Penetrations Near Flues and Chimneys

Why this matters

The space around a flue where it passes through a ceiling, a floor or a roof looks like the most obvious air leak in the house, and it is. It is also the one gap that must not be filled with foam, caulk, insulation or wood, because it is a required air space between something hot and something that burns. Insulation crews, air-sealing crews and every other trade passing through the attic are tempted to close it. When they close it with the wrong thing, they have built a slow fire: wood and foam held against a hot surface for years until the day the appliance runs long or the vent runs hotter than usual.

The rule in one line

Anything inside the clearance must be noncombustible, and the clearance comes from the listing or the code for that vent type. Every detail below is that line applied to a particular vent.

Clearances by vent type

These are the figures a tech meets most. Each carries its owner; where the listing and a general figure disagree, the listing on the vent in front of you governs.

Two reading notes. First, "commonly" means you read the label. Type B pipe carries its clearance printed on its outer jacket; a factory-built chimney has a metal label near the ceiling support. Second, the 1 inch and 2 inch figures are air space, not a gap to be packed. R1003.18 says the air space around a masonry chimney is not to be filled, except for the noncombustible fireblocking at floors and ceilings that R1003.19 requires.

The condensing appliance row is the exception that proves the rule. A PVC or polypropylene vent from a high-efficiency furnace carries cool, wet exhaust, and the appliance manual commonly permits zero clearance and ordinary sealants. That vent is still a vent, and its manufacturer's instructions govern how it passes through each layer.

Why foam near a flue is a fire

Spray and can foams are combustible plastics. Wood, kraft facing, cardboard baffles and cellulose are combustible. A Type B vent's outer jacket runs warm to hot whenever the appliance fires, and hotter during a long firing cycle, when the vent is partly blocked, or when a connector comes loose and the vent is starved of dilution air. Its listing clearance was set by testing at those worse conditions, which is why a gap that looks generous on a mild day is not slack.

There is also slow damage. Wood held for years near a hot surface darkens and chars at its face, a process called pyrolysis, and fire investigators commonly treat that charred layer as more easily ignited than fresh wood. That is the honest reason "it has been fine for ten years" is not evidence that it is fine. Foam adds a second problem: when it does burn, it gives off dense toxic smoke.

What may go in the clearance, and what may not

May:

• Sheet metal, cut to fit around the vent and fastened to the framing, to close the ceiling or floor opening around it. Sheet metal is noncombustible, so it may come closer to the vent than the clearance allows wood to.
• High-temperature sealant rated for the purpose, at the joint between the sheet metal and the vent or chimney.
• The vent manufacturer's listed firestop spacer at each floor and ceiling a Type B vent passes through, and its attic insulation shield where its instructions call for one, which keeps loose insulation out of the air space.
• Noncombustible fireblocking at a masonry chimney's floor and ceiling lines, self-supporting or laid on metal strips or lath across the gap (R1003.19).

May not: foam of any kind, ordinary caulk, fiberglass with a facing, cellulose, wood blocking, cardboard or plastic baffles, or duct tape. Unfaced mineral wool is noncombustible but is still insulation sitting in a required air space, so it does not belong there unless the listing allows it.

Ordinary foam or caulk is fine outside the clearance: at the joint where your sheet metal collar meets wood framing, for example, which is already the required distance from the vent.

The same rule at the roof and the wall

The clearance does not end at the attic floor. Where a Type B vent leaves through the roof, the hole cut in the deck has to keep the listed air space to the wood, and the vent gets its own listed roof flashing and storm collar, both metal. Reroofing crews are the usual offenders here: underlayment and shingles get trimmed tight against the vent, or a rubber pipe boot gets pulled over it because it was on the truck. Both put combustibles inside the clearance at the hottest part of the vent's exposed run. See related: How to Seal a Roof Penetration That Will Not Leak.

A direct-vent appliance terminating through a sidewall carries two different sets of numbers, and they get confused: a clearance to combustibles at the wall pass-through, set by the vent kit, and clearances from the terminal to windows, doors, air intakes and grade, set by the appliance manual and the fuel gas code (IFGC or NFPA 54, as adopted). Sealing the wall penetration well does nothing for the second set.

Worked example: a water heater job that found two problems

An HVAC tech replacing a gas water heater on an existing 4 inch Type B vent goes up into the attic to check the vent's route, which is good practice whenever the appliance under a vent changes. Before going up, the old heater was off and its vent cool, and a personal CO monitor was clipped on for the work around the appliance below.

Problem one: foam against the B-vent. An insulation contractor had foamed the ceiling penetration around the vent, and the foam was bonded directly to the vent's outer jacket for about 6 inches up from the ceiling. The label on the pipe printed a 1 inch clearance. Measured clearance: zero. Required: 1 inch. Short by the full 1 inch.

The tech looked first for darkening on the foam's inner face and on the nearby joist: none. Then the fix:

• Cut the foam away from the jacket with a hand saw and knife. No heat: hot-knifing polyurethane foam releases toxic gases.
• Cleared the foam back from a 4 inch vent with a 1 inch ring, so nothing combustible remained within 1 inch of the jacket.
• Cut a sheet metal plate to cover the ceiling opening, fastened it to the joists, sealed the metal to the vent jacket with high-temperature sealant, and sealed the metal-to-wood edges with ordinary foam, since those edges were well over 1 inch from the vent.
• Wrapped a sheet metal dam around the vent rising above the insulation level so the blown cellulose could not slide back into the air space.

Problem two: the chimney framing. The house also has an interior masonry chimney for a fireplace. The attic floor header framing around it sat 3/4 inch from the chimney face. The code air space for an interior masonry chimney is 2 inches, so the framing is 1-1/4 inches too close (2 minus 3/4). That is how the house was built, not something the tech did, and moving framing is not a water heater call. The tech confirmed no insulation or debris was packed in the gap, photographed it with a tape in frame, and wrote it up for the owner as a recommendation for a chimney professional to evaluate. The tech did not close the gap, because sealing a too-small air space hides it.

Back downstairs. New heater in, vent connected, then the draft checked at the draft hood during a firing with the burner running several minutes, and the CO monitor read in the room. Both results went on the ticket.

What changes the answer

• Category III or IV vents. Stainless special-gas vent and plastic vent systems follow their own listings and the appliance manual, which can permit zero clearance or demand more. Read the manual before you seal anything around them.
• A vent in a rated assembly. A vent through a garage ceiling or a unit separation also needs the rating restored, with a method that respects the vent's clearance. See related: Ductwork Through Fire-Rated Assemblies; Dwelling Unit Separations in Townhouses and Duplexes.
• Signs of heat damage. Charred wood, melted foam or scorched facing around a vent mean the appliance or the vent has been running hot. That is a combustion and venting diagnosis, not an air-sealing task, and the appliance stays off until it is understood.
• Solid-fuel appliances. Wood stove and fireplace connectors run far hotter and carry an 18 inch single-wall clearance. Treat anything there as a chimney professional's call.

How to verify you got it right

With a tape, measure from the vent's outer surface to the nearest combustible at every side, at the ceiling line and just above the insulation, and compare against the clearance printed on the vent or set by the code row above. Write the measured figure next to the required one. Confirm the only materials inside that ring are metal, listed parts and high-temperature sealant, and that insulation is held back by a dam or shield. Photograph the finished collar from two sides. See related: The Penetration Record SOP.

References

• IRC R1003.18 and R1003.19 (masonry chimney clearance and fireblocking) and M1803.3.4 (solid-fuel connectors), as adopted by your jurisdiction.
• The fuel gas code your jurisdiction has adopted (IFGC or NFPA 54) for vent and connector clearances; vent and chimney listings to UL 441 and UL 103.
• Appliance, vent and chimney manufacturers' installation instructions, which govern listed clearances.
• See related: How to Air Seal Penetrations Into the Attic; How to Restore Attic Insulation After You Work in It; Fireblocking and Draftstopping.