Island Sink Venting: Loop Vents and the AAV Option

Why this matters

An island sink has no wall behind it to run a vent up, and that is where kitchen remodels go wrong: a vent that rises a few inches and stops, a loop built with sanitary tees that fills with grease, a foot vent with no cleanout buried under the floor. Both model codes have a specific detail for it, they write it differently, and only one of them accepts an air admittance valve as the easy way out. This card draws the loop vent, states what each code asks for in substance, and lays out one island from rough-in to the vent tie-in. The step-by-step install of an AAV where it is allowed belongs to 'How to Install an Air Admittance Valve Where It Is Allowed'.

The loop vent

The idea is the same under both codes: the vent rises from the fixture drain as high as the cabinet allows, turns and comes back down, joins the drain below the floor, and gets its air from a foot vent that runs to a wall and up to the vent system. Because the looped section sits below the sink's flood level rim, it can fill with water during a stoppage, so it is built like a drain.

                 +----------+  loop top, close
   SINK          |          |  under the counter
    |            |          |
   trap          |          |
    |     vent ->|          |  return vent, down,
    +=====>=====tee         |  drainage fittings
     trap arm    |          |
  ===============|==========|========== FLOOR
                 |          +--wye--> FOOT VENT to
      fixture    |          |   CO    wall, then up
      drain      |          |         to vent system
                 +====>=====wye======> DRAIN
                            return    to stack,
                            joins     1/4 in per ft

Parts, named once. The vent riser leaves the fixture drain at the tee and rises. The loop top is the return bend under the counter. The return vent comes back down through the floor. The foot vent branches off the return below the floor and runs to the nearest wall, where it rises to join the vent system. The cleanout (CO) sits on the foot vent so every vent pipe below the flood level rim can be rodded.

What the IPC asks, in substance

IPC 2021 Section 916, island fixture venting, as adopted by your jurisdiction:

• Which fixtures. Island venting is limited to sinks and lavatories. A residential kitchen sink with a dishwasher, a disposer, or both, is expressly permitted.
• The rise. The vent connects to the fixture drain as an individual or common vent would, and rises vertically to above the drainage outlet of the fixture before it offsets horizontally or turns down.
• Built as drainage below the flood level rim. The vent below the flood level rim of the fixture is installed as drainage piping under Chapter 7, except for sizing, which follows the vent sizing rule in Section 906.2.
• The low point. The lowest point of the island vent connects full size to the drainage system, either to a vertical drain or to the top half of a horizontal drain.
• Cleanouts allow rodding of all vent piping below the flood level rim, and rodding in both directions through a cleanout is permitted.
• The tie-in. The vent (or branch vent for several island fixtures) extends at least 6 in above the highest island fixture served before it connects to the outside vent terminal.

What the UPC asks, in substance

UPC 2021 Section 909, special venting for island fixtures, as adopted by your jurisdiction:

• The trap is roughed in above the floor, and the vent extends as high as possible, not less than the drainboard height, then returns downward and connects to the horizontal sink drain immediately downstream of the vertical fixture drain, through a wye-branch fitting.
• A foot vent is taken off the vertical fixture vent by a wye immediately below the floor and runs to the nearest partition, then up through the roof, or connects to other vents at least 6 in above the flood level rim of the fixtures served.
• Drainage fittings are used on the vent below the floor, with slope back to the drain of at least 1/4 in per ft.
• The return bend under the counter is a one-piece fitting or a 45, a 90 and a 45 in that order.
• The island sink drain upstream of the returned vent serves no other fixture.
• An accessible cleanout goes in the vertical portion of the foot vent.

Check the current edition for exact wording; the UPC detail is more prescriptive about fittings than the IPC's, and inspectors under it look for the 45-90-45.

The air admittance valve alternative

An air admittance valve is a one-way valve that opens to let air into the drain when negative pressure develops and closes under positive pressure and when idle. Under the island it replaces the loop and the foot vent.

IPC: accepted, with conditions. IPC Section 918 accepts an individual or branch AAV listed to ASSE 1051, in substance: located in an accessible, ventilated space (an island cabinet qualifies if the valve is reachable and the space gets air), installed at least a stated height above the drain it serves (read the edition; it is a few inches), within the developed length limits for the vent, and with at least one vent to the outdoors elsewhere in the drainage system to relieve positive pressure, which an AAV cannot do.

UPC: not a general vent. The UPC base code does not recognize an AAV as a general venting method. Where a UPC jurisdiction accepts one under an island, it is by local amendment or a written alternate-method approval from the authority having jurisdiction. Without that, the loop vent is the detail.

The trade-off even where it is allowed. An AAV is a mechanical part with a service life, it has to stay accessible forever (not behind a glued-on cabinet back), and it does nothing for positive pressure from a busy stack. A loop vent has no moving parts. On a large kitchen where the floor below is open, the loop is often the better long-term detail even under the IPC.

Worked example: an island over an open basement

IPC jurisdiction, 2021 edition as adopted. A kitchen remodel moves the sink to a new island 9 ft from the nearest partition wall. Double-bowl sink with a disposer on one bowl and a dishwasher discharging to the disposer. Unfinished basement below, joists running toward the wall. Drain and vent in PVC.

Fixture check. A residential kitchen sink with disposer and dishwasher is permitted under island venting. Pass.

Drain. 1-1/2 in trap under the sink, 2 in fixture drain dropping through the floor to a 2 in horizontal run at 1/4 in per ft toward the stack, 14 ft away.

Vent riser and loop. The vent leaves a 2 in sanitary tee in the fixture drain (a horizontal-to-vertical turn going up, as a vent) and rises in the sink base. Cabinet interior height under the countertop is 34 in from the floor; the loop top is set at 31 in, above the sink's drainage outlet at about 26 in. Above the drainage outlet: pass.

Return and joint to the drain. The return comes down through the floor in drainage fittings and joins the top half of the 2 in horizontal drain through a wye, 18 in downstream of where the fixture drain drops. Full-size connection at the low point: pass.

Foot vent. A wye on the return just below the floor sends a 2 in foot vent along the joist bay 9 ft to the partition, sloped back toward the return at 1/4 in per ft: 9 ft times 1/4 in per ft = 2-1/4 in of fall over the run, so the foot vent is 2-1/4 in higher at the wall than at the wye. At the wall it turns up and rises inside the partition to tie into the existing vent at a height that clears the island sink's flood level rim by more than the required 6 in. Cleanout on the foot vent where it turns up at the wall, accessible from the basement.

Sizing. Section 906.2 in substance sizes a vent at not less than half the drain it serves and never under 1-1/4 in, so 1-1/4 in would meet the minimum for a 2 in drain. The crew runs 2 in throughout so the whole loop rods with the same cable as the drain, which is a choice, not a requirement.

The alternative priced in time. An AAV in the sink base would replace the return, the 9 ft foot vent and the rise inside the partition to the tie-in, and a wall to open. It would also put a valve in the cabinet that has to stay reachable, behind a sink with a disposer and dishwasher hoses in front of it. With the basement open, the crew builds the loop.

What would change the answer. On a slab, the foot vent and its cleanout would be under concrete, and the AAV becomes the realistic IPC detail. Under the UPC with no amendment, the same slab job still gets the loop, with the cleanout brought up through the floor in an accessible box.

Where island vents fail later

• The loop that holds water. A return vent or foot vent run level, or sagging between hangers, holds water and grease from every backup. Over a few years the low spot closes, the vent stops passing air, and the island sink starts gurgling and draining slowly with a clear drain line. Slope and support the below-floor vent like a drain.
• The cleanout that got covered. A foot vent cleanout boxed in when the basement was finished means the next stoppage in the loop is a ceiling to open. The cleanout is part of the detail, not an extra.
• The AAV that stuck. An AAV that fails closed starves the trap of air (gurgling, slow drain, trap siphoned); one that fails open lets sewer gas into the cabinet, with an odor that comes and goes. Either way the valve is replaced, and the house's open vent to the outdoors is confirmed while you are there.

How to verify you got this right

• The loop top is above the sink's drainage outlet (IPC) or at least at drainboard height (UPC), measured, not estimated.
• Every fitting below the flood level rim is a drainage pattern fitting and the below-floor vent slopes back to the drain.
• The return joins the drain full size at the top half of a horizontal drain or a vertical drain, and no other fixture drains into the island drain upstream of it.
• The foot vent has an accessible cleanout and ties in at least 6 in above the highest island fixture's flood level rim.
• Where an AAV is used, the code on the job file accepts it, it is listed to ASSE 1051, and it is reachable with the cabinet finished.

References

• IPC 2021 Sections 906.2, 916 and 918, as adopted by your jurisdiction.
• UPC 2021 Section 909, as adopted by your jurisdiction.
• ASSE 1051, binding through the code sections that reference it.
• See related: How to Install an Air Admittance Valve Where It Is Allowed.
• See related: Where the IPC and UPC Disagree on Service and Remodel Work; DWV Fittings for Each Change of Direction.