Our temps have been anywhere from 20 - 40. I have them in my garage which is very drafty and still just as cold. And it is possible that I could be running them too long but I let the chicks tell me if they need the lamps or not. If they are in a pile and trying to get warm, I will turn on the heat lamps.
How many meaties did you raise?
Having 100 broilers to start with, 1 heat lamp didn't do the trick. I think if I had a small number of meaties, then I would not have had to use more than 1 heat lamp. I had 3 lamps going at one time. :/
What you see in those pics are 52 CX and 2 DP extras. One heat lamp. You don't have to heat the whole brooder, just one end and you can lower or raise that heat to get more or less. I wouldn't have added another heat lamp if I had double the chicks, I'd have just moved it a little further down my brooder and pointed it back towards the "warm" end to make for a larger area being warmed.
These were raised in a open air coop with just plywood on the top of the brooder but that was taken away after the first week.
I think next time you could get chicks in the early spring and save yourself the money of continuing the heat lamps for a longer period. Or just get them earlier in the fall so they would be done before temps got too cold. Getting them outdoors for exercise early in their life is also a way of keeping them warm....a moving bird is one that is generating heat for themselves. A bird that just stands in one spot or moves from the feeder and back is not doing enough to generate heat.
There are all sorts of ways to cut the cost of raising meat birds and I've been studying on them all and will continue to pare that down to a bare minimum. This spring I'm only doing 10 and they will be brooded under a heating pad brooder along with some DP chicks....minimal cost, and they will harden off quickly because they will be forced to go out in the cold to eat and drink and poop.