Are roof vents needed, and why do you need them?
Short answer. Yes, most homes with a conventional attic need roof vents, because the attic has to release the heat and moisture that build up under the roof. The exception is an attic deliberately built as a sealed, unvented space with spray foam under the roof deck, which follows its own code rules.
Why roof vents are needed
- Moisture. Showers, cooking, and everyday living push water vapor into the attic through ceiling gaps. Without airflow it can condense on the cool decking in winter, leading to mildew, rusted nails, and softened OSB.
- Heat. A Georgia attic in July can reach temperatures far above the outdoor air. Venting carries some of that heat away, which reduces heat stress on shingles and on ductwork that often runs through the attic.
- Warranty and code. Building codes set a minimum vent area for vented attics, and shingle manufacturers expect proper ventilation for full warranty coverage.
How many vents do you need?
The count depends on attic size and the type of vent, not on a fixed number. Code asks for net free vent area of 1/150 of the attic floor area, or 1/300 when the exhaust is properly placed high in the attic. A 1,500 square foot attic at 1/300 needs about 720 square inches in total, split roughly evenly between intake and exhaust. That might be about 20 feet of ridge vent rated at 18 square inches per foot, or seven or eight box vents, plus matching soffit vents. Always check the rating printed on the vent you are using.
When vents are not the answer
Cathedral ceilings and sealed spray foam attics follow different rules, and adding vents to a sealed attic can cause problems. Read roof ventilation basics and attic heat and shingles for the full picture.