Is attic ventilation necessary?

Two legitimate designs, nothing in between

Building science recognizes two ways to build an attic. A vented attic keeps the insulation on the ceiling below and treats the attic as outdoor space, so it needs air moving in low at the eaves and out high near the ridge. An unvented, conditioned attic moves the insulation up to the roofline, usually with spray foam, and seals the attic into the house. The International Residential Code allows both, but it sets strict insulation rules for the unvented version in Section R806.5. The problem homes are the ones caught in between: insulation on the floor with no working vents, or spray foam with leftover vents letting humid air in.

What happens without ventilation in Georgia

• Summer heat: a floor-insulated attic with no airflow under dark shingles runs far hotter than one that breathes, which speeds up shingle aging and loads any ductwork running through the attic.
• Moisture: household humidity leaks up through light fixtures, the attic hatch and wall top plates. Without airflow it condenses on the cool underside of the deck on winter nights, leading to stained plywood, rusty nail tips and eventually soft decking.
• Warranty trouble: shingle manufacturers generally require ventilation that meets code, and an attic with none gives them a reason to limit coverage.

How to tell which you have

Look into the attic on a sunny afternoon. If insulation is on the floor, you should see daylight at the eaves and a vent near the peak. If foam is sprayed on the underside of the roof deck, there should be no open vents at all. Either picture is fine. A mix of the two is worth having checked. For how intake and exhaust work together, see roof ventilation basics, and for the heat side, attic heat and shingles.