bpratt
Saint Mike
Costs might be another useful item ...
Not just to you Mike, but for others reading and the OP.
A basic whirly bird goes for around $60 or so at the big hardware shops, the solar powered ones start at around $250 at some of the big hardware shops.
They go up to around $800 for the more powerful versions.
I guess with the smallest solar powered one equalling 12 times the volume of a standard whirly bird, to get the same volume of air moved with standard whirly birds would cost you nearly $720 for 12 whirly birds.
12 whirly birds would look pretty ugly on your roof though, which is why people only put one or two up there.
The long and short of it is if you want to get good airflow through your roof that you will need to ensure you have plenty of good inlets and outlets to move plenty of air.
Thanks, that is a good way of comparing the two options. I'm going to ask Metricon what they will slug me for the solar option.