Rain Screen Wall Assemblies with a Drained Air Gap

The Problem It Solves

Water gets past siding at butt joints, trim, corners and fastener holes, and wind pushes it in harder. What matters is whether it has anywhere to go. When the cladding sits tight against the weather barrier, the thin film of space between them holds water by capillary action, and the back of the boards and the face of the membrane stay wet long after the rain stops. Calgary is dry on average, with 325 mm of annual rain in the code’s climate table, but its one-day rainfall design value is 103 mm, and a wall has to cope with the storm, not the average.

Homeowners hear two opposite things about rain screens: that the code requires one, or that they’re pointless in a dry city. Neither is right. The Alberta building code requires a capillary break only where the moisture index exceeds 0.90 below 3400 degree-days, or 1.00 at 3400 or more. Calgary sits at 5000 degree-days with a moisture index of 0.4, so the code doesn’t force a rain screen cavity on a house here. Much of the confident advice online is written for wetter climates and doesn’t apply.

That doesn’t make the cavity optional in every case. Some products require it regardless of climate, and a contractor who skips it because “Calgary doesn’t need it” can put the wall outside the conditions its materials were evaluated under. HardiePanel vertical siding must go over a drained and vented space “in all areas”. Tyvek’s housewrap evaluations call for a 10 mm air space unless the cladding is deemed not to need one. The Western Red Cedar Lumber Association warns that cedar over rigid foam can trap moisture that stains and buckles the siding.

How It Works

A rain screen puts a deliberate gap between the cladding and the weather barrier, open at the bottom to drain and at the top to vent. Water that gets past the siding hits the membrane, runs down the open cavity and leaves at the base instead of wicking along the back of the boards. Air moving through the gap dries the back of the cladding and the face of the membrane. The code’s two planes of protection still apply: the cladding is the first plane, and the membrane and flashings behind the cavity are the second.

We build the gap one of two ways. Vertical furring strips, at least 19 × 38 mm, are screwed through the membrane into the studs, giving fibre cement or cedar a solid nailing line and leaving a cavity about 19 mm deep between strips. Where depth is tight at windows or trim, a vented, open drainage matrix mat at least 10 mm thick and 80% open can serve instead. On walls getting exterior insulation, the furring that holds the cladding through the foam creates the cavity.

So why build one where the code doesn’t ask? Because the benefit is drainage and drying, and Calgary walls still get wet. Fibre cement and wood-based claddings do better when their backs aren’t held damp against a membrane, and a cavity lets synthetic housewrap sit behind fibre cement within its evaluated conditions. It also gives the wall a margin for workmanship: a missed lap or an aged sealant joint has somewhere to drain.

What Decides the Specification

Cladding type and orientation set the layout. Horizontal lap siding nails straight into vertical furring on stud lines, which keeps the drainage path clear from base to top. Vertical cladding such as HardiePanel or board and batten needs a nailing layout that still lets water drain, and we settle it before strapping goes on. Heavier claddings need furring and fasteners chosen for the load.

Depth is the other decision. The code’s minimum air space is 9.5 mm and a drainage mat’s is 10 mm, but standard furring gives more, which drains and vents better and also pushes the face of the wall outward. That extra depth has to be picked up at every window, door, soffit and corner with deeper trim or returns, and cavities deeper than 25 mm bring fire blocking into the design. Where the cavity is combined with exterior insulation, the furring depth is added to the board thickness when we plan trim and fasteners.

Around Calgary

On an inner-city infill in southwest Altadore finished in HardiePanel board and batten, the cavity isn’t a design choice: HardiePanel requires it. On a northwest Edgemont home from the 1980s or 1990s being re-clad in new cedar, a rain screen is what we recommend, particularly over added foam. West of the city, homes on exposed lots in Cochrane take more wind-driven rain than a sheltered lot, and the cavity gives water that gets past the siding a clear path back out.