I've got two on my greenhouse roof and could'nt do without them. The sun shines on the greenhouse first thing in the morning and one of them is usually open if i am not up early.
I also have double doors which also help the flow of air.

__________________

And when you're back stops aching,
And you're hands begin to harden.
You will find yourself a partner,
In the glory of the garden.

Rudyard Kipling.