Roof Ventilation Calculator
Proper attic ventilation prevents heat buildup, moisture damage, and ice dams. Enter your attic square footage to find out how much intake and exhaust vent area you need.
Why Attic Ventilation Matters
Attic ventilation controls temperature and moisture. In summer, attic temps can hit 150°F without ventilation, radiating heat into living spaces below and baking roof shingles from the underside. This heat accelerates shingle aging and drives up air conditioning costs.
In winter, warm indoor air rises and meets cold roof decking. Without ventilation, condensation forms, soaking insulation and promoting mold growth. Ice dams form when heat escapes through the roof, melting snow that refreezes at the eaves. Proper ventilation keeps the attic temperature closer to outside air, preventing these problems.
Balanced ventilation combines intake vents low on the roof with exhaust vents at or near the peak. Air enters cool, heats as it rises, and exits naturally through convection. This continuous cycle removes heat and moisture without mechanical fans in most climates.
Intake vs. Exhaust Balance
The 50/50 split between intake and exhaust is critical. If exhaust capacity exceeds intake, the system creates negative pressure that pulls conditioned air from the house through ceiling penetrations. This wastes energy and defeats the purpose of insulation.
Conversely, too much intake and not enough exhaust means air can't exit fast enough. The attic stays hot and humid despite open vents. Check that both intake and exhaust NFA meet code minimums, and verify they're as close to equal as possible.
Soffit vents are the most common intake method. Continuous soffit vents run the length of the eave, while individual rectangular vents space every few feet. Both work, but continuous vents provide more consistent airflow. For exhaust, ridge vents win on new construction, while gable vents or roof-mounted vents retrofit existing homes.
Common Ventilation Mistakes
Mixing vent types at the same level causes short-circuiting. For example, gable vents at both ends of the attic create cross-flow that bypasses the central attic space. Air enters one gable and exits the other without rising through the attic cavity. Always pair low intake with high exhaust.
Blocking soffit vents with insulation is another frequent error. When insulation fills the eave space, air can't enter even though vents are present. Install baffles or rafter vents to maintain an air channel from soffit to ridge, ensuring insulation doesn't obstruct airflow.
Powered attic fans seem like a shortcut, but they often do more harm than good. If intake ventilation is insufficient, the fan pulls air from the house instead of outside, wasting energy. Passive ventilation with proper intake and exhaust works better in most cases and costs nothing to operate.
Frequently Asked Questions
What is net free area (NFA)?
NFA is the actual open area for airflow after accounting for screens, louvers, and baffles. A vent might be 12×12 inches but provide only 60 square inches of NFA.
How much ventilation does my attic need?
The standard is 1 square foot of NFA per 150 square feet of attic floor (with vapor barrier) or 1 per 300 without. Split this 50/50 between intake and exhaust.
Where should intake vents be located?
Install intake vents low on the roof, typically in the soffit or eave overhang. Cool air enters here and rises as it heats, exiting through higher exhaust vents.
What types of exhaust vents are best?
Ridge vents work well because they run the entire roof peak, providing even exhaust. Gable vents, turbine vents, and powered fans are alternatives.
Can I have too much attic ventilation?
Rarely. Excess ventilation usually just means more airflow. The bigger mistake is unbalanced ventilation, such as too much exhaust and not enough intake, which can pull air from the house instead.