7.4 Fireplaces, Recessed Luminaires, Air-Sealed Boxes, Combustion Rooms & Vapor Retarders (R402.4.2–R402.4.6, R402.1.1)

7.4 Fireplaces, Recessed Luminaires, Air-Sealed Boxes, Combustion Rooms & Vapor Retarders (R402.4.2–R402.4.6, R402.1.1)

Quick Answer: New wood-burning fireplaces shall have tight-fitting flue dampers or doors and outdoor combustion air; UL 127 factory-built fireplace doors shall be tested and listed for that fireplace. In Climate Zones 3 through 8, open-combustion fuel-burning appliances with open combustion-air ducts shall be outside the thermal envelope or in a sealed, insulated room meeting basement-wall R-values from Table R402.1.3, with a fully gasketed door; a combustion-air duct through conditioned space shall be at least R-8. Recessed luminaires in the envelope shall be IC-rated, labeled ≤ 2.0 cfm at 1.57 psf (75 Pa) per ASTM E283, and gasketed or caulked to the finish. Air-sealed boxes shall meet NEMA OS 4 at the same 2.0 cfm / 75 Pa cap. Wall vapor retarders follow IRC R702.7 or IBC 1404.3; Class I films are not the air barrier on below-grade walls.

R402.4.3 fenestration leakage (0.3 / 0.5 cfm/ft²) is Chapter 6. This section is the rest of R402.4 plus the moisture pointer in R402.1.1. Independent OpenExamPrep material for ICC Exam 79 uses the national 2021 IECC first printing.

The charging sentence in R402.4 still says "R402.4.1 through R402.4.5" even though R402.4.6 air-sealed boxes was added in 2021. Score R402.4.6 anyway; it is a real subsection.

R402.4.2 Fireplaces

New wood-burning fireplaces shall have:

1. Tight-fitting flue dampers or doors, and
2. Outdoor combustion air.

Where using tight-fitting doors on factory-built fireplaces listed and labeled in accordance with UL 127, the doors shall be tested and listed for the fireplace. An aftermarket glass door that was never listed with that firebox does not satisfy the sentence. Site-built masonry fireplaces still need the tight-fitting damper or door and the outdoor combustion-air supply.

Outdoor combustion air is a dedicated outdoor-air path to the firebox, not "the room is leaky so the fire can breathe." A fireplace that draws replacement air from the house fights the air barrier you just tested in R402.4.1.2.

R402.4.4 Rooms containing fuel-burning appliances

This section applies in Climate Zones 3 through 8 where open combustion air ducts provide combustion air to open-combustion fuel-burning appliances. Those appliances and the combustion-air opening shall be located outside the building thermal envelope or enclosed in a room that is isolated from inside the thermal envelope.

If a room is used, all of the following apply:

• The room shall be sealed and insulated in accordance with the envelope requirements of Table R402.1.3.
• Walls, floors, and ceilings of that room shall meet not less than the basement-wall R-value in that table.
• The door into the room shall be fully gasketed.
• Water lines and ducts in the room shall be insulated in accordance with Section R403.
• The combustion air duct shall be insulated where it passes through conditioned space to not less than R-8.

Use the basement-wall cell, not the wood-frame wall cell. A Climate Zone 5 mechanical room with only R-13 cavity and no continuous insulation fails that row; Climate Zone 5 basement walls are 15ci or 19 or 13&5ci, not the CZ4-except-Marine 10ci or 13 pair and not the wood-frame R-30 / 20&5ci menu.

Exceptions

1. Direct-vent appliances with both intake and exhaust pipes installed continuous to the outside.
2. Fireplaces and stoves complying with R402.4.2 and IRC R1006.

A sealed-combustion furnace with coaxial or two-pipe venting is Exception 1. It does not need the isolated room. An atmospheric water heater with a combustion-air duct from outdoors into a laundry room that is inside the envelope, in Climate Zone 5, does. Either move the appliance and its opening outside the envelope, or build the gasketed, basement-R, R-8-duct room.

Climate Zones 0 through 2 are outside R402.4.4. Do not fail a CZ2 open-combustion water heater against this section. You may still fail it against mechanical or fuel-gas rules; those are not this energy section.

R402.4.5 Recessed luminaires

Recessed luminaires installed in the building thermal envelope shall be sealed to limit air leakage between conditioned and unconditioned spaces. They shall be IC-rated and labeled as having an air leakage rate not greater than 2.0 cfm (0.944 L/s) when tested in accordance with ASTM E283 at 1.57 psf (75 Pa). They shall be sealed with a gasket or caulked between the housing and the interior wall or ceiling covering.

Table R402.4.1.1 already required airtight, IC-rated cans that are buried or surrounded with insulation. R402.4.5 adds the number, the test standard, the label, and the housing-to-finish seal. IC-rated means the fixture is listed to be in contact with insulation. A non-IC can that needs a 3-inch clearance cannot be buried and does not belong in an insulated envelope assembly.

Field scenario: unlabeled can light

A cathedral ceiling is part of the thermal envelope. The electrician installed six recessed housings. None of them has an IC mark or a leakage label. The contractor offers to caulk the trim rings at final.

Reject the installation. Envelope recessed luminaires shall be IC-rated and labeled at not greater than 2.0 cfm at 1.57 psf (75 Pa) per ASTM E283. Caulking unlabeled, non-IC housings does not create that listing. Replacement with labeled airtight IC cans, gasketed or caulked to the interior covering, and then buried or surrounded with insulation, is the repair. Surface-mounted or IC-rated airtight remodel housings listed for the assembly are the usual field fix; empty "can-less" wafer lights still have to be evaluated as envelope penetrations if they punch the air barrier.

Do not confuse this 75 Pa / 2.0 cfm can test with the 50 Pa blower-door test. They are different pressures and different objects.

R402.4.6 Electrical and communication outlet boxes (air-sealed boxes)

Electrical and communication outlet boxes installed in the building thermal envelope shall be sealed to limit air leakage between conditioned and unconditioned spaces. They shall be tested in accordance with NEMA OS 4, Requirements for Air-Sealed Boxes for Electrical and Communication Applications, and shall have an air leakage rate not greater than 2.0 cfm at 1.57 psf (75 Pa). They shall be marked "NEMA OS 4" or "OS 4" in accordance with NEMA OS 4. They shall be installed per the manufacturer's instructions and with any supplied components required to achieve compliance with NEMA OS 4.

The 2.0 cfm / 75 Pa number matches the can-light cap, but the standard is NEMA OS 4, not ASTM E283. An unmarked plastic box with a foam gasket the electrician cut from a door-sweep is not OS 4. Missing the supplied flange, membrane, or cable-entry seals is an installation fail even if the box body is marked OS 4.

Table R402.4.1.1 still lists an air barrier behind the box as an alternative to air-sealed boxes on exterior walls. When the box itself is the sealing strategy — and whenever R402.4.6 applies to boxes in the thermal envelope — score the NEMA OS 4 mark, the 2.0 cfm rating, and the kit parts. When the air barrier continues behind a standard box so the box is not a hole, you are on the table's first method.

R402.1.1 vapor retarder, and Class I on below-grade walls

R402.1.1: wall assemblies in the building thermal envelope shall comply with the vapor retarder requirements of IRC Section R702.7 or IBC Section 1404.3, as applicable. The IECC does not reprint the Class I / II / III tables. Open the occupancy-appropriate building code. IRC R702.7 for typical IRC dwellings; IBC 1404.3 when the residential building is under the IBC.

Table R402.4.1.1 adds the air-barrier conflict that Chapter 4 only flagged: Class I vapor retarders shall not be used as an air barrier on below-grade walls, and they shall be installed per IRC R702.7. A 6-mil polyethylene sheet on the interior of a basement wall may be a Class I vapor retarder where R702.7 requires or allows that class. It is not the air barrier. The air barrier on that wall has to be a material or coating that actually seals (concrete that is detailed as the barrier, a listed membrane, sealed gypsum, or another continuous barrier). Using the polyethylene as both "the vapor retarder and the air barrier" is a Table R402.4.1.1 fail on below-grade walls.

In an unvented crawl, the Class I covering on exposed earth is the opposite case: Table R402.4.1.1 does call that covering a vapor retarder/air barrier, installed per R402.2.10. Do not apply the below-grade-wall prohibition to the crawl-earth sheet.

Inspection sequence

1. Wood-burning fireplace: damper or listed doors, plus outdoor combustion air; UL 127 doors listed for that firebox.
2. Climate Zones 3–8 open-combustion appliances: outside the envelope, or a gasketed basement-R room, with combustion-air ducts through conditioned space at R-8. Skip this section for direct-vent and for fireplaces already under R402.4.2 / IRC R1006.
3. Envelope cans: IC, ≤ 2.0 cfm ASTM E283 label, gasketed or caulked to the finish, buried or surrounded.
4. Envelope boxes: NEMA OS 4 or OS 4 mark, ≤ 2.0 cfm, supplied components — or a continuous air barrier behind the box.
5. Walls: IRC R702.7 or IBC 1404.3 vapor-retarder class and location; no Class I film counted as the below-grade wall air barrier.

Independent OpenExamPrep study material for Exam 79 treats an unlabeled can in a cathedral ceiling as a listing fail, not a caulk-and-pass item.