Every Vent Is a Hole in Your Building: Sealing Exhaust Penetrations

Count the vents on the outside of your building. Dryer vents, bathroom exhaust, range hoods, makeup air intakes. Each residential unit contributes several, so a hundred-unit building carries a few hundred of them.

Every one is a deliberate hole through the wall assembly. The water barrier that protects the rest of the building was cut, and something had to be built around the opening to keep water out.

Vent penetrations produce a disproportionate share of leak complaints for their size, and the reasons are specific and repeatable enough that a council can check for them.

Why a small hole causes a big problem

A wall face sheds water across its entire surface. Rain runs down, spreads out, and leaves at the bottom.

A penetration interrupts that. Water running down the wall reaches the hood and has to go around it, which concentrates flow at the edges of the opening. Anything that fails at that concentration point receives far more water than an equivalent area of plain wall ever would.

There is also the matter of what the penetration passes through. On a rainscreen wall, the duct crosses the cladding, the drainage cavity, the water-resistive barrier, and the structure behind it. Each of those layers has to be maintained around the opening, and the water-resistive barrier in particular has to be sealed to the sleeve so water in the cavity drains past the penetration rather than into it. That is a detailing problem, and detailing problems are where envelopes fail, as covered in the guide on flashing details that fail.

The four ways vent penetrations leak

The sleeve slopes the wrong way. The duct sleeve through the wall should tilt slightly downward toward the outside, so any water that gets into the duct runs out rather than back toward the building. Installed level, it holds water. Installed sloping inward, it actively channels water into the assembly.

This is easy to get wrong during construction and hard to correct afterward, because fixing it means opening the wall around the sleeve. It is also invisible from outside, which means a building can have this defect at every vent on an elevation and nobody knows until leaks appear on the same floor across several units.

Sealant is doing a flashing’s job. The correct detail has a head flashing or an integrated hood flange that directs water running down the wall around and past the opening, with sealant as a secondary seal behind it.

What frequently exists instead is a hood screwed to the wall with a bead of sealant around its perimeter and nothing behind it. That works until the sealant fails, and sealant always eventually fails. When it does, there is no second line of defence and water goes directly into the assembly. The relationship between primary and secondary defences is exactly the logic set out in the guide on rainscreen versus face-sealed walls, applied at the scale of a single opening.

The damper stops closing. Exterior hoods have a damper that opens when the fan runs and closes when it stops, keeping weather and pests out. When it sticks open, the opening is exposed continuously, and on an elevation facing wind-driven rain that means water entering the duct directly.

Dampers stick for ordinary reasons: accumulated lint, corrosion, paint from a previous coating job applied over the mechanism, or physical damage. Painting a damper shut during a recoat is a genuinely common error, which is one argument for hoods being addressed deliberately during coating work rather than treated as something to cut in around.

The sealant joint has simply failed. Sealant at a penetration works harder than sealant elsewhere. It sits at a junction between dissimilar materials, usually metal and cladding, which move at different rates with temperature. It takes concentrated water flow. On a dryer vent it also takes warm humid exhaust. That combination shortens its service life relative to a joint in the middle of a wall, which means penetration sealant should be inspected on a tighter cycle than field joints, using the signs described in the post on when a building needs sealant replacement.

Why dryer vents are the worst of them

Dryer vents carry warm, moist air and lint, and each of those causes a specific problem.

Lint accumulates in the duct and around the damper. That is the damper-sticking mechanism above, and it makes dryer vents the most likely of all penetrations to be sitting open in a rainstorm.

The moist exhaust condenses inside the duct in cold weather. Warm humid air meeting a cold duct wall produces liquid water, which then has to go somewhere. If the sleeve slopes outward, it drains harmlessly. If it does not, it runs back toward the building, and the building has a leak that only appears in cold weather and has nothing to do with rain. That pattern confuses investigations, because it does not correlate with rainfall. Separating condensation-driven water from a rain leak is covered in the guide on condensation versus leak.

Lint in a duct is also a recognized fire risk, and a restricted duct makes a dryer work harder and vent moisture into the unit instead of outside, which raises interior humidity and contributes to the condensation problems described in the guide on humidity management in winter.

So dryer duct maintenance sits at the intersection of fire safety, indoor air quality, and building envelope. Most stratas already have it on a schedule for the safety reason. The envelope benefit comes along with it.

What to look for

Several checks require nothing more than walking around the building and looking up.

Staining below a vent. A streak or discoloration on the wall under a hood means water has been running out of or around the penetration. It is the clearest visible early warning available, and it means the problem has been developing for a while.

Hoods that are damaged, loose, or missing. A hood that has come away from the wall at one edge, or has lost its damper entirely, is an open hole. These are surprisingly easy to spot from ground level with binoculars.

Sealant condition at the perimeter. Cracking, splitting, or separation from either the hood or the wall.

Painted-over dampers. Visible if the hood has been coated along with the wall and the damper no longer has a clean shadow line at its edge.

Interior complaints clustering. Several units on the same elevation reporting water near an exterior wall, particularly at the same relative location within each unit, points at a repeated detail rather than an isolated failure. That pattern is worth acting on, because it usually means a construction-era detail that exists at every similar penetration on the building.

Fixing them properly

Where the hood detail is sound and only the sealant has failed, the repair is a normal recaulk: remove the old material completely, clean the substrates, prime if the sealant requires it, and install new sealant with proper joint geometry. The difference between doing that properly and applying a cap bead over the existing material is set out in the post on caulking removal versus cap bead.

Where the underlying detail is wrong, sealant alone is not a fix. Correcting it means removing the hood, establishing proper flashing, sealing the sleeve to the water-resistive barrier, correcting the slope if it can be reached, and reinstalling with the sealant as a secondary seal rather than the only one.

That is more invasive, and it is why the right time to do it is during work that already has the wall open or the access mobilized. A recladding gives full access to the sleeve and the barrier. A coating renewal gives access to the hood and the perimeter, which is enough to correct flashing and sealant even if the sleeve slope cannot be changed.

Handling penetrations deliberately during a coating project also avoids the specific failure of dampers painted shut, and it means the new sealant is installed as part of a specified scope with acceptance criteria, rather than as an afterthought by whoever is nearest.

Where this belongs in planning

Vent penetrations are rarely a line item in a maintenance plan, and they should be. There are hundreds of them on a typical building, they fail faster than field sealant, and they concentrate water at points that lead directly into the assembly.

Three practical steps. Include exterior hood inspection in routine envelope walkarounds, checking dampers, seals, screens, and staining. Keep dryer duct cleaning on its schedule for safety reasons and treat the exterior hood check as part of the same program. And when a coating or recladding project is being scoped, make sure penetrations are named in the scope with a defined treatment, rather than left to be worked around.

A few hundred small openings, each one a place the water barrier was cut. They deserve more attention than they usually get.

Sources and further reading

• BC Building Code, Division B Part 5 (Environmental Separation) and Part 6 (Heating, Ventilating and Air-conditioning), referenced 2026
• BC Housing, Building Enclosure Design Guide, penetration detailing, referenced 2026
• Canada Mortgage and Housing Corporation, Best Practice Guide: Wall Penetrations and Flashings, referenced 2026
• National Fire Protection Association guidance on clothes dryer exhaust systems, referenced 2026
• Allweathercoating field inspection records, Metro Vancouver, 2026