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Industry standard for proper attic ventilation recommends the following. Please check your local building code requirement before selecting which rule to apply.

 

"1 in 150" rule: For no vapor retarder type of attic insulation – no paper, plastic or aluminum layer between the attic floor and insulation layer. Recommends 1 sq. foot of ventilation for every 150 sq. feet of attic space divided 50 / 50 between the inlets and outlets.

"1 in 300" rule: For vapor retarder equipped attic insulation (for example fiberglass blankets/bats with paper facing), Recommends 1 sq. foot of ventilation for every 300 sq. feet of attic space divided 50 / 50 between the inlets and outlets.

 

Enter the total square footage of your attic to calculate the attic ventilation.

 

Attic Square Footage

 

Proper attic ventilation assumes a balance of airflow between inlet (soffit/eave) and outlet (ridge). An equal or greater amount of sq-in of net free area must exist at the soffit in relation to the ridge. Installers should ensure during installation that the soffits are not obstructed or else proper air flow will not exist.