Rainscreen Assemblies and Furring Behind Metal Siding
Metal cannot dry outward. That single fact is why the cavity behind metal siding matters more than behind any other cladding — and why furring depth, orientation and venting are engineering decisions.
A rainscreen is not a product. It is an arrangement: cladding on the outside, a gap, and a properly detailed water and air barrier behind. The gap is what makes it work. Behind metal siding that gap is more important than behind almost any other cladding, for reasons that come down to a single material property — metal does not let water vapour through it in either direction.
The Four Jobs a Cavity Does
It is worth being precise about what the gap is actually accomplishing, because different assemblies emphasize different functions.
1. Drainage
Any water that gets past the cladding — through a lap, around a fastener, through a reveal joint — lands in the cavity and runs down the face of the barrier to an exit at the base. Without a cavity, that water sits in contact with the sheathing. A drainage gap only needs to be a few millimetres to work, which is why drainage mats and dimpled membranes are effective for drainage alone.
2. Pressure Moderation
This is the mechanism the name comes from. Wind-driven rain is pushed into a wall by the pressure difference across the cladding. If the cavity behind is vented to the outside, air pressure in the cavity rises to match the pressure on the face of the cladding. With little pressure difference across the cladding, there is far less force driving water inward through joints. The rain still lands on the wall; it just is not being pushed through it.
3. Back-Ventilation and Drying
Airflow up through the cavity carries moisture away — from the back of the cladding, from the barrier surface, and from the sheathing. This is the function that needs a real gap, not just a drainage plane, and it is the one that matters most behind metal.
4. Capillary Break
Two wet surfaces in contact will draw water between them by capillary action. An air gap breaks that path completely, so water cannot wick from the back of the cladding into the sheathing.
Why This Matters More Behind Metal
Wood, fibre cement and even vinyl assemblies all have some capacity to dry outward. Metal has none. It is a vapour barrier hung on the outside of the wall, and that creates two specific conditions worth understanding.
The first is drying direction. Anything that gets behind a metal panel — a bit of wind-driven rain, a construction-stage wetting, moisture that diffused out from inside — cannot leave through the cladding. It has to drain out the bottom or be carried away by air moving through the cavity. Remove the cavity and you have built a wall with a vapour barrier on both faces, which is the classic recipe for trapped moisture in a cold climate.
The second is night-sky cooling and condensation. On a clear night a metal panel radiates heat to the sky and can fall several degrees below the surrounding air temperature. Humid air in contact with that cold back surface condenses on it. This is not a defect — it is exactly why dew forms on a car roof — and it means liquid water appears on the back of metal cladding on nights when it has not rained at all. A vented cavity dries that condensation off the next morning. A sealed cavity holds it against the sheathing.
If you remember one thing about metal cladding assemblies: the cavity is not there to handle rain leaks. It is there because metal cannot dry outward and because condensation forms on its back face on clear nights. Those two facts make the cavity part of the system, not an upgrade to it.
How Deep Does the Cavity Need to Be?
It depends on which functions you want. A few millimetres provides drainage and a capillary break. Meaningful back-ventilation needs more — enough cross-section for air to actually move by buoyancy and wind. Common practice for a genuinely ventilated cavity behind metal in a wet, cold climate is nominal 3/4" strapping, which is both readily available and generous enough to move air.
- ✓Nominal 3/4" (1x3 or 1x4 strapping) — the practical standard for a ventilated rainscreen behind metal on a house.
- ✓Nominal 1-1/2" (2x strapping) — used where extra-deep furring is needed to match a masonry plane, or where a second layer of cross-furring is required.
- ✓Drainage mats and dimpled membranes — thin, easy to install, good drainage and capillary break, limited back-ventilation. A reasonable compromise on retrofits where thickness is constrained.
- ✓Furring depth also determines fastener length. Longer fasteners into structure through furring, and much longer again through exterior insulation.
Orientation: Which Way Do the Strips Run?
Water drains vertically and air rises vertically, so the cavity needs a continuous vertical path. That makes vertical furring the natural choice — and it works directly under horizontal siding, since panels run across the strapping.
Vertical siding creates a conflict, because vertical panels need horizontal support. Horizontal strapping alone blocks the vertical drainage and ventilation path. There are three ways out of it.
- ✓Cross-furring: vertical strapping first, horizontal strapping over it. Keeps a continuous vertical cavity behind the horizontal strips at the cost of thickness and one more layer of labour.
- ✓Notched or spaced horizontal furring: horizontal strips with deliberate gaps or notches so water and air can pass. Simpler, but the gaps have to be real and consistent, not occasional.
- ✓A drainage mat behind horizontal furring: gives a continuous drainage path even where the strips are solid.
Furring Materials and Thermal Bridging
Wood strapping is the common choice and should be preserved or treated where it may see moisture. Metal hat channel is stiffer and dimensionally stable, but steel conducts heat well, so a wall furred with steel channels over insulation has a thermal bridge at every strip — worth avoiding on a wall where continuous exterior insulation is doing real work. Composite and plastic furring exists specifically to solve this and costs more.
Where exterior continuous insulation is part of the wall, fastening becomes the key design question. Long screws through insulation into studs work up to a point; beyond that thickness, clip-and-rail systems designed for the purpose are the reliable route, and they also reduce thermal bridging compared with a continuous metal furring member.
Terminating the Cavity Top and Bottom
A cavity that is not open at both ends is not ventilated. At the bottom it must be open enough to drain and to admit air; at the top it must be open enough to exhaust. Both openings need insect screening — a perforated closure, formed vent trim or purpose-made screened strip. Fine mesh window screen clogs quickly and is not the right product.
Interruptions matter too. Every window head, band course and horizontal transition is a potential cavity blockage. Where a flashing crosses the cavity it should be detailed to carry water out while leaving a path for air — otherwise the ventilated cavity becomes a series of sealed pockets, each with its own moisture to deal with.
What This Costs You
A rainscreen adds material, labour and wall thickness. The strapping has to be shimmed flat, which is real time. Every window and door gets deeper, so jamb extensions, extended sills and relocated fixtures are part of the job. The wall gets thicker at the corners and the eave, which changes the roof edge detail. None of this is a reason to skip it behind metal — it is a reason to plan and price it properly rather than discovering it mid-project.
Frequently Asked Questions
Can metal siding go directly on sheathing without furring?
Some products permit it, and it is still done. It gives up drainage, back-ventilation and the ability to shim a flat plane, and it puts a vapour-impermeable cladding in direct contact with the barrier and sheathing. In Southern Ontario's climate, a drained and vented cavity is the substantially better assembly.
Do the furring strips need to be pressure treated?
Use preserved or treated strapping where it may see repeated wetting — near grade, at the base of the wall, and at deck and roof intersections. In a well-detailed, ventilated cavity the strapping stays dry most of the time, but the base of the wall is where water collects and treated material is cheap insurance.
Will a vented cavity make the house colder?
No, provided the insulation and air barrier are inboard of the cavity and continuous. The cavity is outside the thermal control layer. What does cause a problem is wind blowing through insulation because the air barrier is incomplete — that is an air barrier issue, not a cavity issue.
Does the cavity get filled with insects or rodents?
Not if it is screened at every opening — bottom, top, and around penetrations. Use a rigid perforated closure or formed vent trim rather than loose mesh, and check it where the cavity meets the soffit, which is the opening most often left unfinished.
How wide can furring be spaced?
It is set by the panel, not by habit. Each profile has a maximum support spacing based on gauge, profile depth and wind load. Standard framing spacing often works, but check the manufacturer's table for the specific panel and the exposure rather than assuming.
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