How to Flash an Exterior Wall Penetration Into the WRB
Why this matters
Every vent cap, hose bib, line set and outdoor box you put through a wall is a hole in the layer that keeps rain out of the framing. A bead of caulk at the siding makes it look sealed on the day you leave. Caulk cracks within a few seasons, and from then on the only thing between wind-driven rain and the sheathing is whether you lapped the flashing into the water-resistive barrier the right way. Get the laps backwards and the detail funnels water behind the wrap, where it rots sheathing for years before a soft spot or a stained ceiling gives it away.
Before you cut: the hazards at this job
- Height. Most of these are ladder work. Set the ladder at about a 1 to 4 angle with the rails extending 3 feet above the landing point if you step off, and keep both hands free for the cut by working from a stable stance, not an overreach. A second story or a slope below the wall usually means staging rather than a ladder.
- What is in the wall. Scan and check from inside before you drill: wires, pipes and gas lines run in exterior walls. See related: How to Scan a Wall Before You Cut or Drill.
- What the cut releases. Fiber-cement siding releases respirable crystalline silica when cut with a power saw; use shears or score-and-snap, or a saw with integrated dust collection operated per its instructions, which OSHA's construction silica standard (29 CFR 1926.1153) recognizes in its Table 1. Painted siding on a house built before 1978 may carry lead; on rental and child-occupied housing the EPA renovation rule (40 CFR 745 Subpart E) sets work practices and certification for the occupants' protection, and your workers' exposure falls under 29 CFR 1926.62. Treat unknown paint as lead and do not dry-sand or power-cut it without those controls.
The rule the whole procedure serves
Water runs down. Every piece in a wall detail must lap so that water running down the face of it lands on the outside of the piece below. At the top, the WRB above laps over the flashing. At the bottom, the flashing laps over the WRB below. Sealant fills gaps; laps move water. A detail that depends on sealant to keep water out has already failed, it just has not happened yet.
WRB ABOVE .......................... slit in the WRB,
folds DOWN ....+--------------+...... taped last
over the ....| top flange |...... sits UNDER the
top flange ..::+--------------+::.... WRB flap
..::| |::....
side tapes ..::| ( vent ) |::.... side tapes
go on first ..::| |::.... go on first
and run past ..::+--------------+::....
top and foot ....| bottom flange|...... lies OVER the
WRB BELOW ....+--------------+...... WRB, untaped
Steps
Find out what the WRB is. Pull the siding course at the location, or look at the edge of an existing opening. It may be housewrap, asphalt felt, a sheathing panel whose taped seams are the WRB, a fluid-applied membrane, or, on older houses, nothing at all. Why: the lap sequence, the tape that will stick, and where the "WRB above" even is all depend on it. Skip it and you will tape to the wrong surface or slit a panel-type WRB that should have been patched instead.
Use a mounting block or a flanged sleeve, never a bare hole. Choose a block sized for the penetrant, with a flange all the way around and a face the siding can butt to. Vinyl siding manufacturers require one so the siding can expand around the penetration. Why: the flange is what laps into the WRB; a bare hole has nothing to lap. Skip it and the only seal is caulk to the siding.
Cut the cladding to the block's footprint and stop. Remove only enough siding to set the block, keeping the WRB around it as intact as you can. Why: every inch of wrap you cut is another lap to rebuild. Skip the care and you will be patching torn wrap with tape, which is a weaker lap than the original.
Cut the WRB to the penetration, plus one horizontal slit above. Cut the hole the size of the penetrant. Then cut a horizontal slit just above where the block's top flange will sit, as wide as the flange. Why: the slit lets you tuck the top flange under the wrap above. Skip it and you will tape the top flange onto the face of the wrap, which is a reverse lap held shut by adhesive alone.
Set the sleeve or duct with a fall to the outside and seal it at the sheathing. The penetrant should slope down toward the exterior. Seal the gap between penetrant and sheathing. Why: the slope drains anything that gets in, and the sheathing seal is an air seal. Skip the slope and water that reaches the sleeve runs into the wall.
Install the block: top flange under the flap, sides and bottom over the wrap. Fasten it per its instructions. Why: this is the lap rule in one move. Skip it by putting the block entirely over the wrap and taping all four edges, and the top edge becomes a ledge that sends water behind the tape.
Tape the sides first, then the top, and leave the bottom open. Side tapes run from past the bottom flange to past the top flange; then the WRB flap is folded down over the top flange and the slit taped. Leaving the bottom edge untaped, as most window and penetration details do, gives water that gets behind the flange a way out. Why: each later tape laps over the earlier one. Skip the order and the top tape ends up under the side tapes, opening a path at each upper corner. Press the tape hard with a roller; self-adhered flashing tape bonds by pressure and is weakest where it was only smoothed by hand.
Seal the annular gap between the penetrant and the block. Use an exterior sealant compatible with the block and the penetrant. Why: this keeps bulk water and insects out of the sleeve. It is the backup, not the defense.
Reinstall the cladding to the block. Butt the siding to the block, leaving the clearance the siding manufacturer calls for (vinyl needs room to move and is never caulked tight to the block). Skip it and vinyl buckles around the block in summer.
Worked example: a bath fan vent through lap siding
A 1994 house, wood lap siding with a 5 inch exposure over housewrap on OSB. The new bath fan needs a 4 inch vent cap through the wall, in a stud bay located from inside and confirmed clear by scanning.
The tech pulled the lap course at the location and found housewrap in good shape, with a surprise: an abandoned cable hole 8 inches above the planned spot, closed only with a gob of caulk, which had shrunk away from one side. The OSB around it was dark. An awl went in about 1/8 inch with light pressure there, against barely marking the sound OSB a hand's width away. That softening is the failure this procedure prevents, already sitting in the wall.
The fork. Soft sheathing is not something to flash over. The tech stopped, showed the owner, and photographed it. Probing outward found the soft area was about the size of a palm and stopped well short of the stud. The owner approved a small sheathing patch as a change, with the new vent moved down one stud bay so its block would not land on the patch's seam. Had the softening reached the framing, the job would have stopped for a carpenter and the vent waited.
With the sheathing repaired and wrapped, the vent went in:
- Hole in the OSB at 4-1/4 inches for a duct of 4 inch outside diameter, leaving a 1/8 inch gap around it, sealed at the sheathing before the block went on.
- Duct falling about 1/4 inch toward the outside over its short run through the wall.
- A housewrap slit just above the block's top flange, as wide as the flange.
- Side tapes, 4 inch wide, from 2 inches below the bottom flange to 2 inches above the top flange; the flap folded down over the top flange and the slit taped across, lapping over both side tapes. Bottom left untaped.
- Annular gap at the cap sealed; siding cut to butt the block with a sealed joint, as wood siding allows.
The abandoned cable hole got the same treatment as a new penetration: a flanged blank flashed into the wrap, then the siding patched. It was never going to stay sealed with caulk alone.
What changes the method
- Panel sheathing with taped seams as the WRB. There is no flap to tuck under. Lap the top flange under a new strip of the panel's own tape running above, or follow that product's penetration detail exactly.
- No WRB at all. Common on older houses. Flash the block to the sheathing with a self-adhered flashing patch large enough to lap under the siding course above, and record that the wall has no WRB, because the next penetration will meet the same thing.
- Stucco, brick and EIFS each have their own drainage plane logic. See related: Stucco and EIFS and the Hole You Cut.
How to verify you got it right
Before the siding goes back, look at the detail and trace a drop of water from above it down to the wrap below. At every edge it should cross onto the outer surface of the next piece. If at any edge it would run behind something, that edge is reverse-lapped. Photograph the taped detail before it is hidden; it is the only proof anyone will ever have. If the wall is accessible from a basement or an unfinished room, a hose test directed from above, run for several minutes, with someone watching inside, settles it for the price of the time. Enter the penetration in the job record. See related: The Penetration Record SOP.
References
- IRC R703 (exterior coverings, the WRB and flashing applied shingle-fashion), as adopted by your jurisdiction.
- WRB, flashing tape, mounting block and siding manufacturers' installation instructions.
- 29 CFR 1926.1153 (respirable crystalline silica, construction), 40 CFR 745 Subpart E and 29 CFR 1926.62 (lead), as they apply to the job.
- See related: The Control Layers a Penetration Goes Through; Flashing + Weatherproofing Details Reference; How to Run a Line Set or Cable Through an Exterior Wall; The Wall That Leaked Only in Wind-Driven Rain.