Balanced attic ventilation is a design principle stating that intake ventilation, usually at the soffit, should roughly match exhaust ventilation, usually at the ridge, so air can flow through the attic efficiently in both directions. An attic with plenty of exhaust but too little intake, or the reverse, moves far less air than the total vent area suggests, since the smaller side becomes the bottleneck. Codes and manufacturer guidelines typically specify a target ratio of net free area to attic square footage to achieve it.
Attic ventilation only works as a system, not as a collection of individual vents, and balance is the principle that ties the two halves of that system together. Cool air has to come in low, typically through soffit vents, at roughly the same rate that hot air is leaving high, typically through a ridge or other exhaust vent, for the whole attic to actually circulate.
The word balance is doing real work here. It is entirely possible to install plenty of exhaust vent area and still end up with poor ventilation, if the intake side cannot supply air fast enough to replace what the exhaust is pulling out. The smaller of the two sides effectively caps how much total air can move through the attic, no matter how generous the larger side is.
Building codes and shingle manufacturers generally specify ventilation requirements as a ratio, expressed in net free area, relative to the attic's square footage, and many of those guidelines call for that area to be split roughly evenly between intake and exhaust rather than loaded onto one side.
A common real-world imbalance happens during a re-roof: a ridge vent gets added or upgraded as part of the new roof, but nobody checks whether the existing soffit intake, possibly original to a much older house, can actually keep pace with the new exhaust capacity.
The fix is rarely about adding more vents indiscriminately. It is about auditing both sides of the system, intake and exhaust, and sizing whichever side is falling short so the two work together instead of one being a bottleneck for the other.
An attic that still runs hot after a ventilation upgrade is a strong sign the balance, not the total vent count, is the actual problem.
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