34.1 Envelope, Duct Pressure & Blower-Door Testing

34.1 Envelope, Duct Pressure & Blower-Door Testing

Trade Area H Energy Management is 5 percent of the Class A trade exam and 5 percent of Class B. The official outline's first numbered cluster is energy testing: duct pressure tests, blower-door tests, and the envelope effects those results have on humidity, equipment runtime, and the load you already calculated in Chapter 16. The 2026 Air Conditioning CBT list that supports this chapter is Florida Building Code — Energy Conservation, 2023 (8th Edition), Energy Efficient Building Construction in Florida (2024), and SMACNA Energy Systems Analysis and Management, 2nd Edition (2014). Class B's 25-ton / 500,000 Btu per-system cap does not shrink these test numbers. A 3-ton changeout in a new 2,000 ft² house still needs a legal envelope and a legal duct tightness result.

Quick Answer: Run a blower-door test at 50 Pa (0.2 in. w.g.) and keep ACH50 ≤ 7 in every Florida climate zone. Run a duct leakage test at 25 Pa (0.1 in. w.g.) and, on the prescriptive path, keep total leakage ≤ 4 CFM25 per 100 ft² of conditioned floor area with the air handler in place. ACH50 is not Manual J natural ACH. A duct test is not required when ducts and the air handler are entirely inside the thermal envelope.

Why the envelope and the ducts are one energy system

Florida Climate Zones 1A and 2A are warm-humid. Uncontrolled outdoor air is not just extra sensible load. It is grains of moisture that the coil has to condense. A leaky envelope (high ACH50) raises both infiltration CFM and latent load. A leaky supply in a vented attic dumps cooled, dehumidified air into 110–130°F attic air. A leaky return in that same attic pulls hot, wet attic air into the air handler, raising entering wet-bulb, cutting latent capacity, and making the house feel clammy even when the thermostat is satisfied. Energy Efficient Building Construction in Florida (2024) treats attic-duct leakage as a Florida-specific energy and IAQ failure, not a cosmetic seal-class issue.

SMACNA Energy Systems Analysis and Management (2nd, 2014) puts the same idea in audit language: measure the distribution system, then the envelope, then the plant. Guessing “the mastic looked wet” is not a test.

Blower-door testing — R402.4.1.2

R402.4 Air leakage (Mandatory) requires the thermal envelope to limit leakage per R402.4.1 through R402.4.5. R402.4.1.1 plus Table R402.4.1.1 is the continuous air barrier and insulation-installation checklist (boot-to-drywall, recessed cans, electrical boxes, attic hatches). The test that proves the barrier is R402.4.1.2 Testing:

• Leakage shall not exceed seven air changes per hour in Climate Zones 1 and 2, and three air changes per hour in Climate Zones 3 through 8.
• Every Florida county on Table R301.1 / C301.1 is 1A or 2A. The exam number in Florida is 7 ACH50, not the national IECC 5 ACH50 rumor and not a Zone 3 Panhandle 3 ACH50.
• If a dwelling tests tighter than 3 ACH50, provide whole-house mechanical ventilation per R403.6.1 and FBC Residential M1507.3.
• Test method: ANSI/RESNET/ICC 380, reported at 0.2 inch w.g. (50 Pa).
• Testers: individuals defined in F.S. 553.993(5) or (7), or licensees under F.S. 489.105(3)(f), (g), or (i) (Class A air-conditioning, Class B air-conditioning, or mechanical), or an approved third party. Class C (h) is not listed.
• Written, signed report to the code official. Test after all envelope penetrations exist.
• Exception: testing is not required for additions, alterations, renovations, or repairs where the new construction is less than 85 percent of the building thermal envelope.

R-2 dwelling units and multiple attached single-family dwellings may instead use C402.5 (the R402.4 exception). Do not apply that commercial-style path to a detached house just because the builder prefers it.

During the test (R402.4.1.2 protocol): exterior windows and doors closed but not taped beyond weatherstripping; exhaust/intake/flue dampers closed but not extra-sealed; interior doors open; exterior doors for continuous ventilation and HRVs closed and sealed; HVAC off; supply and return registers fully open. If the attic is air-sealed and insulated at the roof deck, open the attic hatches and add attic volume to the test volume.

Convert the fan reading to ACH50:

(\mathrm{ACH}{50} = (\mathrm{CFM}{50} \times 60) / V)

(\mathrm{CFM}{50} = (\mathrm{ACH}{50} \times V) / 60)

Volume (V) is the conditioned volume used in the report (and the extra attic volume when R402.4.1.2 item 7 applies). 7 ACH50 is a 50-pascal test limit, not the natural ACH in Manual J. A teaching conversion is that natural ACH is on the order of ACH50 divided by about 15 to 20, depending on climate and shielding. Plugging 7 ACH into (Q_s = 1.08 \times \mathrm{CFM} \times \Delta T) as if the house leaked that hard at outdoor wind pressure overstates infiltration by an order of magnitude and is how a 2.5-ton house becomes a 5-ton submittal.

Worked envelope example. Hillsborough County (Zone 2A), 2,200 ft², 8 ft ceilings, (V = 17{,}600) ft³. Limit:

(\mathrm{CFM}_{50,\max} = (7 \times 17{,}600) / 60 = 2{,}053\ \mathrm{cfm})

A measured 3,000 cfm at 50 Pa is ACH50 = ((3{,}000 \times 60) / 17{,}600 = 10.2) — fail. A measured 1,800 cfm is ACH50 = 6.1 — pass, still above 3 ACH50 so the extra whole-house-ventilation trigger in R402.4.1.2 for sub-3 ACH50 homes is not in play, but R403.6 ventilation still applies on its own terms.

Duct pressure testing — R403.3.2 through R403.3.6

R403.3.2 Sealing (Mandatory): ducts, air handlers, filter boxes, and building cavities that form the primary air path are ducts and plenums. Seal per C403.2.9.2 (commercial provisions referenced into residential) and prove tightness. Verify by ANSI/RESNET/ICC 380 with the same tester classes as the blower door (553.993(5) or (7), or 489.105(3)(f), (g), or (i)) to be “substantially leak free” per R403.3.3.

R403.3.2.1: air handlers need a manufacturer's designation of no more than 2 percent of design airflow leakage when tested to ASHRAE 193. That is a factory enclosure rating, not a substitute for the field duct test.

R403.3.3 Duct testing (Mandatory) — total leakage at 0.1 inch w.g. (25 Pa):

1. Rough-in: include the manufacturer's air handler enclosure if it is installed. Tape or otherwise seal all registers.
2. Postconstruction: entire system including the air handler enclosure; registers sealed.

Exceptions:

1. No duct leakage test where ducts and air handlers are located entirely within the building thermal envelope.
2. Duct testing is not mandatory for buildings complying by Section R405, unless the compliance report takes credit for leakage and indicates Qn to the outside less than 0.080, where Qn = duct leakage to the outside in cfm per 100 ft² of conditioned floor area at 25 Pa. Default leakage on a Florida R405 run is that 0.080 Qn. Claim a tighter system, and you owe the test report.

R403.3.4 Duct leakage (Prescriptive) — when you do test on the R402 path:

Worked duct example. Same 2,200 ft² house, attic supply and return, air handler in the attic (so Exception 1 does not apply). Postconstruction cap = (4 \times 22 = 88) CFM25. A 140 CFM25 total-leakage result is 6.4 cfm per 100 ft² — fail. If that 140 cfm leaked to a 120°F attic against 75°F return air, the extra sensible hit is on the order of (1.08 \times 140 \times 45 \approx 6{,}800) Btu/h — more than half a ton of “mystery” load that never shows on a square-foot rule of thumb.

R403.3.1 (Prescriptive) insulation: attic supply/return R-8 for ducts ≥ 3 in. diameter, R-6 under 3 in.; elsewhere R-6 (≥ 3 in.) and R-4.2 (< 3 in.). Ducts completely inside the envelope are excepted from that insulation table. R405.2 still requires R-6 on supply and return not completely inside the envelope (R-8 if site-wrapped supply).

R403.3.5 (Mandatory): building framing cavities shall not be used as ducts or plenums. Panned joists and stud bays as returns fail this section even if the blower door passed.

R403.3.6 Air-handling units: do not install the air handler in the attic when the home complies by Section R402 (prescriptive envelope). Attic air handlers are allowed on R405 only if the service panel is within 6 feet of attic access, a condensate-failure alert or shut-down device is installed, the access opening is large enough to replace the air handler, and a 16-point all-caps notice is posted on the electric service panel telling the owner the air handler is in the attic.

Florida HVAC scenario

Gulfshore Mechanical, a certified Class B firm, finishes a 2,200 ft² slab house in Hillsborough County (Zone 2A) with flex in a vented attic and the air handler on the attic platform. The salesperson told the owner “Florida doesn't do blower doors.” R402.4.1.2 does. The rater (a 553.993 energy rater) measures 3,000 CFM50 (ACH50 10.2) and 140 CFM25 total duct leakage (6.4 / 100 ft²). Both fail. Moving the air handler and ducts into a sealed mechanical closet inside the envelope would have invoked R403.3.3 Exception 1 for the duct test — it would not have waived the 7 ACH50 envelope test. The Class B qualifier may perform the tests under 489.105(3)(g); a Class C service grandfather may not. The 25-ton Class B cap is irrelevant on this 3-ton system; the energy-test numbers still govern the CO.

Traps: (1) Entering 7 ACH50 as Manual J infiltration. (2) Testing ducts at 50 Pa or the envelope at 25 Pa. (3) Skipping the duct test because the job is “performance path,” then claiming Qn 0.03 on Form R405. (4) Putting the air handler in the attic on a prescriptive R402 house. (5) Using panned returns (R403.3.5). (6) Treating Escambia as Zone 3 so you only have to hit 3 ACH50 — Florida has no Zone 3 county on the 2023 table.