they are in stays toasty.

I use heat lamps but others use other methods. The heating pad cave is really popular right now but I suspect you have a lot of chicks, it may be challenging getting a heating pad big enough. You might want to investigate a hover. There are some similarities. Think of a shallow box, turned over so it holds heat in since warm air rises. You can make those as big as you wish. Raise it just enough so the chicks can get under it. At that age you may find they spend more time on top of it than under it.

I’ve had chicks 5-1/2 weeks old go through overnight temperatures in the mid-20’s with no supplemental heat. They were raised in my brooder so they played enough in the cold end to get acclimated and were fully feathered out. The coop they were in had good breeze protection and great ventilation up high. Also there were several of them so they could huddle for warmth if they needed to.

You obviously have electricity out there, they could have been out there 4-1/2 weeks ago.

Good to know. The last thing I want is them to die. The reds are all feathered out at this point, or 90% anyways. The CX are 90% or more feathered out. I'm gonna set them up out there after work. They will have a heat lamp where they can easily stay warm so I suspect things will work out fine.

In Wisconsin and have CX about the same age in unheated garage under a brooder plate, last week I began shutting the plate off during the day. They huddled for a day or two but got over it and are fine. Plate comes on maybe an hour before the florescent lamp turns off just so it can warm up and they can figure out where to bed down.