Hello,
We have a 4000 sq ft roof, and the original builder installed six Ross 150 passive vents (the big turtle or mushroom shaped things) on the back side of the house, but the roof was setup for ridge vent installation.
Question is whether ridge vents will provide more ventilation than those Ross 150s? Is it s bad idea to have both installed?
Thanks.
Ridge vent will not work properly if turtle are installed. You would need to remove the turtle vents for the ridge vent to do its job.
With out seeing the house or a diagram of it you can’t determine what ventilation you need. You need to look at the length of the ridge and brand of ridge vent, different products provide different air flow, and compare it to the combined total of the existing vents you have.
If your not having any humidity or extreme heat issues in your attic, your ventilation is doing its job.
Those are 150 sq in nfa(net free area) vents that your home has, so you have 900sq in nfa. Premium ridge vent would have around 18 to 20sq in nfa per foot. So if you had over 45’ of ridge vent you could have more ventilation.
Thanks for the answer.
It seems like I have about 65’ of ridge vent installed, but I noticed that about 15’ of that doesn’t have the opening cut on both sides of the ridge board, if that reduces the effective ventilation to 1/2 of length to 7.5’, for total of ~58’.
Either way, going by your answer, it sounds like the ventilation is not worse but might have just ended up being about the same for more money(?). 
And thanks for your answer MPA, I missed it earlier. I have GAF Cobra 3 ridge vent, if that matters.
I think I misunderstood, did you roofers install both the vents and ridge vent? That would be a bad idea.
No, they removed the Ross 150s and added GAF Cobra 3 ridge vents.
I caused the confusion when I separately wondered if you could have both for more ventilation but it sounds Iike they would conflict with each other.
I see, so in that case your fine. Ridge vent may be more common in your area and they’re just doing whats normal for your area. It is more attractive to some people, but ofcourse aesthetics are subjective.
Ridge vent is so uncommon here, I used some on a new build house and the owners freaked out and asked why I didn’t put any vents in.
You should be fine with the old vents removed and 58+’ of cobra 3. That is a good ridge vent opposed to the roll type. Sounds like your roofer did you right.
We commonly switch old turtle style vents out for ridge vent, providing the ridge is long enough.
Thanks. I measured the distance again and it seems I was wrong, it actually comes out to be about 65’ of Cobra 3, so it sounds good.
With my attic floor square space, GAF’s ventilation calculator says I need 60’ (1/300 rule) of Cobra 3 for effective ventilation, so it comes in nearly exactly with their calculation (a little more, being OK, I assume).
Cobra 3 provides 18 sq in NFA per foot, so that would be 1,170 vs. 900 before (unless I am misunderstanding something here).
I appreciate all the information, and I strive to expand my knowledge about anything relating to the home!