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The 50/50 Balance Rule

The 50/50 Balance Rule

Ventilation Basics from the Ventilation Experts

Ventilating metal roofs isn’t hard, but it is a science. Knowing how ventilation works and the math behind it will help you give your customers superior performance that will save energy and cut utility costs in the long run.

The ventilation experts at Marco have put together this quick primer as a reminder on ventilation basics:

Properly ventilated attics harness the chimney effect; the power of convection pulling in cooler air entering at the underside of the eave or near the eave by pushing warm, moist air up and out of the attic space at the ridge vent.

Properly ventilated attics also utilize the wind effect. This is where the wind flows over the ridge as it speeds up, creating a natural vacuum above the ridge which pulls hot, moist air out of the attic.

The mechanics of these two air movements require a proper amount of intake and exhaust for the system to work. The rule: balance your ventilation with 50% for intake and 50% for exhaust. Under no circumstances should the amount of exhaust ventilation exceed the amount of intake ventilation!

When figuring a job’s ventilation needs remember this: one square foot of vent area is needed for every 150 square feet of attic floor space. The bare minimum according to code is one square foot of vent area for every 300 square foot of attic floor space with a vapor retarder (please check with your local building official).

Once you have the ventilation needs calculated, make sure you take the same care in selecting your ventilation materials. Look for superior quality like Marco’s exclusive nonwoven polyester ventilation solutions that deliver long-lasting resiliency and breathability to help prevent moisture absorption that can lead to costly rework, repairs and high energy costs. You can download Marco’s ventilation brochure here.

To learn more about Marco, visit To contact Marco directly, you can email them here or call 1-800-800-8590.