Ocala Envelope Leakage vs. Duct Leakage Testing: Two Different Tests, Two Different Code Requirements

Florida new construction has two distinct mandatory leakage tests, and they get confused with each other constantly. The blower door test (also called the envelope leakage test) measures how much outside air leaks through the home’s exterior envelope. The duct leakage test measures how much conditioned air leaks out of the HVAC duct system. They use different equipment, different procedures, different code limits, and different acceptance criteria.

This article explains both tests, what they measure, why both are required, and which one matters for which part of the construction process — written specifically for Ocala and Marion County builders. For a deeper dive into how both tests fit into the new construction process, see our Marion County new construction energy testing guide.

The Two Tests at a Glance

Test	What it measures	Code limit	When performed
Envelope leakage (blower door)	Air leakage through the home’s exterior shell	≤5.0 ACH50	After drywall, before final
Duct leakage	Air leakage from the HVAC duct system	≤4 CFM25 per 100 sq ft conditioned area	After ducts installed

Both are required by the 2023 Florida Building Code, Energy Conservation chapter for new residential construction in Climate Zone 2 (which includes all of Marion County).

The Envelope Leakage Test (Blower Door)

What it measures: Total air leakage through the home’s exterior envelope — walls, ceiling, floor, windows, doors, and all penetrations. The test treats the house as one big box and measures how much air leaks through that box.

Equipment: A calibrated fan (blower door) installed in an exterior doorway, plus a digital manometer to measure pressure differential.

Procedure: The fan creates -50 Pa pressure differential between inside and outside (pulling air out of the house). The fan’s airflow at that pressure is the CFM50 result, which is converted to ACH50 using the home’s conditioned volume.

Code limit: ≤5.0 ACH50 for Florida CZ2 new construction — this is the absolute ceiling under any compliance path per FBC-EC R402.4.1.2. The prescriptive testing limit for Climate Zones 1 and 2 is 7 ACH50, but the 5.0 ceiling overrides it. Below 3 ACH50 triggers the mechanical ventilation verification requirement.

Where the leakage typically comes from: Plumbing stack penetrations, electrical penetrations, recessed lighting, bath fan ducts terminating in attic, attic access hatches, sliding glass door tracks, HVAC line set penetrations, top plate gaps. See our article on why Ocala homes fail blower door tests for detailed fixes.

Who has to pass it: Every new residential construction in Marion County, plus substantial additions (typically >50% of original conditioned area).

The Duct Leakage Test

What it measures: Air leakage from the HVAC duct system itself — supply trunks, return ducts, branch lines, register boots, plenum connections, and all duct joints. This is measured separately from envelope leakage because conditioned air leaking from ducts inside an attic is energy lost outside the conditioned envelope.

Equipment: A calibrated pressurization fan (often called a Duct Blaster, a brand name now used generically) plus a digital manometer.

Procedure: The duct system is sealed at all register locations using tape or magnetic covers. The fan is connected to the air handler or a return register. The duct system is pressurized to +25 Pa (the standard test pressure). The fan’s airflow at +25 Pa is the CFM25 reading, which is divided by conditioned floor area to give CFM25 per 100 sq ft.

Code limit: ≤4 CFM25 per 100 sq ft of conditioned area. This applies for either:

• Total Leakage at rough-in (before drywall, while ducts are accessible) — this measures leakage to anywhere
• Leakage to Outside at final (post-completion, with the air handler running) — this measures only leakage to outside the conditioned space

Most Florida builders test for Total Leakage at rough-in because it’s the easier test to do and the limit is the same.

Where the leakage typically comes from: Plenum connections at the air handler, branch duct connections to the trunk, register boot connections to ductwork, panned joist returns (sometimes prohibited in Florida), supply runs through unconditioned attic space. Duct leakage is also the single biggest energy waste source we find in existing Florida homes — finding it early in new construction prevents that problem from being baked into the build.

Who has to pass it: Every new residential construction in Marion County, plus replacement HVAC systems where ducts are altered.

Why You Need Both Tests

Some builders ask why both tests are required since they seem related. The answer is they’re measuring different things:

Envelope leakage measures air movement between inside and outside the conditioned space.

Duct leakage measures air movement between the duct system and anywhere outside that duct system.

A home can have an extremely tight envelope (1.5 ACH50) and still have leaky ducts that lose 15% of conditioned air to the attic. The envelope test wouldn’t catch that — the air leaving the ducts technically went into the attic, which the envelope test treats as outdoors. So the duct test is needed to catch the duct-specific leakage.

Conversely, a home can have very tight ducts (2 CFM25 per 100 sq ft) and still have a leaky envelope (7 ACH50) because of penetrations that have nothing to do with the duct system. The duct test wouldn’t catch envelope leakage — different system entirely.

The code requires both because both are sources of energy loss and both contribute to HVAC system inefficiency.

When Each Test Is Performed

Duct leakage test (rough-in): After ducts are installed but before drywall and insulation are completed. At this stage, the ducts are accessible and any leaks can be sealed before being hidden. This is the more common test schedule in Florida.

Duct leakage test (final / leakage to outside): Performed after the home is fully complete, with the air handler running. The test measures only leakage to outside the conditioned space (ducts inside the envelope don’t count). This test is harder to set up but sometimes required for specific code paths.

Envelope leakage test (blower door): After drywall is hung and finished, after exterior doors and windows are installed, after all bath fans and other exhaust ducts are installed. Typically at the trim-out / paint-ready stage. Before final inspection. See our test day walkthrough for what the visit looks like.

Common Causes of Each Test Failing

Envelope test failures are usually caused by:

1. Unsealed plumbing stack penetrations
2. Bath fan ducts terminating in the attic
3. Top plate gaps
4. Non-IC-AT recessed lights
5. Attic access hatches without weatherstripping

Duct test failures are usually caused by:

1. Plenum connection leaks at the air handler
2. Branch duct connections sealed with cloth tape that’s degraded
3. Register boot connections where the boot meets the ceiling drywall
4. Supply runs joined with screws but not mastic-sealed
5. Return air panned into floor joists (sometimes prohibited per Florida code)

The two failure patterns are completely different because the systems are completely different. A team that’s great at building tight envelopes might still have leaky ducts, and vice versa.

How B&I Testing Performs Both Tests for Marion County

B&I Testing performs both blower door and duct leakage tests throughout Marion County using ANSI/RESNET/ICC 380 procedures and calibrated Energy Conservatory equipment. We can perform both tests on the same site visit if scheduling allows (typically duct test at rough-in, blower door at final).

Each test produces a signed report that meets Marion County’s certificate of occupancy requirements.

If you need either test (or both) for an Ocala project, call (352) 756-2986, email brandon@banditesting.com, or reach us through the contact form.

Frequently Asked Questions

Are envelope leakage and duct leakage the same thing? No. They measure different systems. Envelope leakage is air movement through the home’s exterior shell. Duct leakage is air movement out of the HVAC duct system specifically.

Can one test be substituted for the other? No. The 2023 Florida Energy Code requires both tests for new construction. They’re independent compliance checks.

Which test is more likely to fail in Florida new construction? Both can fail. In our experience at B&I Testing, blower door failures are slightly more common than duct test failures, mostly because more trades touch the envelope (plumbing, electrical, HVAC, framing) than touch the ducts (typically just HVAC).

Do I need separate technicians for each test? No. Most testing companies, including B&I Testing, perform both tests with the same technician using different equipment.

What if I only need one test? For new construction in Florida, you’ll almost certainly need both. For renovations or partial additions, requirements vary. Call (352) 756-2986 to discuss your specific project.