The insulation choice for a Florida home is one of the highest-leverage energy decisions a homeowner makes. Florida HVAC systems run roughly 9–10 months a year at high load. The single biggest determinant of attic temperature and therefore AC efficiency is the insulation strategy in the attic — and the fiberglass-versus-spray-foam choice has very different implications for how the attic, ductwork, and HVAC system all work together.
This is not a like-for-like swap. The two materials lead to fundamentally different building science strategies.
The strategy split
Before comparing materials, understand the strategy split:
- Fiberglass strategy = vented attic. Insulation lays on the attic floor (the ceiling of the conditioned space below). The attic itself is unconditioned and vented to outside — soffit vents in, ridge vents out, hot air rises and exhausts. AC ducts that run through this attic are running through 110–140°F summer air, which means the ducts lose 15–25% of their cooling capacity to that hot environment. Air leaks in the ceiling plane let conditioned air escape into the vented attic.
- Spray foam strategy = conditioned attic. Closed-cell foam sprayed on the roof deck (underside of the roof sheathing) brings the attic inside the thermal envelope. The attic stays close to indoor temperature — often within 5–15°F of the living space. AC ducts in this attic are now in a cool, conditioned environment, so they retain their full capacity. Air sealing happens at the same time as insulation.
For Florida specifically, where AC ducts almost always run through the attic, the conditioned-attic strategy with spray foam is dramatically more efficient. Most FL homes built since 2010 in higher-end markets use it. Most older FL homes use the vented-attic-with-fiberglass strategy and could benefit enormously from a conversion.
When closed-cell spray foam wins
Spray foam is the right pick for most FL retrofit projects:
- Air sealing happens automatically. A vented FL attic typically has 200–400 sq inches of leak area through ceiling penetrations (recessed lights, AC registers, plumbing chases, top plates). Air sealing those by hand is tedious and incomplete. Spray foam seals everything it touches. After a foam job, the attic is essentially airtight relative to the living space.
- Massive attic temperature reduction. A conditioned attic with closed-cell foam on the roof deck stays 30–40°F cooler than a vented attic on a hot FL afternoon. That means AC ducts in the attic deliver cold air without parasitic loss, and the AC compressor cycles 15–25% less.
- R-value per inch is higher. Closed-cell foam delivers R-6 to R-7 per inch. Fiberglass batt is R-3.1 to R-3.4 per inch. For the same R-value, foam is roughly half the thickness — which matters in low-attic FL retrofits where there is not 14 inches of vertical space available.
- Vapor barrier built in. Closed-cell foam is itself a vapor retarder. In humid FL conditions, that protects roof sheathing from condensation-driven rot — a real problem in vented attics where humid outside air contacts cooler interior surfaces. We see roof decks in FL homes failing at year 20 from accumulated moisture damage; closed-cell foam essentially eliminates that risk.
- No nutrient value for mold or rodents. Polyurethane foam is inert. Mold does not grow on it. Rodents do not nest in it. Fiberglass, in wet conditions, will host mold; in dry conditions, it is a perfectly comfortable rodent nesting material. In a FL attic where humidity and small openings to the outside are routine, this matters.
When fiberglass wins
Fiberglass is the right pick in specific FL situations:
- You are simply adding R-value to an existing vented attic. If your home already has functional soffit vents and ridge vents, your ducts are sealed and insulated separately, and you just want to bump R-19 to R-30 or R-30 to R-49 (the current FL energy code minimum for Climate Zone 2), blown fiberglass is cheap and fast. The conditioned-attic conversion only makes sense if you are doing a comprehensive retrofit.
- Tight upfront budget. Blown fiberglass for a 2,000 sqft FL attic runs $1,600–$4,000. Closed-cell spray foam runs $6,000–$13,000. If you cannot fund the foam project, adding fiberglass to existing insulation is meaningfully better than nothing.
- Crawlspaces and floor cavities. Where vapor barriers, fire performance, and ease of inspection matter more than air sealing — and where the geometry makes foam application awkward — fiberglass batts are the standard.
- DIY scope. Blown fiberglass can be DIY (rent a blower from a big-box store for ~$40/day, buy bags at $25–$45 each). Spray foam is professional-only — the chemistry, ventilation, and equipment are not appropriate for homeowner installation.
- Recently built FL home with good envelope. Newer FL homes (post-2015) often have well-sealed envelopes from construction and may not need the air-sealing properties of foam. A code-compliant fiberglass install in those homes is fine.
Florida-specific concerns
A few FL-specific items that affect the decision:
- Existing ductwork location. If your AC ducts are in the attic, conditioned-attic with foam saves 15–25% on cooling bills. If your ducts are in conditioned space (closet, dropped ceiling, mechanical room), the savings are smaller. Most FL homes have attic ducts, which is why the foam strategy is so common.
- Hurricane and wind uplift. Spray foam on the roof deck adds adhesion between the sheathing and the rafters/trusses. Some research suggests this provides modest secondary uplift resistance in hurricanes — beyond the primary fastening pattern. Not a primary reason to choose it, but a nice side effect.
- Humidity and dehumidification. Closed-cell foam in a conditioned attic can sometimes create the opposite problem from a vented attic — the attic becomes a moisture sink where infiltration humidity accumulates. Some FL spray-foam installations require a small dehumidifier in the attic ($300–$600 plus install) to keep humidity below 60%. Discuss with your foam contractor before install.
- FL code requires thermal barrier in living spaces. Closed-cell foam in exposed living spaces (basement-equivalent areas) requires an intumescent fire-rated coating. In attics and concealed spaces, this is not required. Fiberglass has no such requirement.
- Hot-roof concerns are largely a myth. Some critics worry that a "hot roof" (unvented roof deck under conditioned-attic foam) causes shingle degradation. Actual measurements show shingle temperatures on hot-roof installations are 5–15°F higher than vented roofs — well within manufacturer warranty parameters. Both major shingle manufacturers (GAF, Owens Corning) explicitly warranty their products on unvented foam-insulated roofs.
The 20-year cost picture
For a 2,000 sqft FL home attic:
Fiberglass retrofit scenario (existing vented attic, add to R-49):
- Year 0 blown fiberglass: $2,500
- Years 1–20 AC bills (baseline): $3,000/year cumulative = $60,000
- 20-year total: ~$62,500
Closed-cell foam conversion scenario (full conditioned-attic):
- Year 0 foam + duct sealing + small dehumidifier: $10,000
- Years 1–20 AC bills (20% lower): $2,400/year cumulative = $48,000
- 20-year total: ~$58,000
Foam wins the long-term math by a modest margin and delivers a meaningfully more comfortable, quieter, and lower-humidity attic. The break-even is typically year 9–13 depending on HVAC use intensity.
When to pick closed-cell spray foam
- Your AC ducts run through the attic (almost all FL homes).
- You are doing a comprehensive retrofit, not just adding R-value.
- You want maximum attic temperature reduction.
- Roof deck sheathing condition is good (foam does not save deteriorated decking).
- Budget supports the $6,000–$13,000 install.
- You plan to own the home 8+ years (payback horizon).
When to pick fiberglass
- Existing functional vented attic — just need more R-value.
- Tight budget — under $4,000 for the insulation project.
- DIY-inclined and you want to do it yourself.
- Crawlspace or floor cavity application (fiberglass is standard there).
- Newer FL home with already-good envelope where foam ROI is smaller.
For typical FL retrofits where the AC and duct strategy supports it, closed-cell spray foam delivers the largest energy and comfort improvement. For budget-driven or simpler scope work, fiberglass remains a perfectly fine FL choice.