A gable vent is a vent, often louvered and either triangular or rectangular, installed high on the flat wall of a gable end to let hot attic air escape. It is an older style of passive exhaust ventilation, common on homes built before ridge venting became standard, and it can work well on its own or alongside other vents depending on the attic's layout. When a roof is later fitted with a full ridge and soffit vent system, gable vents are sometimes sealed off so the two exhaust paths do not work against each other.
Before ridge vents became the standard way to exhaust attic heat, gable vents were the common answer. Set high on the flat, vertical wall at each gable end, they let hot air escape out the side of the attic rather than out the top of the roof.
A gable vent is usually a louvered opening, triangular to match the shape of the gable or rectangular, sized to move a meaningful volume of air out of the attic space behind it.
On a simple attic with soffit intake and no ridge vent, a pair of gable vents on opposite ends can create a real, if less efficient, cross-breeze that helps clear out heat and moisture.
The complication comes when a home has both gable vents and a ridge vent. Air naturally takes the path of least resistance, and a ridge vent's draw can actually pull air in through a gable vent instead of up from the soffit, short-circuiting the intake-to-ridge airflow the system was designed around.
For that reason, when a roof is upgraded to a full ridge and soffit system, a roofer will often recommend sealing off the old gable vents so the new system works the way it is supposed to.
For a homeowner, having both types of exhaust vent is not automatically better ventilation and can sometimes work against itself.
Any time ridge venting is added or upgraded, ask whether the older gable vent should stay open or be sealed off, rather than simply leaving both in place.
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