We have approximately 1800 square feet of attic space. Ranch style 1-story home. We have triangle gable vents at each end and 3 whirly bird vents spaced across top of roof. Attic stays pretty hot from April through November. We are in the Mississippi Delta where outdoor temps reach 100 in the heat of the summer. Should we have soffit vents installed using the correct formula for the number of soffit vents required for the 1800 square feet? Are soffit vents okay since we already have gable vents and whirly birds?

  • How many square feet of vent space?
    – FreeMan
    Mar 28, 2022 at 23:14

1 Answer 1


Attic Ventilation shall be “cross ventilation “ and shall be 1/150th the area that is to be vented. (See ICC R806 Vents.) So your 1800 square feet of house requires 1800 / 150 = 12 square feet of vents, unless you can use one of the following exceptions, then it’s half that.

However, there is one exception: The attic ventilation can be reduced to 1/300th the area to be vented provided one of the following items are met:

  1. a Class I or II vapor retarder is installed on the “warm-in-winter” side of the ceiling, or

  2. not less than 40% and not more than 50% of the ventilation is located in the upper portion of the attic. The upper portion is defined as: “Upper ventilation shall not be lower than 3’ below the ridge or highest point of the roof. “ (See ICC R806.1.2.)

Also, the Code says you shall maintain a 1” clearance between attic insulation and framing.

So, you can see there is clearly a benefit to having ridge vents, gable vents, etc. , but in a proportion that allows COMPLETE CROSS-VENTILATION in both directions to the attic.

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