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Question DetailsAsked on 7/23/2012

hi! I am getting different ventilation options from the roofers ( ridge vents, o'hagins to electric fan). Which do you recommend?

I have a attic volume of about 3400 square foot. A loud attic fan. Hip roof and some eve vents ( about every 8 feet). I live in California and the summers get really hot.

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3 Answers


I have no idea what an ohagin is. Perhaps you can explain?

There is no such thing as a one size fits all attic ventilation system. Therefore I am very pleased that you gave us some details about the roof/attic space.

When you say that you have 3,400 square foot of volume that is a little confusing to me. In my mind volume would be cubic foot, not square foot. However can I assume you meant 3,400 square foot (at the foot print)?

Ok I will assume 3,400 square feet foot print of attic space, not cubic, not volume, but simply square footage.... According to Air Vent, Inc. you would need one of the following MINIMUM:

91 linear foot of Shingle Vent II ridge vent

or 11 mushroom breather vents


30 16"x8" soffit intake vents or 182 linear foot of continuous soffit intake vent.

If you wanted attic fans then you would want at least two fans, 1100 CFM minimum each


39 16"x8" soffit vents or 240' continuous intake.

Note: if you choose a fan, I would also highly suggest each fan be wired with a humidistat which will allow the fan to operate when it is moist, not just when it is hot.

Note the size of the intake vents matters. Also there is something called Net Free Area (NFA) that all vents have a rating. A cheap chitty ridge vent may have less NFA than the Shingle Vent II mentioned above.

If your roof is steeper than a 6/12 pitch you would need more than what is listed above.

Do not mix and match ventilation systems. If you choose to go with a ridge vent, you must remove your fan. When in doubt, mushroom vents are a great option. My rule of thumb is ridge vent is the best, but is finicky so only with equal or greater intake and seldom ever on hip roofs.

I like solar fans, and if you choose solar fans you'd need 1 extra.


Answered 8 years ago by ReliableAmericanRoof


So simple answers,

A ridge vent is requires no energy,like a fan (here runs about $10 a foot) & must have adequate soffit ventilation to feed it. Air like water takes the path of least resistance, so no mixing of systems,,Use roofers math on # of new soffit vents you need installed.

I like running out vent to edge of roof in non hip roof installation, looks cleaner. You must feed the ventilation system you install with outside air or you run risk of pulling conditioned air from inside the home. Fyi venting your attic is necessary year round (in the winter it gets the moisture out of your insulation allowing it to function at a higher efficiency).

If you are do it selfer I suggest installing a radiant barrier on the inside of a non finished attic ONLY on the attic rafters (not on top of the insulation).

I experienced a 50% reduction in a/c costs in my home with it last year.

Jim Casper

Old Energy Conservation Contractor

(not a roofer)


Answered 8 years ago by jccasper


Thanks for your input!! It brought up realy important stuff! The area of my roof is 2520square feet. ( sorry i calcualted wrong). I calculated I need 2400 square inch of ventilation. My soffit venting is 1008 square inch. i don't have a moisture barrier between the ceiling and the insulation. I have a hip roof of 24 feet, which is not enought for a ridge vent because of the lack of moisture barier. Is that right? Passive venting is my first choice but I don't know if it is possible. Is it? Or should I go with solar fans ( thanks for sugesting that company)?

:-) anjali

Answered 8 years ago by Ang. B

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