Attic Ventilation
by Richard
(Fernandina beach fla.)
Should roof/ridge vents span the entire length of the roof, and should there also be vents in the gables at each end of the roof? (Soffit ventilation is a given).
Answer from Green Energy Efficient Homes
The US Federal Housing Administration (FHA) recommends at least one square foot of ventilation openings for every 300 square feet of attic floor space. The ventilation should be divided between top and bottom vents. So for a house that is 20 x 30 feet square with a roof covering the entire area, you would have a 600 square foot attic floor space that requires 2 square feet of ventilation - one square foot at the top and one square foot at or near the bottom. A roof ridge vent provides about one eighth of a square foot of ventilation per linear foot. If the roof ridge of that 20x30 house is on the 20 foot dimension and the ridge vent runs the full length, then you would have 2.5 square feet of ventilation, which is more than twice what you need at the top. You'd then need an equivalent amount in the soffits or low down on the roof surface.
For your particular house you would need to calculate the attic floor area and the roof ridge length, then determine what length of the roof ridge needs venting. The warmer your climate, the more beneficial added venting is - remember that venting in cold weather when the heat is on can actually cool your house down. But given that you live in Florida you would probably benefit from more venting than the FHA recommendation, in which case roof vents spanning the entire length of the roof probably make sense.
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