Attic ventilation is one of the most under-specified items on Florida residential roofs. The code mandates a minimum; the minimum is often inadequate; the consequences accumulate slowly enough that owners do not connect them to the original under-specification.
A properly ventilated attic in Florida summers runs 15-30°F cooler than a poorly ventilated one. That cooling reduces HVAC load, extends underlayment life, and prevents the condensation issues that can damage roof decking from below.
How soffit-to-ridge ventilation works
The system has two openings:
- Soffit vents at the eaves, allowing cool air to enter the attic
- Ridge vents along the peak, allowing hot air to exit
The physical principle is stack ventilation: hot air rises and exits at the ridge; cooler air is drawn in at the soffits to replace it. The cycle is continuous and passive — no mechanical equipment, no power consumption.
What the code requires
The Florida Building Code requires net free vent area equal to 1/150th of the attic floor area, or 1/300th if a vapor barrier is in place on the attic floor.
For a 4,000-square-foot house with conditioned floor area roughly matching the attic plan area, that is about 25 square feet of net free vent area at 1/150th, or 13 square feet at 1/300th. Distributed between soffit and ridge.
Why the minimum is often inadequate
The code minimum is a code minimum. It is designed to prevent the worst outcomes (moisture damage, ice damming where applicable), not to optimize cooling.
For Florida specifically, we typically specify 1.5x to 2x the code minimum. The cost differential is small. The benefit is a noticeably cooler attic and a reduced HVAC load.
What can go wrong
Imbalanced ventilation
If the soffit vents are blocked or undersized relative to the ridge vents, the system pulls air from elsewhere — sometimes from the conditioned space below. This creates negative pressure that can pull humid air into the attic from indoors.
Both soffit and ridge must be sized together. The soffit area must equal or exceed the ridge area to prevent backflow.
Blocked soffit vents
Common in older homes: the soffit vents are intact but blocked by insulation that has migrated over them. The ventilation is non-functional. Visible from inside the attic.
Ridge cap on the ridge vent
Sometimes a contractor will install ridge vent material and then cap it with standard ridge tile that blocks the vent. The vent material is in place; the ventilation is not.
We have walked attics where every component was installed correctly individually and the system was functionally non-ventilated because of one of the above.
What to look for
Three quick checks:
- From the attic: can you see daylight through the soffit vents? Yes = unblocked. No = blocked.
- From outside, at the peak: is the ridge cap raised slightly above the field, with a visible gap underneath? Yes = ridge vent is functional. No = either not installed or sealed off.
- Attic temperature on a summer afternoon: should be within 10-15°F of outdoor temperature. If it is 30°F+ above outdoor, the ventilation is inadequate.
What we install
On every roof we touch:
- Continuous soffit vents at the eaves (intake)
- Continuous ridge vents at the peak (exhaust)
- Sized to 1.5x code minimum
- Verified balanced (soffit area ≥ ridge area)
- Insulation baffles where insulation could migrate into the soffit
The cost differential over code-minimum is in the range of $400-$1,000 on a typical residential project. The cooling benefit is felt every summer day.
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