SmartInsulationCalculator

Attic Ventilation & Baffle Calculator

Adding insulation without keeping the soffits clear cooks your roof deck. This gives the ventilation area your attic needs and the baffle count to protect it.

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How this calculator works

  • Net free area = attic floor area ÷ 150 (IRC R806.2); ÷ 300 only under the exception — balanced vents (40–50% exhaust within 3 ft of the ridge) plus, in climate zones 6–8, a Class I/II vapor retarder on the warm side. A vapor retarder alone does not qualify.
  • Split 50% intake at the soffits, 50% exhaust at the ridge
  • Baffles = soffit length in inches ÷ rafter spacing

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Frequently asked questions

Do I need baffles if I blow insulation in the attic?

Yes, anywhere insulation meets the soffit. Without baffles the loose fill blocks the intake vents and the whole ventilation path stops working.

Can an attic have too much ventilation?

Unbalanced ventilation is the real problem — more exhaust than intake pulls conditioned air up through ceiling leaks. Keep intake at least equal to exhaust.

Why do I need baffles if I am adding insulation?

Because insulation blown to the eaves blocks the soffit vents, and a vented attic with blocked intake cannot work. Baffles hold a channel open from the soffit up past the insulation so air can still enter. Skipping them is how an insulation upgrade produces ice dams and a damp attic the following winter.

How many baffles do I need?

One per rafter bay that has a soffit vent below it, along every vented eave. They are not something to space out — a bay without a baffle is a bay where the intake is blocked, regardless of how well the neighbouring bays breathe.

Do I need baffles with spray foam at the roofline?

Not in the same way. A sealed unvented roofline assembly deliberately has no soffit-to-ridge airflow, so there is nothing to keep open. This is a different roof design rather than a variation on the same one, and mixing the two — foam at the roofline plus vented soffits — gives you the disadvantages of both.

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Net free vent area required

1,152 sq in

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