Ventilation and Moisture Control
Ventilation and Moisture Control
This domain tests attic ventilation ratios, balanced intake and exhaust, and moisture details that prevent condensation and ice dams.
Core rules
- IRC R806.1 generally requires attic ventilation area of at least 1/150 of the vented ceiling area. The common exception is 1/300 when intake and exhaust are balanced and other conditions are met; net free area, not gross vent size, is what counts.
- Provide roughly 50/50 intake at the soffits and exhaust at the ridge or near the ridge. Keep soffit vents clear with baffles so insulation does not block airflow.
- Exhaust bathroom and kitchen fans outdoors, never into the attic. Seal attic air leaks and insulate properly; ventilation alone does not stop ice dams.
- In cold climates, combine air sealing, insulation, ice barrier at eaves, and ventilation.
Worked example
A 1,500-sq-ft attic needs 1/150 ventilation: 1,500 ÷ 150 = 10 sq ft of net free area. Split 50/50, that is 5 sq ft intake and 5 sq ft exhaust. Convert to square inches: 10 × 144 = 1,440 sq in. total; 720 sq in. each. If the ridge vent provides only 300 sq in. net, the design is short and needs more exhaust or a different ratio.
Traps
Using 1/300 without balanced intake/exhaust, counting gross vent dimensions, blocking soffits with insulation, and venting bath fans into the attic are common errors.