Quick question regarding venting of my attic. I have two gable vents and a ridge vent, but home inspection recommends soffit vents be installed. I've calculated how many soffit vents needed, without gable vents. However, I wanted to know if having gable vents will affect the number of soffit vents needed? Thanks for your help!
Your attic ventilation should be a loop from bottom to top, so that convection and draft work with the system. Soffit venting should typically be at least a third of the total area, which is why you often see alternating perforated panels in aluminum soffits. In short overhang scenarios I've used full venting.
You should consider closing your gable vents, assuming you have adequate ridge vents. The gable vents short-circuit the loop and can result in dead zones.
Basically it's like this: The airflow and natural thermal lifting in your attic result in outflow at the ridge. That creates lower pressure in the attic, which must be accommodated. You want it accommodated by the soffit vents, not the gable vents, so that you have a continuous flow across the entire attic area.
Attic Ventilation shall be “cross ventilation “ and shall be 1/150th the area that is to be vented. (See ICC R806 Vents.)
However, there is one exception: The attic ventilation can be reduced to 1/300th the area to be vented provided one of the following items are met:
1) a Class I or II vapor retarder is installed on the “warm-in-winter” side of the ceiling, or
2) not less than 40% and not more than 50% of the ventilation is located in the upper portion of the attic. The upper portion is defined as: “Upper ventilation shall not be lower than 3’ below the ridge or highest point of the roof. “ (See ICC R806.1.2.)
Also, the Code says you’ll maintain a 1” clearance between attic insulation and framing.
So, you can see there is clearly a benefit to having ridge vents, gable vents, etc. , but in a proportion that allows COMPLETE CROSS-VENTILATION to the attic.