How to Use This Tool
Enter the attic floor area and what venting you already have. The total is easy arithmetic; the balance between intake and exhaust is the part that decides whether the ventilation works or backfires.
The quantity rule
Net free area equals attic floor area divided by 300 where there is a vapour retarder on the warm side and the venting is balanced, or divided by 150 where there is not. For a 1,500 sq ft attic that is 5 sq ft or 10 sq ft respectively — 720 or 1,440 square inches.
1,500 sq ft attic, 1/300 rule total NFA 1500 / 300 = 5.00 sq ft = 720 sq in intake 60% = 432 sq in exhaust 40% = 288 sq in soffit vent at 9 sq in per ft -> 48 ft needed ridge vent at 18 sq in per ft -> 16 ft needed
Why exhaust without intake is worse than nothing
A ridge or roof vent is a hole that air leaves through. It cannot create air; it can only pull it from wherever the path of least resistance leads. With open soffits, that is outside air entering low and sweeping the underside of the roof deck, which is exactly what you want.
With the soffits blocked, the nearest available supply is the heated house below, drawn up through the attic hatch, recessed light fittings, plumbing penetrations and every gap in the ceiling plane. You are then exhausting air you paid to heat and pushing household humidity into a cold attic, where it condenses on the sheathing.
So adding roof vents to a house with no working intake makes both the heating bill and the moisture problem worse. Intake first, always.
Insulation is the usual culprit
The most common way intake disappears is an insulation upgrade that packs the eaves solid. Baffles exist to hold a clear channel from the soffit up over the top plate, and a house that gained insulation and then started forming ice dams very often lost its ventilation on the same afternoon.
Net free area is not the size of the hole
Louvres and insect screening block a large share of a vent's opening. A 16 x 8 in gable vent measures 128 square inches but might provide only around 56 of net free area. The real figure is printed on the product or in its data sheet, and estimating it from the outside dimensions overstates it by roughly half.
Do not mix exhaust types
Ridge vents and powered or static roof vents on the same roof plane can short-circuit each other: the roof vent becomes an intake for the ridge vent, ventilating a small area vigorously and the rest not at all. Choose one exhaust type per roof plane.
