How do you determine whether your attic has proper ventilation?
Short answer. Determine proper attic ventilation two ways: by the numbers, adding up the net free area of your existing intake and exhaust vents and comparing it to the code minimum for your attic size, and by the evidence, checking the attic and roof for signs of trapped heat and moisture. A properly vented attic passes both tests.
The numbers test
- Measure the attic floor area.
- Calculate the requirement: area divided by 300 (balanced, high exhaust) or 150, times 144.
- Inventory exhaust: feet of ridge vent, number of box vents, turbines, fans, and gable vents, using the label ratings.
- Inventory intake: soffit vents and edge vents, leaving out any that are painted over or blocked.
- Compare. Is each half close to the requirement? Is intake at least equal to exhaust? Is there more than one exhaust type?
The evidence test
- Decking: dark staining, mildew, or delamination on the underside, especially on north slopes and near the ridge.
- Nails: rusty nail tips, or frost on them on cold mornings, point to condensation.
- Insulation: damp, matted, or stuffed into the eaves.
- Daylight: you should see light at the eaves and through a ridge vent slot from inside.
- Heat: an attic that stays hot long after sunset, and upstairs rooms that are hard to cool.
- Roof surface: shingles that curl, blister, or age faster on some slopes than others, which can point to heat trapped below.
- Ice dams in the North Georgia mountains during snow events.
Interpreting the results
If the numbers look fine but the evidence is bad, look for a moisture source: bath fans venting into the attic, air leaks, or a roof leak. If the numbers fall short but the attic is dry, plan the upgrade for the next reroof. If both fail, correct intake first, then exhaust. See attic heat and shingles and what to expect from a roof inspection.