Building science

How a South Florida attic actually works

This is the explanation we use before a bid. It is about vented house attics in Palm Beach County and Broward County. It is not a sales page for a product, and it is not a promise about your power bill. The quotes are prepared by Russell Y, a building scientist who is BPI certified and certified to perform blower door testing.

The roof is not the insulation

A barrel-tile roof in Boca Raton, Delray Beach, or Parkland shades the wood deck. The air gap under the tile and the vented attic above the ceiling are supposed to let heat out. They do not keep the bedrooms cool by themselves. The layer that faces the rooms is the attic floor: the drywall ceiling, the leaks in it, and whatever is lying on top of it.

Four inches of settled fiberglass is a common find in houses built to an older minimum. At about R-2.2 per inch, that is around R-9. The rooms feel it after lunch, when the deck is hotter than the outside air and the air conditioner runs without catching up. Adding tile, a new shingle, or a radiant barrier on the rafters does not replace the missing depth on the floor.

The code number and the number we install

Palm Beach and Broward are Climate Zone 1A. The 2023 Florida Building Code, Energy Conservation, Table R402.1.2, sets the prescriptive ceiling insulation minimum at R-30. The same table allows a ceiling U-factor of 0.035. R-30 of continuous insulation is in that neighborhood. A thin, lumpy blow does not meet it just because the proposal says R-30.

Our blown-in standard is R-38. That is above the zone 1 floor, and it is the zone 2 ceiling number in the same Florida table. It is not a luxury package and it is not R-49 or R-60. Some Florida pages print R-49 to R-60 as the attic target because that is what the unamended national model code says for a colder zone. The Florida book does not. Past R-38 you are buying more bags. You are not fixing an open hatch. New fiberglass is about R-2.5 per inch, so R-38 from a bare ceiling is about 15 inches. New cellulose is about R-3.6 per inch, about 10.5 inches from bare. Over the R-9 already in a typical attic, the new fiberglass gap is about 12 inches and the cellulose gap is about 8 inches. The planner on this site does that math from the depth you type in. The bid is still the attic we see.

One attic, with the arithmetic shown

Take a common house: 1,800 square feet of attic, four inches of settled fiberglass. Four times 2.2 is 8.8, which we call about R-9. R-38 minus 8.8 is 29.2 of R-value still missing. Divide 29.2 by 2.5 and the new fiberglass is about 11.7 inches, which we install as about a foot, level. Divide 29.2 by 3.6 and new cellulose is about 8 inches. On this site's planner that size of gap is a planning range of $2,100 to $3,350. It is not a bid. A bigger attic, a tight hatch, or fill that has to come out first moves the number.

Written by Cardinal Insulation of Boca Raton LLC, the crew that does the work. The shop is 4263 NW 1st Ave, Boca Raton. Blown-in insulation is the first service. We carry general liability and we will send the certificate if a property manager asks. We do not claim a pest-control license. If animals are still in the attic, a licensed pest company gets them out before we close the holes.

The edges fail first

Soffit vents on a tile roof push air along the eaves. That air is doing its job. It also shoves loose fill off the top plates, which is exactly where the ceiling meets the wall and where the leaks are. A baffle holds the insulation back so the vent stays open and the plate stays covered. Without the baffle, the middle of the attic can look deep while the perimeter, the part over the outside walls, is bare.

Depth markers, about one every 300 square feet, are how you check this from the hatch without crawling the whole attic. A high pile next to the opening is the easiest place for a crew to leave material. It is also the least useful place. Read the marker at the far side.

Five holes the hose will not close

Insulation slows heat moving through the material. It does not stop air. These are the openings we close before a bag is opened:

1. The hatch or the pull-down stair. An uninsulated plywood lid is a hole the size of a person. The energy code treats it as part of the ceiling: weatherstripped, and insulated to the ceiling around it.
2. Top plates. The gap where interior walls meet the attic is open framing in a lot of these houses. Air from the rooms uses it all afternoon.
3. Can lights that are not rated to be covered. Non-IC fixtures keep their clearance. IC fixtures can be sealed and then buried. Covering the wrong one is a fire problem, not an insulation upgrade.
4. Bath fans that dump into the attic instead of through a dampered duct to the outside. The fan is then pushing wet room air onto the deck.
5. Plumbing stacks, wire penetrations, and the open chase over a dropped soffit. Each one is small. Together they are the reason a deep attic still feels like the upstairs is outside.

A whole-house blower-door result, 7 air changes per hour at 50 pascals in Zone 1, tests the entire house. An attic job is not that test. We seal the ceiling plane. We do not hand you an ACH50 number unless a blower door was actually run. The air-sealing page is the longer version of this list.

One boundary, not two

A vented attic gets blown-in on the floor. The attic stays hot on purpose. Spray foam on the underside of the roof, with the soffit and ridge vents closed, is the other legal assembly: an unvented attic, used when a bonus room or the ducts should sit inside the house. Florida's roof ventilation section allows that unvented attic when the assembly matches the rule. In Zones 1 and 2A the condensation table lists R-5 of air-impermeable insulation so the sheathing stays on the right side of the dew point. R-5 is about two inches of closed-cell, roughly R-13. That is an air-seal coat. It is not Table R402.1.2, and it is not R-38.

Do not foam the deck and leave the attic vented. That mixes the two assemblies. Open-cell foam is about R-3.6 per inch and can hold water if the roof leaks. Closed-cell is about R-6.5 per inch and resists water better. Neither one repairs a roof. Foam also smells while it cures. Fiberglass and cellulose do not.

What a finished attic will not fix

Supply ducts in the attic are insulated on their own, at least R-8, and they still sit in hot air if the attic stays vented. Insulating the floor does not move those ducts inside. A disconnected supply, a crushed flex run, or a return that is sucking attic air will keep the bill high after the ceiling is at R-38.

Wet insulation, rodent nests, and a deck that has been rained on are not a top-up. Dry fill stays and is counted at the thickness you can measure. Wet or contaminated fill comes out first, at its own price. Sanitizing the deck after that removal is a third price. It is not mold remediation for the rooms below, and it is not a roof repair.

Sources

• 2023 Florida Building Code, Energy Conservation, Eighth Edition, Table R402.1.2. Climate zone 1 ceiling minimum: R-30. Climate zone 2 ceiling minimum: R-38. Effective December 31, 2023.
• Settled fiberglass figured at about R-2.2 per inch. New blown fiberglass about R-2.5. New cellulose about R-3.6. Open-cell foam about R-3.6. Closed-cell foam about R-6.5. These are the planning numbers on this site, not a product label from a bag we have not opened.
• Attic depth markers on the order of one every 300 square feet, so the depth can be read from the hatch. The edges, not the pile by the opening, are the check.

Questions

The shop is 4263 NW 1st Ave, Boca Raton, FL 33431. Call (561) 654-7698. The install pages are blown-in insulation, air sealing, spray foam, and removal.