Natural Ventilation
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Natural ventilation occurs when air enters soffit vents, travels unobstructed along the underside of the roof deck, and exits through a ridge vent, gable vent, and/or hat vent. Mechanical ventilation that is temperature-controlled is also effective at reducing heat buildup inside an unconditioned attic space. Proper natural ventilation can be achieved by following the U.S. Department of Housing and Urban Development (HUD) "Minimum Property Standards" (MPS 403-3):
As a general rule, one square foot of net free vent area per 300 square feet of attic floor or area to be vented is recommended.
In the rare situation where no vapor retarder is used and proper distribution of under eave and ridge vents cannot be achieved, one square foot of net free vent area should be provided for each 150 square feet of attic floor or area to be vented.
For a balanced system, ventilation should be equal at the under eave (intake) and ridge (exhaust).
In cases where a balanced system cannot be achieved, always provide more than 50% of the total required ventilation at the under eave and the remainder at the upper portion of the roof.
Natural Ventilation is an exert from What to Do about Ice Dams by Jeff Calcamuggio
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