How to Close a Penetration You Made Through the Air Barrier

Why this matters

An electrician running one new circuit from a basement panel to a bathroom fan can cross the air barrier four or five times in an afternoon: up through the floor, through a top plate into the attic, through the ceiling at the fan, and out through the rim joist or the garage wall for whatever else is on the ticket. Each hole is small. Together, after a few trades have been through, they are why an air-sealed house tests leaky again two years later, why the second floor ceiling grows a gray stain at the top plate, and why the inspector fails a garage wall that passed at drywall.

The trade that drilled the hole is the only one that knows it is there. Once the insulation goes back and the drywall is patched, nobody else will find it until something goes wrong. This card is for electricians, plumbers and low-voltage techs: what layer you just crossed, what closes it, and what you write down.

The four layers you cross, and what each one is

Before choosing a sealant, name the layer. One hole can be two of these at once.

Where the air barrier actually runs in a given house is its own subject; see related: Where the Air Barrier Actually Is in a House. In a multifamily building, a wall or floor between units or between a unit and a corridor is often a rated assembly, and a penetration there needs a listed firestop system tested for that assembly and that penetrant (tested under ASTM E814 or UL 1479, and specified by whoever owns the rating). If you are not sure whether an assembly is rated, the building's drawings or the building department say; do not guess with foam.

Steps

1. Before you drill, look at what you are drilling into. A top plate may have a cable or pipe already in the bay. A garage wall may carry a gas line. An exterior wall may have a water line close to the sheathing. Look from the attic or basement first, and where a cable is possible, turn off and lock its circuit and prove it dead with a tester checked on a known live source before and after. Before you remove your lock and re-energize, look at the bit's path for a nicked jacket and confirm nobody is working on that circuit. What breaks if skipped: the bit finds the conductor or the pipe, and the penetration you have to close is now a repair.

2. Drill the smallest hole that passes the work. Every eighth of an inch of annular gap is sealant you have to place and a leak if you do not. A 3/4 inch hole for one cable leaves a gap that caulk bridges; a 1-1/2 inch hole for the same cable leaves one that needs foam. Drilling through structural members has its own limits on size and position in your adopted code; the framing rules govern before the air barrier does.

3. Name the layer from the table, and decide whether fire rules apply. At a top plate, at floor level, and around pipes and cables at ceiling and floor level, the IRC treats the opening as a fireblock location (R302.11 as adopted by your jurisdiction; see related: Fireblocking and Draftstopping, which owns the materials list). At the garage separation, penetrations are protected as that fireblocking provision requires (R302.5.3 in the 2021 edition), and ducts in or through the separation have their own stricter rule. What breaks if skipped: a fireblock location sealed with ordinary foam is an air seal that fails inspection, and the rework means opening finished drywall.

4. Pick the material by gap size and layer. A common shop default:

• Gap up to about 1/4 inch: caulk. At a fireblock location, a sealant labeled for fireblocking.
• About 1/4 inch to 3 inches: one-part low-expansion foam; at a fireblock location, a foam the manufacturer lists for fireblocking and your inspector accepts.
• Larger, or an open chase: a rigid patch (drywall, sheet metal) sealed at its edges.
• Plastic pipe that moves with temperature: a flexible caulk or a collar, not rigid foam.
• A hot vent or flue: never foam; sheet metal and a high-temperature sealant that the vent's listing allows, with its clearance kept open.

One-part foam releases isocyanate vapor as it cures and the propellant is flammable: gloves, eye protection, ventilation, and no flame or running appliance burner nearby while it is wet.

5. Seal from the side where the barrier is. For a ceiling penetration under a vented attic, the barrier is the drywall and top plate, so the seal goes on top of the plate in the attic, not on the room side behind a cover plate. For an exterior wall, the interior seal stops air and the exterior seal (a sealant bead or a flashed mounting block) stops water; outside, the cable gets a drip loop so water runs off before the hole. For the garage, seal on the garage side, where the separation's gypsum is.

6. Photograph before you cover. One photo per penetration, close enough to see the seal and wide enough to see where it is (a stud, the panel, the corner). Insulation goes back over the top plate; the drywall gets patched; the photo is the only proof left.

7. Put the insulation back the way it was. Attic insulation pushed aside to reach a top plate is leveled back to its depth. A gap left in the insulation is a cold spot on the ceiling below, and a condensation line in winter.

Worked example: one circuit, one camera, five holes

The job: a new 20 amp circuit from a basement panel to a new bathroom exhaust fan on the second floor, plus a low-voltage camera cable from the same basement to a camera mounted in the attached garage. The house is 1998 construction, two stories, vented attic, basement, attached garage on the first floor. The owner had the house air sealed last year.

The penetration log the tech fills in as the job goes:

PENETRATION LOG   job 2417   tech: R.
#  location           layer         gap    sealed with
1  1st fl bottom plt  fireblock     1/4in  FB caulk
2  2nd fl top plate   air+fireblk   1/4in  FB caulk
3  fan housing, attic air+fireblk   3/8in  FB foam
4  rim joist, N wall  air+water     1/8in  caulk in,
                                           sealant out
5  garage wall        fire separ.   1/4in  FB caulk,
                                           garage side
photos: 5 of 5    insulation restored: yes

Holes 1 and 2. Both drilled 3/4 inch for one 12-2 cable. The flat cable leaves an uneven gap, up to about 1/4 inch beside its flat faces, the top of the caulk range, so fireblock-rated caulk worked in around the cable fills it. At hole 2, the tech pulls back the attic insulation over the plate, seals on top of the plate, and levels the insulation back.

Hole 3. The fan housing sits in a ceiling cutout about 3/8 inch oversize around the box. That gap is the largest hole on the job and the one a sealed house loses most through: it opens straight from the bathroom, the wettest room in the house, into the attic. Low-expansion fireblock foam from the attic side around the housing; the housing's own seams get foil tape where the fan's instructions allow.

Hole 4. The camera cable leaves the basement through the rim joist to run along the outside to the garage. A 3/8 inch hole for a 1/4 inch cable, sealed with caulk on the inside and an exterior-grade sealant outside, with a drip loop. The tech checks the rim bay first for the foam block the air sealing contractor installed last year, cuts a matching notch rather than crushing it, and re-seals the block's edge around the cable.

Hole 5. The cable enters the garage through the shared wall to reach the camera, a 1/2 inch hole in 1/2 inch gypsum for the same 1/4 inch cable. This is the fire separation, so the gap gets fireblock-rated caulk on the garage side. The tech's first plan had been to route the cable through the garage ceiling into the attic; that would have crossed the separation a second time where the ceiling is the separation for the bedroom above, which in a garage with habitable space over it calls for 5/8 inch Type X gypsum under the IRC's Table R302.6. One crossing is better than two.

The count. Five penetrations, five photos, four of the five at fireblock locations or the separation (1, 2, 3 and 5), all four sealed with fireblock-rated material. Hole 4 is an air and water seal only.

What would change it. If the bathroom fan had been in a townhouse whose attic is shared over a unit separation, the ceiling might be part of a rated assembly and hole 3 would need a listed system rather than foam. If the camera had gone on the garage ceiling, the cable would cross the separation under habitable space and the crossing would have to keep the 5/8 inch Type X intact around it.

How you know it held

The insulation is level over hole 2, no light shows around the fan housing from the attic with the bathroom light on, and each photo in the log shows the seal and its location. The five photos and the log go on the job record, which is what a later blower door tester or inspector will ask for when the house tests leakier than last year. Run the light check at the fan housing before the insulation goes back over it; if light shows, re-seal from the attic while the gap is still visible, because once it is buried nobody will see it again.

References

• See related: How to Air Seal Penetrations Into the Attic; Fireblocking and Draftstopping; Where the Air Barrier Actually Is in a House; The Garage to House Fire Separation
• IRC R302.5, R302.6 and R302.11, as adopted by your jurisdiction; 2021 edition numbering
• ASTM E814 and UL 1479, fire tests of through-penetration firestops, which apply where an assembly is rated
• Sealant and foam manufacturers' labels and listings for fireblocking use