brooder in coop

I copied this from the raising chicks section, but thought I would post here just in case.

I was hoping you could give me some ideas or advice.

We made a coop, obviously for the grown chickens in mind. It has windows on the south side to let in heat/sun in the winter. Well we made a section for a brooder. I want to get our 40 chicks outside of the house and into the coop brooder, but I am afraid of the changing temps outside. I have been testing the temps out there and with the heat light, I was able to keep the floor area 80 degrees when the out side was high 30s-low 40s. What I am afraid of, is around noon ish when the sun is out, it will natrualy get up to 80 degrees, and with the heat light on, I would probly end up with fried lil chicken wings. It would not be bad if I was home all day, but I am not always. So around 10 am, the heat in there could be fine, but if I leave, and the sun comes on it, it would be too hot, and I would not be around to adjust the temp. I would have to shut the light off, and open the vents to keep it from getting to hot. These chicks are about 2 weeks old. I put my 5 week olds out there for last night, and it stayed 70-80ish. When they get older, I can just keep the vents more open when its warm, and they can go out side when they want, but if I keep the window open now, they will freeze.
How do you all regulate an outside brooder from the changing heat?
Maybe I should use some reflective insulation on the window and on top of the brooder and try to keep the sun off of it, and try to keep the brooder cooler from the sun, but warm from just the heat light?
Also,
How warm is to warm for chicks? or even older chicks?
For the summer, I am gong to put some reflective insulation on all the southern windows to keep the sun out, and keep screened in windows open on the northern side and open vents on the sides.

Thanks!