This rule is a common benchmark among HVAC professionals. The 1/300 rule is used to calculate just how much intake ventilation an attic needs in order to create a truly healthy home environment.
The general formula is simple: Take the square footage of area your attic covers and divide that by 300. That is the square feet of total ventilation you need. Now, you need to be sure that more than half of that is intake.
However, this is just the first step in determining true intake ventilation needs. Once the square footage is determined, there are additional measurements which must be taken. Specifically, the “net free intake rating” of the type of soffit vents existing within the attic.
Calculating this is a bit more intensive than the average homeowner is willing to handle, but the initial 1/300 Rule is fairly straightforward. To get a complete understanding of an attic’s intake ventilation needs, it is recommended that homeowners consult a certified HVAC professional for full measurements.