Now here comes the true purpose for a balanced ventilation system in your attic. Once this moisture rises into the open attic, there needs to be adequate exhaust ventilation at the ridge as well as adequate intake ventilation at the soffit or bird blocks. Bird blocks are the blocks between the rafters where they rest on the plate of the exterior wall of the building. These bird blocks are usually 2x4’s of wood which space the rafters 2 feet apart. These blocks are either blank or have holes with screen covering the holes to keep birds and bugs out but allow air to flow in as heat rises out of the exhaust vents or ridge vent.  The formula is 18 inches of net free air (NFA) per running foot of ridge to exit at the ridge vent, or can vents, and 9 inches of NFA of intake air per side of the house times 2 sides equals 18 inches of intake air to match the 18 inches at the ridge exhaust. This is a balanced system.

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