Attic ventilation: Soffit Vents or O'Hagin Vents for Intake?

last month

last modified: last month

Hello, my attic ventilation consists of gable vents on the East/West faces of the house and a few bird block soffit vents on the South face. Bird blocks look like the photos shown here:

https://www.garagejournal.com/forum/threads/bird-block-soffit-vent-install.422278

According to the 1/150 rule, my house needs about 4 sqft NFVA (net-free ventilation area) for exhaust and 4 sqft NFVA for intake. For exhaust, I plan to add a set of O'Hagin vents on my cement tile roof and seal off my gable vents so they don't compete. This is the product:

https://ohagin.com/products/products-we-offer/best-vents-for-clay-tile-roofs

But for intake, the existing bird blocks provide only 0.5 sqft NFVA in total, and I'm unsure what is the best approach to take:

Cut out more of the soffit to boost intake NFVA to 4 sqft
Ignore the bird blocks and add the same number of O'Hagin intake vents as there are for exhaust. In this case, I will have a bit more intake than exhaust NFVA overall, and I've read that this is fine (whereas having more exhaust than intake is a problem).
Do some other combination.

Would really appreciate thoughts on this.

Thanks,

James

Featured Answer

3onthetree

last month

The best approach to attic ventilation is passive ventilation, low to high across the entire underside of each plane of roof. Clay tile over cleats, which is then over sheathing with roofing underlayment, is good for the sheathing and tile, but doesn't address the attic.

When you need to rethink attic ventilation, my first question is, what is the problem you are experiencing? Condensation or mold - find the culprit (too little ventilation, bath exhaust, insufficient insulation, no air sealing, blocked vents, etc). Just higher temps - attics can easily be around 20F-30F higher than outside temp, and that is perfectly fine.

O'Hagin vents were created to solve a problem. Namely, when you can't get enough low or high soffit venting and need an alternate location. The downside is they move more air in spots, not across the entire roof plane. There are many houses where gable vents might be adequately performing, each house is a bit different and applying a broad brush for optimal venting may not be necessary because of other details that do not work for the house.

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Comments (7)

Seabornman
last month
Is your clay tile roof over boards with gaps between them? There's ventilation right there. Id spend time and money on air sealing the ceiling.
James Lee thanked Seabornman
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James Lee
Original Author
last month
last modified: last month
I updated my post wording since I found out my tile is actually cement, not clay.

@Seabornman I don't believe there are gaps in my roof boards since I recall only seeing outside light coming in through the gable vents. I will definitely be air sealing though, thank you.

@3onthetree I will be replacing my insulation and air sealing as part of having a new hvac system and attic ductwork installed (only the ductwork is in the attic). My attic does get exceedingly hot in the summer, and now that I'm air sealing the floor of it, I'm thinking a lot more about its ventilation. I see that what I have is not very good: 6 bird blocks and 2 gable vents that depend on the wind but my area isn't particularly windy. For passive exhaust, the only options for me seem to be O'Hagin vents or a ridge vent. I was told ridge vents are more leak-prone so am avoiding them. For intake, is it that you recommend leaning more into soffit vents (i.e. cutting out more holes in the soffits) rather than adding O'Hagins? My understanding from what you wrote is that soffit vents are superior in that they span the length of the roof plane, and I reckon another advantage is that intake would be at the lowest point possible in the roof whereas O'Hagin intakes cannot be installed that low (only concern with bird blocks would be about insulation blocking them so I need to make sure to install soffit baffles).
John 9a
last month
last modified: last month
You said you are also re-insulating, so......if insulating the underside of the roof (spray foam) is an option, you would not need/want attic space venting to the outside. I just had a garage spray foamed and I, at first, assumed I would have the top of the ceiling (attic floor) insulated but was advised I would have better insulating, in my situation, with roof insulation.
James Lee thanked John 9a
James Lee
Original Author
last month
@John 9a That is a good point -no venting would be needed with that approach. For my house, it would not be an option at this time. Appreciate your comment.
3onthetree
last month
I am not aware of ridge vents being more leak prone that any other hole in a roof. Maybe with barrel roof tiles because of the detailing required, but not with flat tiles where they are just a cap on a prefab ridge vent, installed similar to an asphalt shingle.

More openings in the bird blocks would be a better solution for venting than a few low O'Hagins. The math calculation for NFA is somewhat arbitrary - you have to start with some known quantifiable method that would cover any roof layout - so just meeting those calcs doesn't guarantee the best venting, you want to understand the concept behind the math and what it's trying to accomplish.
James Lee thanked 3onthetree
James Lee
Original Author
last month
@3onthetree All excellent points, thank you. I'll definitely keep those in mind.