Attic ventilation requirement

I personally believe that the more ventilation, the better, but is there a certain ventilation requirement for attics, like a certain number of vents, or a certain number of square feet of ventilation per a certain number of square feet of attic? Thanks.

Typically you would like to see 1/300 total vents per attic floor space. For vaulted its 1/150 per sq ft roof area


The 1/300 is the number that applies to me for the report I’m working on.

However, can you expand on the “vaulted” requirement. I’ve never even found an attic when there are vaulted ceilings. Or are we using different terminology?

For a vaulted ceiling you will still have soffit vents and at least roof vents. And no there is no Attic but vents are visible on the roof and soffit.

Not here.

Maybe it’s our desert climate?

R806.2 …shall not be less than 1 to 150 of the area of the space…reduced to 1 to 300 in certain circumstances.

Hey Russell,

If you would like to add some WEB references to your list the following is a very good site:

AirVent will also happily send you free attic ventilation booklets, etc. you may use for clients. They will also provide you with some handy little ventilation slide calculators for those days when the brain goes to mush. And, if you ask nicely (they’re good people so ask nicely) they will even send you mock-ups of ventilation products as they are used. These come as neat little ridge vent sections on mock roof peaks, soffit vents on mock soffit areas. They make nice conversational and demonstration pieces if you ever need to explain to a client or give a presentation about attic ventilation.

By the way, AirVent also has a series of seminars they run around the country regarding attic ventilation. They are generally half day deals with lunch provided. They have very good presentations and they bring engineers with them. Makes for a very informative afternoon and you get to meet some of the local roofing/other professionals from your area.

whats 1/300 mean?

1 square foot of vent for every 300 square feet of attic.:wink:

Thanks, Manny.

am i “Manny”?

thanks Jay


In my area they require 1 sq. ft of free ventilation for every 150 sq ft of attic space. This can be resuced to 1 sq ft of free ventilation for every 300 sq ft of attic space IF there is soffit ventilation.


Interesting. So soffit ventilation isn’t normally considered “free ventilation.” What exactly is “free ventilation”? I’ve never heard the term before.

I believe the free ventilation refers to the total amount of ventilation required. You would then divide that number by 2 to get the amount of square feet for intake vents and the same for the exhaust.

Here’s an example of how I word it:

"The attic space is considered to have inadequate ventilation. Only two gable vents provide ventilation for the structure. For venting to be effective the air has to be able to move. It is recommended that the structure be ventilated with both: A) An intake preferably located as close to the eaves as possible. (Most often this is done through soffit vents. Ensure the soffit vents are not obstructed by insulation.) B) An exhaust preferably located as close to the ridge as possible.
The total net free ventilating area should not be less than 1 to 150 of the area of the space ventilated except that the total area is permitted to be reduced to 1 to 300…” provided mechanical ventilators are used. When only gable vents are used, effectiveness is more determined by wind movement. Area calculations were not conducted; it is recommended that a state licensed general contractor evaluate the adequacy of attic ventilation and add ventilation as deemed necessary. Reference: International Residential Code R806.1/R806.2"

Very nice, Joe. Very, very nice.

Any addition of attic exhaust fans should be followed up by recommending that the draft of all natural vent equipment be tested with a draft meter. Addition of any exhaust motors can cause back drafting.

Also note that the vent area is NET AREA. That is the opening of the vent, less the mesh or screen restriction. The vent Mfg lists this info.