Weather-Resistive Barriers Behind Metal Cladding
Behind an impermeable cladding, the barrier is the wall's real defence and its drying path is one-directional. How to choose and install a WRB that suits a metal exterior in an Ontario climate.
The weather-resistive barrier is the layer nobody sees and everybody depends on. It is the surface that water actually runs down when wind-driven rain gets past the cladding, and it is usually doing double duty as the wall's air barrier. Behind metal siding it carries more responsibility than usual, because the cladding above it can neither breathe nor forgive.
Drainage Plane, Air Barrier, Vapour Retarder: Three Different Jobs
These get used interchangeably in conversation and they are not the same thing. Sorting them out is the beginning of getting the assembly right.
- ✓Drainage plane — the surface liquid water runs down and off. It must be continuous, shingle-lapped, and integrated with every flashing so water is always directed outward.
- ✓Air barrier — the layer that stops air moving through the wall. It must be continuous and sealed at every seam, penetration and transition to the roof, foundation and openings. Air carries far more moisture into a wall than vapour diffusion does.
- ✓Vapour retarder — the layer that controls diffusion. Its permeance and its position in the assembly determine where moisture can accumulate.
One product can serve as all three, and many WRBs do. The point is to know which functions your product is performing and to detail it accordingly. A wrap installed as a drainage plane but never taped is not an air barrier, no matter what the marketing says.
Permeance: Why It Matters More Behind Metal
Metal cladding is a vapour barrier. Nothing dries outward through it. That makes the drying path of everything inboard of it a design question rather than an assumption.
In a cold climate, moisture generally moves from the warm interior toward the cold exterior in winter. If it reaches a surface below the dew point, it condenses. In a typical Ontario wall with a metal exterior, you want the sheathing and the WRB to be able to dry into the ventilated cavity when the weather allows. A vapour-permeable WRB lets that happen. A low-permeance membrane in the same position can trap moisture in the sheathing, because it now cannot dry outward through the WRB or the cladding.
That is the general principle, not a universal rule. Where continuous exterior insulation is used, the sheathing is kept warmer and the calculus changes — a lower-permeance layer outboard of enough exterior insulation can be perfectly sound, because the condensing plane has been moved outward past it. Which is precisely why the insulation strategy and the WRB choice have to be decided together rather than separately.
The order of decisions matters. Choose the insulation strategy first — how much exterior insulation, if any — then choose a WRB whose permeance suits it. Picking a membrane because it is what the crew usually uses, without reference to the insulation, is how walls get built that cannot dry in either direction.
The Main Barrier Types
Each has real strengths, and the right choice depends on the wall, the schedule and the weather during construction.
Mechanically Attached House Wrap
The familiar rolled sheet, fastened with cap nails or cap staples and taped at the seams. Inexpensive, fast, widely available, generally vapour permeable. Its weakness is workmanship dependence: it only performs as an air barrier if every seam is taped, every fastener is capped, and every opening is properly integrated. It also tears, and it flaps in wind if left exposed too long.
Self-Adhered Membrane
A peel-and-stick sheet bonded to the sheathing. Excellent air barrier performance because it is fully adhered with no fastener penetrations, and very good at bridging sheathing joints. It is more expensive, and it is sensitive to installation conditions — surfaces must be clean and dry, and both the membrane and the primer have minimum application temperatures. In an Ontario November that is a real scheduling constraint, and warming a surface with a torch is not an acceptable workaround.
Fluid-Applied Membrane
Rolled or sprayed on and cured in place. Monolithic with no seams to tape, exceptional around complex geometry and irregular substrates, and easy to inspect because gaps are visible. It costs more, requires attention to wet film thickness, and has its own temperature and cure-time limits.
Integrated Sheathing
Structural sheathing with a barrier bonded to the face; joints are taped and the panel does everything. Fast and consistent, with fewer trades and fewer steps to get wrong. It commits you to that manufacturer's system and its detailing.
Shingle Lapping Is Not Optional
Everything on a drainage plane laps like a roof: upper sheets over lower, side laps facing away from prevailing wind-driven rain, and every flashing integrated so water on the barrier is always directed outward and downward. A single reverse lap — a piece of wrap tucked behind the sheet below it — is a funnel that puts water directly into the sheathing, and it will not be visible once the cladding is on.
- ✓Barrier above laps over head flashing, never behind it.
- ✓Jamb flashing laps over sill pan flanges; head flashing laps over jamb flashing.
- ✓Side laps and end laps should meet or exceed the manufacturer's minimum, and be taped where the product calls for it.
- ✓Fastener penetrations get cap fasteners, not staples through bare membrane. A staple leg is a hole; a cap seals around it.
- ✓Every penetration — hose bibs, dryer vents, electrical, gas lines, deck ledgers — gets flashed and sealed to the barrier, not just foamed.
UV Exposure During Construction
Every WRB has a published maximum exposure period before the cladding goes on, typically stated in months. Exceed it and the material degrades — wraps get brittle and tear, adhesives lose grip, fluid-applied products chalk. On a project where framing and cladding are separated by a long stretch, or where the metal panels are on a long lead time, this is worth checking against the actual schedule rather than assuming.
This becomes critical on open-joint and reveal-jointed metal systems, where the barrier stays partially exposed for the life of the building. Those assemblies need a barrier specifically rated for permanent partial exposure, usually black so the joint reads as a clean shadow line. A standard construction wrap behind a reveal joint will fade, degrade and eventually show a manufacturer's logo through the gap.
The Barrier Is Not a Substitute for Flashing
The most persistent misunderstanding on job sites is that a good wrap makes flashing details less critical. The opposite is true. The barrier handles diffuse water running down a surface. It cannot handle concentrated water arriving at a horizontal interruption — a window head, a deck ledger, a band course, a roof-to-wall junction. Those need formed metal flashing with slope, a drip and end dams. Tape and membrane are how you integrate flashing into the barrier; they are not a replacement for the flashing itself.
Cold Weather Installation in Ontario
A great deal of Ontario cladding work happens between October and April, and cold has practical consequences: adhesives and primers have minimum application temperatures, tapes bond poorly to cold or frosty substrates, fluid-applied products need cure time above a threshold, and a frost film on sheathing will defeat adhesion even when the air temperature is acceptable. Mechanically attached wraps are the most tolerant of cold-weather installation, which is one honest reason they remain common here. If a self-adhered or fluid-applied system is specified for a winter job, plan for surface preparation, low-temperature product variants and enclosure — or schedule it differently.
Frequently Asked Questions
Do I need a WRB if there is already a vapour barrier on the inside?
Yes. They do different jobs at different places in the wall. The interior vapour retarder controls diffusion from inside; the exterior WRB is the drainage plane and usually the air barrier. Neither replaces the other.
Can I use roofing underlayment as a wall barrier?
Not as a general substitute. Many roofing underlayments have low vapour permeance, which behind an impermeable metal cladding can leave the sheathing with no drying path in either direction. Use a product intended for wall use, with permeance appropriate to the insulation strategy.
What happens if the WRB gets torn during siding installation?
Repair it before the cladding covers it, using tape or patch material from the same system and lapped so water sheds over the patch. A tear behind finished metal siding is not something anyone will find until there is a problem inside.
Does the barrier need to be black behind reveal-joint metal siding?
Functionally it needs to be rated for the exposure the joints allow. Visually, black is what makes an open joint read as a crisp shadow line instead of showing a pale, printed surface through the gap. Most manufacturers offer a dark UV-stable version for exactly this application.
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