Honestly, they might be too hot in there with a heat lamp in that small space. You don't need a heat lamp, especially at 45 degrees. 10 month old chickens have plenty of feathers to keep themselves warm under a shelter.
I agree. Hens and roosters are actually comfortable in 45 degree weather without a heat lamp. Ever worn a down coat? Imagine if you could only shed it once or twice a year. My flock went out this morning in 37 degree weather and only went back to the coop to lay, drink or see if I had a treat when I went to clean.