Inspectors, Do You Show Your Work? Attic Ventilation 101

Whirly Bird _ How Many

Warning :: This topic includes concentration, calculation, and brand-specific data that could be harmful if not used carefully. Enter at your own risk.

Comments and opinions that stretch beyond the Standards of Practice (SoP) for a Visual Home Inspection can land you in a pile of trouble. Don’t get me wrong here, I am not saying an inspector should not know more than the minimum, but when moving beyond that point, you should prepare yourself for that open “can of worms.” With that in mind, inspectors, do you make comments about the number of whirlybird vents, power attic vents, length of ridge vent, the net free area of a cornice vent, or other comments that would include creating complex calculations in an effort to defend your position/opinion? Are you stepping outside/beyond your state’s SoP?

Here’s an example of the needed data to produce a defensible opinion regarding attic ventilation as you consider a “net-free area.”

Net Free Area

Most codes use the 1/300 rule for minimum residential attic ventilation recommendations. This means that for every 300 square feet of enclosed attic space, 1 square foot of ventilation is required – with half the ventilation at the upper portion of the roof (used for exhaust) and the other half under the eaves (used for intake). The following formulas are traditionally used for static roof vents rated for Net Free Area in terms of square inches.

Example: Measure the roof’s Length x Width = (50’) x (30’) = 1500 sq. ft. (complex roofs are not this easy)

1500/300 = 5 sq. ft. of ventilation required >>> 2.5 sq. ft. for exhaust and 2.5 sq. ft. for intake.

Sq. ft. x 144 = sq. in., so (2.5) x (144) = 360 square inches are required for exhaust, and 360 square inches are required for intake.

Do you know the brand name of the installed attic fan? Not all attic fans are created equal. If so, how did you read the nameplate? Most attic fans are mounted too high to read from inside the attic, and most roofs are too steep to walk (but not impossible).

The Lomanco 750 has 50 square inches of free area, so eight (8) 750s would exceed the minimum required for exhaust. Eight 750s yield 400 sq. in. of free area, so the inspector should make sure that the soffit vents are equipped with at least 400 sq. in. of “free” area as used for intake.


If the actual volume of the attic is known, one may divide the volume in cubic feet by six. This will yield the CFM required to provide an air exchange ten times per hour. Again, one should always provide adequate free area in the soffit area (see above).

The volume of Standard Attic (all gable with no hips) = (1/2) x Length x Width x Height
Volume of Hipped Portion of Attic = (1/3) x (Total Length – Ridge Length) x Width x Height
The Height is calculated using the Pitch and the Width and the formula is: Height = Pitch x (some)Width.
(Hint: 4/12 Pitch = 0.33, 6/12 Pitch = 0.50 and 8/12 Pitch = 0.67)

Volume of Attic = (1/2) x Length x Width x Height = (1/2) x 50’ x 30’ x (2/3)15’ = (1/2) x 50’ x 30’ x 10’ = 7500 cu. ft.

Air exchange every six minutes = 7500 cu. Ft. 6 min. = 1250 cfm required.

Volume of Main Ridge = (1/2) x Ridge x Width x Height = (1/2) x 30’ x 30’ x 10’ = 4500 cu. ft.
Volume of Hip Areas = (1/3) x (Length – Ridge) x Width x Height = (1/3) x 20’ x 30’ x 10’ = 2000 cu. ft.
Total Volume of Attic = 6500 cu. ft. 6500 cu. ft. 6 min. = 1083 cfm required.

Do’s/Don’ts/Good Rules of Thumb

  • Lack of undereave soffit ventilation. This is the #1 reason for weather infiltration. There should be an equal or slightly greater amount of free area in the soffit regardless of the type of exhaust system used. Make sure that the soffit vents are not covered by insulation, light should be visible in the soffit area when one is standing in the attic.
  • Do not mix different exhaust products on a single home, i.e., do not use roof vents with ridge vents, ridge vents with power vents, turbines with roof vents, etc.
  • Avoid using ridge vents or roof vents near gable vents. Depending on the wind direction, the gable vents can act as exhaust vents and cause the ridge or roof vents to act as intake vents. If problems occur, make the gable louver non functional.
  • Keep roof vents at a single level on the roof. Do not use them high and low. The lower vents may try to act as intake.
  • Keep roof vents on the same side of the ridge. Do not place them across the ridge from one another. Depending on the wind conditions, one roof vent may try to feed the other roof vent.
  • Do not use roof louvers on the lower part of the roof for intake. Roof louvers are designed to be exhaust vents and may not offer the desired weather protection when being used as intake vents.
  • Avoid placing ridge vents or roof vents on dormers when the dormers are lower than the main ridge and connected to the main attic. If vents are put on lower dormers that are connected to the main attic, separate the dormer from the main attic and let the dormer be a “mini”attic.
  • If ridge vents are used on homes with multiple ridgeline heights, it may be desirable to separate the attic areas where the ridgelines change. This may be done with plastic sheeting or roofing felt.
  • Cut the hole(s) correctly. Holes that are cut too large can lead to weather infiltration. This is especially true for ridge vents since some of the internal baffling may be rendered ineffective.
  • If lanced or perforated soffit panels are used, the ones that provide maximum ventilation should be used. In most cases, the panels should be used continuously around the soffit area. Panels with holes typically have more free area than panels that are lanced. Also, some lanced panels are often not lanced cleanly and can cause more resistance to air flow.
  •  IMPORTANT: Attics Require 50 % Intake and 50 % Exhaust!

Call me today for a specific area inspection of your attic’s ventilation (or lack of it). 

Northtowne Builders, LLC – Now that’s a smart move!


Thanks to Lamanco for product specifics and calculations.

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One thought on “Inspectors, Do You Show Your Work? Attic Ventilation 101

  1. Dude! Awesome stuff. Please keep writing more things like this. I really like the fact you went so in depth on this and really explored the topic as much as you did. I read a lot of blogs but usually, it’s pretty shallow content. Thanks for upping the game here!

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