This is my brooder in the coop, pretty much set up for your temperatures. When It is below freezing when I brood out here I wrap the whole thing in plastic. That chimney to the left where the heat lamp goes still provides good ventilation. When it is below freezing I use two heat lamps for redundancy, otherwise just one. In the heat of summer I use a lower wattage heat lamp. Some people use heat plates or heating pads. I think I call what Canoe is talking about a "hover". Lots of different ways to do this.
To me the biggest issue with brooding outside is the temperature swings. I've seen it go from the mid 20's F to over 70 F within 36 hours. The brooder needs to have one spot warm enough in the coldest temperatures and a spot cool enough in the warmest temperatures. I achieve that with ventilation and distance. This brooder is 3' x 6'. I've seen frost in the far end but the end the chicks were on was toasty.
If you can show us what your coop will look like we might be able to give you specific suggestions. It would help to know how you would provide the heat, heat lamp, pad, plate, whatever. There are little tricks for each that can make them easier or safer.
To me the biggest issue with brooding outside is the temperature swings. I've seen it go from the mid 20's F to over 70 F within 36 hours. The brooder needs to have one spot warm enough in the coldest temperatures and a spot cool enough in the warmest temperatures. I achieve that with ventilation and distance. This brooder is 3' x 6'. I've seen frost in the far end but the end the chicks were on was toasty.
If you can show us what your coop will look like we might be able to give you specific suggestions. It would help to know how you would provide the heat, heat lamp, pad, plate, whatever. There are little tricks for each that can make them easier or safer.