I prefer to let my broodies brood inside the main coop. Separating broodings makes reintegrating both the hen and chicks more complicated, difficult, and stressful. If you've got a big enough coop, a small crate is great spot for a broody to safely sit on eggs without being bothered by the rest of the flock, but maintains enough contact that they don't consider her a stranger.
And I agree that greenhouses are much too warm to keep chickens in.
Some people might think a greenhouse would be great for chicks, since it is so warm inside them. One giant brooder that maintains 90 degree temps may sound like a dream to some, but it's more of a death trap than a dream.