Yup, warmth is one of the biggest things to have.....I also tend to add some vitamin B to the water supply, this seems to help stop the leg problems...I believe that when they are shipped, especially this time of year, it depleats the B levels....
Originally, I had 100 birds in one of my foaling stalls, but, since then I converted one of the smaller hay barns into a growing area.....it is approx. 14 X 14.....I installed an electric heater, 1500 watts and I also have heat lamps for when I bring in the chicks....We have insulated the walls and ceiling....I will be adding windows fairly soon for natural light and ventilation, there is only a small area that they can have outside....but, as mentioned above, the Cornish Crosses, pretty much eat, sleep and poop.....
When I got the last batch the heat was set at 30 degrees C., with 3 heat lamps, and at week 3, I have them at 20 degrees C. and they are really warm and toasty....
Happy to say, I'm not having mortality problems, like we did with the first 2 batches.....So fingers crossed that we are on track....
Good luck with your building...