Florida's outdoor humidity averages 75-85% through summer and runs above 90% during morning hours much of the year. Indoor humidity in conditioned homes typically runs 45-55%. The gradient between the two is enormous; the place that gradient gets settled is, often, the attic.
When warm humid air meets a cool surface, water condenses. In an under-ventilated Florida attic, the cool surface is the underside of the roof deck — particularly at night, when the deck cools faster than the air around it. The accumulated condensation, over years, damages plywood decking from below.
How to recognize the problem
Three signs:
Water staining on the underside of the deck when viewed from the attic. Not from a roof leak; the staining is from inside.
Soft or punky plywood at specific locations — typically near the ridge or near eaves where ventilation is poor.
Mold or mildew on attic surfaces — joists, insulation, the underside of decking.
If any of these are present, the attic conditions need addressing. The roof deck repair is downstream of the conditions problem.
What causes it
Three failure modes, often in combination:
1. Inadequate attic ventilation
The most common single cause. The soffit-and-ridge system is undersized, blocked, or imbalanced. Humid air cannot escape.
2. Air leaks from conditioned space
Recessed lighting fixtures, attic stairs, plumbing chases, and other penetrations between conditioned space and attic let warm humid indoor air into the attic. The indoor humidity then condenses against the cooler attic surfaces.
3. HVAC ducts in the attic
Standard practice in Florida construction. The ducts radiate cool air; the surrounding attic surfaces stay relatively cool. Humid attic air condenses against the ducts and the surrounding deck.
What to do
Improve ventilation
Soffit and ridge ventilation, balanced and unblocked. We have written separately about this. The single largest improvement on most homes.
Air-seal the ceiling
Close penetrations between conditioned space and attic. Foam-seal around recessed lights, attic-stair openings, plumbing pipes, and electrical penetrations.
Insulate the ducts
If HVAC ducts are in the attic, ensure they are insulated to the rated R-value. Bare or under-insulated ducts radiate cold and drive condensation.
Consider unvented attic conversion
In some cases — particularly newer construction with HVAC in the attic — the right answer is to convert the attic to an unvented, conditioned space, with insulation on the underside of the roof deck and the attic brought into the building envelope.
This is a larger project, but it eliminates the humidity-gradient problem entirely. Worth considering on home renovations of any scope.
Where roofing fits
When we open a roof for re-roof or repair, we inspect the deck for condensation-related damage from below. If we find it, we report it with photographs and recommendations.
The roofing scope itself does not fix attic humidity. The roof repairs the deck; the attic work prevents the problem from recurring. Both need to happen for the long-term outcome to be right.
We do not do attic-side work directly. We coordinate with insulation and air-sealing contractors when our roof scope reveals the attic side needs attention.
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