The decision
Which insulation belongs in a South Florida house
Three products get sold as if they were interchangeable. They are not. A vented attic over a flat ceiling gets blown-in on the floor. An open wall during construction can take batts. A roof that should sit inside the house gets spray foam on the deck, and the vents get closed. Pick the boundary first. The product follows.
Prepared for Cardinal Insulation of Boca Raton LLC, 4263 NW 1st Ave, Boca Raton. The quotes are written by Russell Y, a building scientist who is BPI certified and certified to perform blower door testing. A blower door result is reported only when the test is run.
The table, with the Florida code next to it
Source for the code column: 2023 Florida Building Code, Energy Conservation, Eighth Edition, Table R402.1.2. Effective December 31, 2023. The per-inch figures are the planning numbers Cardinal uses. They are not a bag label we have not opened.
| Product | Per inch | Where it goes | Where it does not |
|---|---|---|---|
| Blown fiberglass, new | About R-2.5 | Vented attic floor, after the seal | A roof deck, a wet attic, an air seal |
| Fiberglass already up there | About R-2.2 | Counted, if it is dry, then topped | Treated as R-3.7 because a flyer said so |
| Blown cellulose, new | About R-3.6 | A vented attic floor that can take the weight | A substitute for sealing the hatch |
| Fiberglass batts | About R-3.1 to R-3.7 if they fit the bay | Open walls and floors while the cavity is empty | A top-up over slumped attic batts |
| Open-cell spray foam | About R-3.6 | Roof deck, vents closed | The floor of an attic that stays vented |
| Closed-cell spray foam | About R-6.5 | Roof deck, or a thin air-seal coat | Called R-38 because two inches went on |
Zone 1 ceiling minimum in that Florida table: R-30. Zone 2 ceiling minimum: R-38. Palm Beach County and Broward County are zone 1. Cardinal's blown-in standard is R-38, which is above the zone 1 floor and equal to the zone 2 ceiling. It is not R-49 and it is not R-60. Those show up when someone copies the unamended national model code for a colder climate and calls it Florida.
Blown-in is the attic-floor job
The usual house in Boca Raton, Delray Beach, Boynton Beach, and the Broward cities we drive is a vented attic under tile. The rooms are cooled. The attic is not. Insulation belongs on the ceiling of the rooms, level, after the leaks are sealed. Fiberglass is the common top-up because it is light and it matches the fiberglass already there. Cellulose packs denser and weighs more. We use it when the ceiling can take it, not because the R-per-inch looks better on a proposal.
From a bare ceiling, R-38 is about 15 inches of new fiberglass or about 10.5 inches of new cellulose. Four inches of settled fiberglass is about R-9. The gap is about 12 inches of new fiberglass or about 8 inches of new cellulose. A thick pile by the hatch and a bare edge at the soffit is not that number. Markers, about one every 300 square feet, are how you read it from the opening.
Batts are for an open cavity
A batt that fills a stud bay, cut to the bay and not jammed past its thickness, does the job it was made for. An attic full of batts from 1985 has slumped, pulled off the plates, and left the wires proud. Laying a second batt on top of that does not fill the gaps. Blowing over dry batts does, after the top plates, the cans, and the hatch are sealed. If the batts are wet or full of nests, they come out. They do not get buried.
Foam is the roof-deck job
Spray foam on the underside of the roof, with the soffit and ridge vents closed, makes an unvented attic. That is how a bonus room, or a set of ducts, moves inside the house. Florida allows that assembly when it matches the roof-ventilation section. In zones 1 and 2A the condensation control listed for air-impermeable insulation is about R-5. Two inches of closed-cell is about R-13. That keeps the sheathing on the right side of a dew point. It does not satisfy the energy-code ceiling by itself, and it is not an R-38 attic.
Do not foam the deck and leave the attic vented. That mixes the two assemblies. Open-cell can hold water if the roof leaks. Closed-cell resists water better. Neither one repairs a roof. Foam smells while it cures. Blown fiberglass and cellulose do not. People and pets leave for a foam day. A blown-in day does not require that.
What a real quote has to show
- Square feet of the attic, not a county-wide price per foot.
- Inches already there, and which product, so the existing R-value is honest.
- The product being added, and the inches required to reach R-38.
- Whether the attic stays vented. If it stays vented, the product is blown-in. If the vents close, the product is foam, and that is a different price.
- Removal and sanitizing as their own lines, if the fill is wet or contaminated. Not folded into a top-up.
A planning range for the 1,800-square-foot example above is $2,100 to $3,350. That is the gap from about R-9 to R-38 in fiberglass. It is not your bid. A larger attic, a long hose, a tight hatch, or a removal changes it. We do not quote a dollar amount off the FPL bill. Ducts in a vented attic are still in the heat.
Questions
Is blown-in better than batts in a South Florida attic?
On a vented attic floor, yes, as a top-up. Batts leave gaps at the trusses, the wires, and the eaves. Blown fiberglass or cellulose covers those after the leaks are sealed. Batts still belong in an open wall cavity during construction. They are a poor second layer on a settled attic.
What R-value per inch should a Florida quote use?
New blown fiberglass is about R-2.5 per inch. Settled fiberglass already in the attic is about R-2.2. New cellulose is about R-3.6. Open-cell foam is about R-3.6. Closed-cell foam is about R-6.5. A quote that prints R-3.2 to R-3.8 for both fiberglass and cellulose is using one band for two materials.
Does Florida require R-49 or R-60 in the attic?
No. The 2023 Florida Building Code, Energy Conservation, Table R402.1.2, lists a climate zone 1 ceiling minimum of R-30 and a climate zone 2 ceiling minimum of R-38. Palm Beach County and Broward County are zone 1. Cardinal installs R-38 and checks the depth. R-49 and R-60 are national-model numbers for colder zones.
When is spray foam the right product?
When the roof deck should be inside the house: a bonus room, or an attic whose ducts should stop baking, with the soffit and ridge vents closed. It is the wrong product for a vented attic over a flat ceiling. Two inches of closed-cell is about R-13, an air-seal coat, not R-38.
How much does a typical top-up cost?
A planning example, not a bid: 1,800 square feet with 4 inches of settled fiberglass, about R-9. The gap to R-38 is about 12 inches of new fiberglass. On this site's planner that gap is $2,100 to $3,350. Removal, a tight hatch, and spray foam are separate prices.
The shop is 4263 NW 1st Ave, Boca Raton, FL 33431. Call (561) 654-7698. Read how a South Florida attic works, then the install pages: blown-in, spray foam, and air sealing.