You can read more about this in my post here. If you have a very powerful fan, having some extra soffit vents will balance out any negative pressure created in the attic. In the case of an active ventilation system ie, when you have attic fans, the amount of air intake is regulated by the size of the fans. So having too many soffit vents doesn’t have any impact on passive attic ventilation.
Here the volume of the intaken air is the same as that of the exited air, there is no way the attic will develop a positive pressure due to the excess number of soffit vents. In a passive ventilation system, the cool air enters the attic through the soffit vents due to a negative pressure created inside the attic by the wind blowing over the exhaust vents. Having too much intake ventilation is not a problem it is having too many exhaust vents that is the problem. Yes, you can have too many soffit vents as long as they are divided equally between both sides of the roof. If you are using 1/4 inch mesh screens with rain louvers then the vent you cut on the soffit should have an area of 2 x 6 sq.ft = 12 sq.ft.
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In one of our above calculations, we found that the total net free area of soffit vents for a 1500 sq.ft attic space is 6 sq.ft. To know the total size of the soffit vent to cut, multiply the net free area calculated with the corresponding multiplier of the soffit cover from the above table. So here is a size guide I saw on the website which I find very useful Soffit cover typeĬontinuous soffit with 1/4 inch ventilation holesĬontinuous soffit with 1/8 inch ventilation holes
In many cases, the net free area of a vent cover is not specified. This effective area is the net free area, and it changes with the type of vent cover you use. So if you cut a 3 sq.ft vent on your soffit and use a vent cover over it, the effective area through which the air can flow will be much lesser. These meshes obstruct a portion of the airflow. Soffit vents are covered by vent covers which are basically holes/meshes drilled on a cover. The net free area of a vent is the area available for the air to pass. To find the number of soffit vents you need, divide the total soffit vent area of the house by the ‘Net Free Area’ of the soffit vent cover. Soffit vents are covered with ‘covers’ to prevent insects and such things from entering the attic.
The total soffit vent area should be equally divided between the soffits on both sides of the roof, and they should be spaced equally. Soffit vent area = (60/100) x (1/300) x Total attic floor area = (1/300) x Total attic floor area.Įxample: If the total attic space is 1500 sq.ft in size then the total soffit ventilation required is 3 sq.ft. If the above two conditions are met, then our formula for soffit vent calculation changes to 40 to 50% of the vent area is located not less than 3ft from the ridges, and the rest of the vents should be located at the bottom one-third of the attic space.The roof should use a vapor retarder in its warm-in-winter side.So if you have an attic of 1500 sq.ft then the soffits will take up 6 sq.ft of net free area.īut, the IRC also maintain that the attic ventilation needed is only 1/300th of the attic space if both of the two conditions below are met Ie, Soffit ventilation = (60/100) x (1/150) x Total attic area = (1/250) x Total attic area. So in this case the soffit ventilation area would be 60% of the 1/150 sq.ft of the total attic area. However, in practice, many experts recommend using 60% of the total net free area for intake ventilation. Since soffit vents are intake vents and since most houses use them, let’s assign this 50% to soffit vents. Soffit vent size calculation (Why it is not 1 sq.ft per 150)Īs per the International Residential Code, a ventilated attic needs a net free vent area of 1/150 sq.ft of the total attic area.Įxample: If an attic is 1500 sq.ft in size then it needs a net free vent area of 10 sq.ft.Īs per the code, this net area should be divided equally between intake vents and exhaust vents. Can soffit vents be used on a flat roof?.Soffit vent size calculation (Why it is not 1 sq.ft per 150).
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