spacing soffit vent panels
I don't know of any code but for your shingle warranty sake I would balance the airflow out. Read this it's from the Alcoa vinyl company's website. Pay attention to the last sentence if anything.
Determining Your Ventilation Requirements
When determining soffit ventilation options, your choices are many. Each has its own net-free area. Net Free Area Defined: The net free vent area is the actual, unobstructed area where air can freely flow from outside, to inside, to outside. For every 750 cfm of airflow, it is recommended that there be a minimum of 1 sq. ft. of attic venting, i.e. roof vents, gable vents, soffit vents, ridge vent. This allows the air flow of the fan's exhaust air into the attic and out of the home freely.
To calculate your net free area, add up your total number of vents in feet. If your vent(s) are screened take approximately 1/2 away from your total. For example:
You have four 1 sq. ft vents, and they are screened. You would have a 4 sq ft gross of venting, however since the screens will block air flow, you would need to subtract approximately 2 sq. ft of venting from your gross, giving you a 2 sq. ft net free area.
The importance of a properly balanced ventilation system is hard to exaggerate. Experts say inadequate intake ventilation causes 95% of all ventilation problems and will typically void the shingle warranty.
So what is balanced ventilation? Balanced ventilation can be defined as equal amounts of air per foot coming in through the continuous soffit vents and going out at the ridge vent. More air coming in at the soffit will allow the ridge vent to do a better job of removing hot air at the peak. If the ridge vent has more air flow than the soffit, the ridge vent could back draft and pull hot air, debris, rain, etc into the attic rather than expel as designed. More air flow will create more air exchanges which will serve to lower attic temperatures, akin to lowering car windows or opening the front and back door of a house, creating a strong pull-through of ventilation.
So how does this stack up in the real world? An example of a simple gable design home with a 40' ridge: ridge vent net free area = 40ft x 18sqin/'nfa = 720 sq.in of net free area; therefore, you need at least 720 sq.in of net free area coming in at the soffit. Soffit vent net free area = 40ft(2) x 9sqin/' NFA = 720 sq .in. of net free area, so the system is balanced.
What's wrong with this picture? This assumes an ideal world where no insulation blocks the air of the soffit vent and you can run the soffit vent the full length of all available horizontal soffit areas. Builders typically break up the front of the home with design features, leaving maybe half of the front or less available to ventilate
In this situation, as long as half of the front could be ventilated, you can still balance with a 12.7sqin/' NFA continuous vent; the calculation would be (40 + 20) x 12.7 = 762 sq.in of net free area. Having more air flow at the soffit than ridge creates no problems. These net free area calculations are based on .030 vent material thickness as standard. Be careful with products providing airflow resistance inherent in product designs. We recommend running ridge and continuous soffit vents from end to end in all available horizontal areas for best aesthetics and ventilation .
Last edited by James Con; 10-29-2008 at 12:20 AM.
Reason: added info