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2021 IECC Section C402.5

Lesson 5: Air Leakage and Air Barriers

IECC C402.5, Table C402.5.4 · 14 practice questions

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Reading

Air that leaks through an envelope carries heat, humidity, and dust with it, day and night. Section C402.5 attacks the problem with a continuous air barrier that is verified or tested, and with separate rules for the places where leakage concentrates: windows and doors, loading docks, vestibules, recessed lights, and large operable openings. Several of these rules stop at Zone 2 or start above it, so this lesson flags which apply in Zone 2A. Local adoption governs the edition (Lesson 1).

Continuous air barrier critical joints and a vestibule plan Left: a wall and roof section with a continuous air barrier line running along the roof deck, down the exterior wall face, and across the slab. Four numbered critical joints are marked: one, the roof to wall joint; two, a pipe through the wall; three, the window frame perimeter; four, the wall to slab joint. Right: a plan of a vestibule between the outside and the conditioned space, with an outside door and an inside door that are each self-closing and never need to be open at the same time. The vestibule is required in Climate Zones 3 and colder and is exempt in Zones 0 to 2, including Houston in Zone 2A. Continuous air barrier: critical joints and a vestibule plan 1 2 3 4 Outside grade Inside, conditioned space 1 Roof to wall joint sealed across the transition 2 Pipe through the wall sealed around the penetration 3 Window or storefront frame sealed to the wall on all sides 4 Wall to slab or foundation sealed at the base Air barrier line: any break is a leak path Vestibule plan (C402.5.9) Where required: Zones 3 and colder Vestibule Conditioned space Outside Inside door Outside door Rules Enclosed, with self-closing doors Walking through never needs both doors open at the same time Exempt, among others Zones 0 to 2 (Houston is 2A) Spaces under 3,000 sq ft Revolving doors, employee doors
Left: the air barrier is one unbroken line, and the four numbered joints are where it usually breaks. Right: a vestibule is a two-door airlock. It is a Zone 3 and colder rule, so it is shown here for projects such as Pulaski County, Arkansas (Zone 3A), and not for the Houston area.
Trace the air barrier through its transitionsA schematic section highlights the roof-to-wall junction, a sealed penetration, the window perimeter and the wall-to-slab junction. The lines identify places to check and are not a construction detail. Roof-to-wall jointSeal the transitionPenetrationsSeal around the openingWindow perimeterKeep the barrier continuousWall-to-slab jointInspect before concealmentSchematic only. C402.5.1.1.
Trace the air barrier at the four critical joints described in this lesson. Inspect while the barrier is accessible for repairs; use compatible sealing materials and the applicable installation instructions.

Two ways to comply

Section C402.5 lets the envelope comply by meeting Sections C402.5.1 through C402.5.11.1, or by passing a test under C402.5.2 or C402.5.3. A building that complies by testing must still meet the damper, loading dock, and vestibule sections (C402.5.7, C402.5.8, C402.5.9), so testing is not a way around the entrance and dock rules.

The continuous air barrier

C402.5.1 calls for a continuous air barrier throughout the thermal envelope. It may sit on the inside or outside, inside the assemblies, or be a mix. One exception removes the requirement in Zone 2B, the hot-dry zone. Houston is Zone 2A, so the exception does not apply. The barrier must be continuous across every joint and assembly (C402.5.1.1), and its joints, seams, and transitions between materials must be sealed so they stay in place under wind, stack effect, and fan pressure.

The critical joints

Whole walls rarely leak; the edges do. The figure marks the four spots inspectors watch: the roof-to-wall joint, penetrations, window and storefront frames, and the wall-to-slab joint. C402.5.1.1 requires penetrations to be caulked, gasketed, or otherwise sealed with a product compatible with the materials and able to handle expansion, contraction, and vibration. Joints around a penetration are sealed the same way or taped. Concealed sprinkler penetrations are sealed as the manufacturer recommends, and caulk or adhesive sealant may not be used to fill voids around sprinkler cover plates. Recessed lights and similar objects must keep the barrier intact (C402.5.1.1 and C402.5.10).

Materials and assemblies

Without a test, the barrier is proven by materials or assemblies. A material must leak no more than 0.004 cfm per square foot at 0.3 inch of water (75 Pa) under ASTM E2178 (C402.5.1.3). The code also deems a list of materials to comply when joints are sealed and manufacturer instructions are followed. A few of them:

Deemed air barrier material (C402.5.1.3)Minimum
Plywood or oriented strand board3/8 inch thick
Gypsum board, interior or exterior1/2 inch thick
Extruded polystyrene or foil-back polyisocyanurate board1/2 inch thick
Closed-cell spray foam (density at least 1.5 pcf)1.5 inches
Open-cell spray foam (density 0.4 to 1.5 pcf)4.5 inches
Cast-in-place or precast concrete; fully grouted concrete blockNo thickness stated

An assembly, meaning several layers that work together, must average no more than 0.04 cfm per square foot at 75 Pa (C402.5.1.4). Deemed assemblies include concrete block walls coated with block filler or two coats of paint or sealer, clay masonry at least 4 inches nominal, and a parge, stucco, or plaster coat at least 1/2 inch thick.

Verification and testing

Where the barrier is not tested, C402.5.1.5 requires three steps by the code official, a registered design professional, or an approved agency: review the construction documents, inspect the air barrier during construction while it is still accessible for repair, and, when the design professional or agency did the inspecting, give the owner and the code official a final commissioning report listing deficiencies and corrections.

Testing has two forms. Dwelling and sleeping units are tested at 50 Pa and may leak no more than 0.30 cfm per square foot of unit enclosure (C402.5.2). Where a building has fewer than eight units, every unit is tested. With eight or more, the greater of seven units or 20 percent are tested, including a top-floor unit, a ground-floor unit, and the largest unit, and each failing unit adds two more tests. Other buildings are tested at 75 Pa and may leak no more than 0.40 cfm per square foot of envelope area (C402.5.3). If the result lies between 0.40 and 0.60, a diagnostic with smoke tracer or infrared imaging, sealing of leaks, and a report satisfy the section.

Worked example. A small clinic has 6,000 square feet of envelope area. A fan test reads 2,100 cfm at 75 Pa: 2,100 / 6,000 = 0.35, which passes. A reading of 2,700 cfm gives 0.45, which is over 0.40 but not over 0.60, so the diagnostic route applies. A reading of 3,900 cfm gives 0.65 and fails. For an apartment building with 40 units, 20 percent is 8, which is more than 7, so 8 units are tested; with 30 units, 20 percent is 6, so the seven-unit floor governs.

Which buildings must test in Zone 2A? In the text of C402.5.1.2, Group R and I buildings go to unit testing with no size exception in Zone 2A. Other groups go to whole-building testing, but buildings over 5,000 square feet are excepted in Zone 2A. Buildings that do not test meet the materials or assemblies path plus verification. Because these items interlock, ask the code official which path a project is using.

Fenestration and door leakage

Products must be lab-tested and labeled for air leakage (C402.5.4). Field-fabricated assemblies sealed under C402.5.1 are exempt, as are buildings that comply through the testing alternative. Table C402.5.4 sets maximums in cfm per square foot, and a note lets windows, sliding doors, and swinging doors show 0.3 instead when tested at 300 Pa:

Product (Table C402.5.4)Maximum leakage
Windows, sliding doors, swinging doors0.20
Curtain walls and storefront glazing0.06
Commercial glazed swinging entrance doors1.00
Revolving doors1.00
Garage doors0.40
High-speed doors1.30

Docks, shafts, dampers, and equipment rooms

Loading dock openings need weather seals that touch the top and sides of a parked vehicle (C402.5.8). Doors from conditioned space to shafts, chutes, stairways, and elevator lobbies must be gasketed, weather-stripped, or sealed, with exceptions for fire-rated and smoke-control doors (C402.5.6). Stairway and elevator shaft vents and other outdoor air intakes and exhausts need dampers (C402.5.7). Rooms with fuel-burning appliances that draw combustion air from outside must be outside the envelope or isolated, but that rule applies only in Zones 3 through 8 (C402.5.5).

Key numbers
  • Barrier exception: Zone 2B only (C402.5.1)
  • Material: 0.004 cfm/sq ft at 75 Pa; assembly: 0.04 cfm/sq ft at 75 Pa (C402.5.1.3, C402.5.1.4)
  • Whole-building test: 0.40 cfm/sq ft at 75 Pa; diagnostic route from 0.40 to 0.60 (C402.5.3)
  • Dwelling unit test: 0.30 cfm/sq ft at 50 Pa; eight or more units test the greater of 7 or 20 percent (C402.5.2)
  • Storefront and curtain wall: 0.06; windows and doors: 0.20 (Table C402.5.4)
  • Vestibules: not required in Zones 0 to 2; spaces under 3,000 sq ft exempt (C402.5.9)
  • Recessed lights: IC-rated, 2.0 cfm maximum at 75 Pa, sealed to the finish (C402.5.10)
  • Operable opening over 40 sq ft: 90 F cooling and 55 F heating setpoints within 10 minutes (C402.5.11)

Vestibules

Building entrances must be protected by an enclosed vestibule with self-closing doors, designed so a person passing through never needs both doors open at once (C402.5.9). A revolving door does not excuse the vestibule on the swinging doors beside it. The exceptions are broad: buildings in Zones 0 through 2, doors not used by the public, dwelling and sleeping unit doors, doors from a space under 3,000 square feet, revolving doors, vehicle and material-handling doors, and doors with a tested air curtain of at least 6.56 feet per second at the floor. In Houston, Zone 2A, no vestibule is required. A project in Pulaski County, Arkansas, is Zone 3A, so the rule can apply there.

In the field

A community center rehab in Zone 2A is stripped to the studs. The inspector walks it before drywall, because C402.5.1.5 calls for inspection while the air barrier can still be repaired. She looks for taped sheathing seams, sealed top-plate and sill gaps, foam around pipe and conduit penetrations, sprinkler drops sealed as the manufacturer says (not caulked at the cover plate), and airtight IC-rated recessed cans. After the ceiling closes, only a fan test can prove any of it, so the walk-through is the cheap moment to find a gap.

Recessed lighting and large operable openings

Recessed luminaires in the envelope must be IC-rated, labeled for no more than 2.0 cfm of leakage at 75 Pa (ASTM E283), and sealed with a gasket or caulk between the housing and the ceiling or wall finish (C402.5.10). Separately, where a space has operable openings to the outdoors larger than 40 square feet, such as a restaurant's overhead glass door, the opening must be interlocked with the HVAC so that opening it raises the cooling setpoint to 90 F and lowers the heating setpoint to 55 F within 10 minutes (C402.5.11). Exceptions cover restaurant food-preparation zones, warehouses with overhead doors where approved, and the first entrance doors of a vestibule.

Practice questions

1. Which climate zone is excused from the continuous air barrier requirement of IECC Section C402.5.1?

  1. Zone 2A, so buildings in Houston and the rest of the Gulf Coast counties are excused from the requirement
  2. Zones 0 through 2, the same zones that are excused from the vestibule rule in Section C402.5.9
  3. Zone 2B, so a Houston building in Zone 2A still needs a continuous air barrier
  4. Zone 3A and colder only, where the vestibule and dock seal rules take over from the barrier rule

2. What is the maximum air permeability of a material used as a continuous air barrier under Section C402.5.1.3?

  1. 0.04 cfm per square foot at 75 Pa, the same as for assemblies
  2. 0.004 cfm per square foot at 75 Pa
  3. 0.40 cfm per square foot at 75 Pa, the whole-building limit
  4. 0.30 cfm per square foot at 50 Pa, the dwelling unit limit

3. In a rehab, an inspector finds that the wall sheathing is 1/4 inch plywood with taped seams and the plans rely on it as the air barrier material. What is the finding?

  1. It is thinner than the 3/8 inch deemed plywood, so it needs test data or a different material
  2. It complies, because any plywood with taped seams is deemed to comply as an air barrier material
  3. It complies, because plywood need only be 1/4 inch thick when the seams are taped and the panels are nailed
  4. It fails only if the building is in Zone 4 or colder, where the code tightens the requirement

4. A 5,000 square foot Group B clinic in Zone 2A has a fan test reading of 1,800 cfm at 75 Pa. The envelope area is 5,000 square feet. What is the result under Section C402.5.3?

  1. 0.36 cfm per square foot, which fails because the limit is 0.30
  2. 0.36 cfm per square foot, which passes the 0.40 limit
  3. 2.8 cfm per square foot, which fails the limit
  4. 0.36 cfm per square foot, which passes only with a diagnostic

5. A Group B office fan test gives 0.52 cfm per square foot at 75 Pa. What does Section C402.5.3 allow?

  1. Nothing at all, because any measured result over 0.40 cfm per square foot fails the section outright
  2. Compliance with no further work at all, because 0.52 is under the 0.60 upper limit of the section
  3. Compliance only if the building is under 5,000 square feet and a second test is done later
  4. Compliance by a diagnostic with smoke tracer or infrared imaging, sealing of leaks, and a report

6. A new Group R-2 apartment building in Zone 2A has 40 dwelling units. How many units must be tested under Section C402.5.2?

  1. 8 units, because 20 percent of 40 is greater than 7
  2. 7 units, because seven is the required number for any building with eight or more units
  3. 40 units, because every unit in a multifamily building must be tested individually
  4. 4 units, because 10 percent of the units is enough when the building is new

7. A dwelling unit with 1,500 square feet of enclosure area reads 420 cfm on a blower door test at 50 Pa. What is the result?

  1. 0.28 cfm per square foot, which fails because the limit is 0.20
  2. 2.8 cfm per square foot, which fails the limit
  3. 0.28 cfm per square foot, which passes the 0.30 limit
  4. 0.28 cfm per square foot, which passes only at 75 Pa

8. A 12,000 square foot Group B office building is planned in Zone 2A. Is whole-building air leakage testing under C402.5.3 required?

  1. Yes, because every commercial building in Zone 2A must be tested under C402.5.3, regardless of size or group
  2. No, because buildings over 5,000 square feet in Zone 2A are excepted; the barrier, materials, and verification rules still apply
  3. No, and no air barrier is needed either, because large buildings in Zone 2A are exempt from C402.5.1 entirely
  4. Yes, but only if the building has a loading dock, in which case the test must cover the dock stories

9. What is the maximum air leakage for storefront glazing under Table C402.5.4?

  1. 0.20 cfm per square foot
  2. 1.00 cfm per square foot
  3. 0.40 cfm per square foot
  4. 0.06 cfm per square foot

10. A 4,000 square foot retail store in Zone 2A has a single front entrance. Does IECC Section C402.5.9 require a vestibule?

  1. Yes, because retail entrances that serve the public always need a vestibule in every zone
  2. Yes, because the entrance door opens from a space larger than 3,000 square feet
  3. No, because the vestibule requirement does not apply in Zones 0 through 2
  4. No, but only because the store is under 5,000 square feet in floor area

11. An inspector finds recessed downlights in the insulated ceiling of a Zone 2A conference room. The cans are IC-rated but have no leakage label and no seal at the trim. What is the finding?

  1. They fail C402.5.10: they need an air leakage label of 2.0 cfm or less and a gasket or caulk to the ceiling finish
  2. They comply, because IC rating alone is enough and the code adds no separate leakage or sealing rule
  3. They comply, because the leakage label and sealing rules apply only in Zone 3 and colder climates
  4. They fail only because they are not vapor tight, which is a moisture rule and not an air rule

12. A restaurant in Zone 2A has a 12 foot by 8 foot overhead glass door that opens to a patio. Under Section C402.5.11, what must the controls do?

  1. Shut off the whole building HVAC system within 10 minutes of the door being opened for any reason
  2. Raise the cooling setpoint to 90 F and lower the heating setpoint to 55 F within 10 minutes of opening
  3. Raise the cooling setpoint to 78 F and lower the heating setpoint to 68 F within 30 minutes of opening
  4. Nothing, because the interlock rule applies only to openings larger than 100 square feet in area

13. A sprinkler contractor fills a gap between a concealed sprinkler cover plate and the ceiling with caulk. What does Section C402.5.1.1 say?

  1. Caulk is required at every cover plate so that no gap is left between the plate and the ceiling
  2. Sprinkler penetrations are exempt from all of the air barrier sealing rules in the section
  3. Caulk is allowed at cover plates if it is rated for movement and applied by a licensed contractor
  4. Caulk or adhesive sealant may not be used to fill voids between cover plates and walls or ceilings

14. An 8,000 square foot Group M pharmacy in Climate Zone 3A has one public swinging-door entrance opening from the sales floor. Under Section C402.5.9, what does the entrance need?

  1. An enclosed vestibule with self-closing doors, unless an exception such as a tested air curtain applies
  2. Nothing, because vestibules are never required for Group M occupancies
  3. Nothing, because the vestibule requirement stops at Zone 3A
  4. A revolving door only, because it replaces the vestibule at the same entrance

Glossary

Air barrier
One or more materials joined together in a continuous manner to restrict the passage of air through the building thermal envelope.
Thermal envelope
The exterior walls, roof, and floors that separate conditioned space from the outdoors and unconditioned space.
Blower door test
A fan installed in a doorway that pressurizes or depressurizes a unit to measure how much air leaks through the envelope.
cfm per square foot
Cubic feet of air per minute leaking per square foot of envelope, the unit used for the leakage limits.
Vestibule
An enclosed entrance space between two sets of self-closing doors that works as an airlock.
IC-rated luminaire
A recessed light fixture approved for direct contact with insulation.
Air curtain
A device at an entrance that blows a stream of air across the opening to reduce infiltration.
Diagnostic evaluation
A survey with smoke tracer or infrared imaging used to find and seal leaks after a marginal test result.
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