How do you measure whether your attic has enough ventilation?

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Measure the attic floor area, add up the net free area of every intake vent and every exhaust vent, and compare the totals to what the code calls for: 1/300 of the floor area when intake and exhaust are balanced, or 1/150 when they are not. If either side falls short, or exhaust is larger than intake, the attic does not have enough working ventilation. Step 1: the attic floor area Use the footprint of the attic, which is usually the footprint of the house under that roof, not the roof area. A 40 by 50 foot ranch has about 2,000 square feet. Treat separate attic spaces, such as one over a garage behind a fire wall, separately. Step 2: inventory the vents Ridge vent: open slot length times the vent's rating per foot. Box vents: count times each vent's rating, often around 50 square inches. Soffit: the panel's or vent's rating times its length or count. Gable louvers: rated area, often smaller than the louver looks because of slats and screen. Ratings are on the vent or its packaging. If they are not visible, a roofer can identify the product or estimate conservatively. Step 3: compare For 2,000 square feet on the balanced path, you need about 960 square inches total, at least 480 of it intake and about 480 of exhaust within 3 feet of the ridge. If all vents are high or all are low, the requirement doubles. Step 4: subtract what is blocked A vented soffit panel over solid plywood counts as zero. So does a soffit bay packed with insulation. Ventilation on paper means nothing if the path is closed. The ratios are explained in roof ven

How do you measure whether your attic has enough ventilation?

Step 1: the attic floor area

Step 2: inventory the vents

Step 3: compare

Step 4: subtract what is blocked

In depth

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