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Rainscreen Siding in Ontario: What the Gap Behind the Siding Does, When the Code and the Product Want It, Strapping Details and What It Costs
Exterior Finishes
Rainscreen: Every Siding Leaks, So Give the Water Somewhere to Go and the Wall a Way to Dry
A rainscreen is not a product. It is a gap, 3/8 to 3/4 in. wide, between the back of the siding and the house wrap, open at the bottom behind a screen and open at the top under the soffit. Water that gets past the siding, and some always does, runs down the wrap and out the bottom instead of sitting against the sheathing. Air moves through the gap and dries both faces. It costs about $2.50 a square foot in strapping on a wall that does not already have it. Brick has had one for a century; the 1 in. air space behind brick is the original rainscreen. The rest of the claddings are catching up.
Why water gets behind siding in the first place
Siding is the first plane of protection in the code’s words, and it is a shingle, not a boat. Wind-driven rain goes up under laps, in at every J-channel, around every penetration and through every joint that opened up when the trim shrank. Then there is the water that was never rain: warm house air leaking out through the wall in January, hitting the cold back of the siding and condensing. On a dark fibre cement or cedar wall in April the sun drives moisture from the wet cladding inward, through the wrap, into the sheathing. That one is called solar vapour drive and it surprised a lot of people in the 1990s.
The second plane of protection, the house wrap or sheathing membrane, is supposed to catch all of that and shed it. It does, as long as the water can actually run down it. Press the siding tight to the wrap and the water bridges across by capillary action and sits there. Fibre cement and wood siding nailed flat on wrap stay wet on the back for days after a storm. The gap breaks the bridge. That is the whole idea, and it is why water behind siding is a bigger problem on a house without a gap.
Which claddings need it and which make their own
| Cladding | Does it need a cavity? | Why |
|---|---|---|
| Vinyl siding | No, in practice | The profile stands off the wall, the laps are loose and the panel is full of weep slots. Vinyl drains and vents itself. Over foam it is still nailed loose. |
| Fibre cement lap | Allowed, not required by the sheet | James Hardie accepts it direct on wrap or on strapping. I strap it on a north wall, a dark colour, and anywhere under a shallow eave, because the back of the board stays wet otherwise. |
| Engineered wood (LP) | Allowed; LP recommends it in wet climates | It is wood fibre and resin. Keeping the back dry is what the 50-year warranty is quietly counting on. |
| Cedar and real wood | Yes | Wood on wrap cups, checks and sheds its finish from the back out. The cedar associations have said strap it for twenty years. Cedar siding. |
| Board and batten, any material | Yes, cross-strapped | Vertical boards need horizontal strapping, which dams the water. The fix is a second layer of vertical strapping under it, or notched horizontals. |
| Steel and aluminum | Usually built in | The panel profile makes a cavity; the trim and closures decide whether it drains. |
| Brick veneer | Yes, 1 in. air space by OBC 9.20 | With weep holes at 800 mm and through-wall flashing at the bottom. Weep holes. |
| Adhered stone | Drainage mat or two layers of paper | Stone is bonded to the wall; the drainage happens in a 1/4 in. mat behind the lath, or it does not happen. |
| Acrylic stucco and EIFS | Drained EIFS, yes; on ICF, the foam face | Face-sealed EIFS with no drainage is the 1990s problem. Moisture behind stucco. |
OBC 9.27 asks for a second plane of protection behind every cladding and a way for water to drain from it; it does not demand a cavity for lap siding in Ontario the way the coastal codes do. The product sheet and the wall decide. “Allowed” on a sheet means the maker will honour the warranty either way; it does not mean the two walls last the same.
How it goes on
- The wrap goes on first, lapped like shingles, seams taped, every window flashed to it with a sill pan and head flashing before any strapping. The cavity does nothing if the water plane behind it is wrong. The flashing details.
- Vertical strapping over every stud, 1×3 or 1×4 at 16 in. on centre, or 3/8 in. plastic furring strips where the siding maker allows the thinner gap. Over exterior foam, the strapping is screwed through the foam into the stud with screws long enough to bite 1 in. of wood.
- Open at the bottom, screened. A perforated closure or insect screen folded over the bottom of the strapping, or the cavity becomes a wasp hotel. Kept 200 mm off grade.
- Open at the top into the soffit space, or vented through a screened gap under the frieze. A cavity closed at both ends is a sealed wet pocket.
- Blocking at every opening and horizontal trim, so the siding has something to fasten to around the window, with the head flashing extended out past the cavity and the trim.
- Jambs and trim built out to the new face. 3/4 in. of strapping moves the siding 3/4 in. away from every window; the brickmould or the extension jamb has to follow.
What it costs
$2.50 a square foot of net wall for 3/8 or 3/4 in. strapping, screened closures and the extra blocking, where the base siding rate does not include a cavity. On the 2,400 sq ft two-storey I use on every page of this site, that is $6,000 before HST. Under mid-grade fibre cement at $18 a foot, $43,200, the strapping is 14% of the siding. Under cedar at $20, $48,000, it is 12.5%, and skipping it on cedar is throwing away the cedar.
The cheaper half-step is a drainage wrap, a crinkled or grooved house wrap that gives 1 to 2 mm of space behind the siding for about $0.25 more than plain wrap. It breaks the capillary bridge, which is most of the benefit on fibre cement, and it does nothing for drying because there is no air moving through 1 mm. I use it under fibre cement on a plain south wall with deep eaves and I strap the rest. Nobody has ever called me back about a strapped wall.
On a vinyl re-side, do not pay for strapping. The panel is its own cavity, and the $6,000 is better spent on the foam under it or on fixing the things that actually go wrong with vinyl.
On an ICF wall
The houses I build are ICF, so the wall is 2.5 in. of foam over concrete with plastic webs every 6 or 8 in. There is no sheathing to rot, which takes most of the urgency out of the question. I still strap fibre cement and wood siding on ICF, screwed to the webs, for the same reason as on a wood wall: the back of the board dries. Stucco goes straight on the foam with no cavity because the foam is the drainage plane and there is nothing behind it that cares. The ICF finishes article goes through all eight claddings on that wall, and this page covers what changes in the fastening.
Questions people ask
Is a rainscreen required by the Ontario Building Code?
Not as a cavity for most lap sidings. OBC 9.27 requires a second plane of protection, a sheathing membrane or equal, with flashing at openings and a drainage path. Brick veneer must have its 1 in. air space under 9.20. The coastal codes require a 10 mm cavity; Ontario leaves it to the product and the builder.
How much does a rainscreen add to a siding job?
About $2.50 a square foot of wall for strapping, screens and blocking, so $6,000 on a 2,400 sq ft two-storey, plus the window trim built out to meet it. A drainage wrap is a cheaper half-measure at about $0.25 extra.
Does vinyl siding need strapping?
No. The panel profile stands off the wall and the laps and weep slots drain and vent it. Strapping under vinyl is money for nothing unless there is foam under it and the nails cannot reach the studs.
Does James Hardie need a rainscreen?
The installation sheet allows direct application over a sheathing membrane and allows strapping. In Ontario I strap it on north walls, dark colours and shallow eaves, and use a drainage wrap elsewhere. Either way the clearances and flashing in the sheet are not optional.
Can a rainscreen cavity let bugs or cold air in?
Bugs, yes, if the bottom is not screened. Cold air, no: the air barrier is the wrap or the sheathing behind the cavity, and the cavity is outside it. A rainscreen does not change the insulation value of the wall.
Put the gap in the number
The exterior cladding cost calculator carries rainscreen strapping as its own line under any siding, so you can see what the wall costs done the way the cedar makers want it. Three quotes for fibre cement that are $6,000 apart often differ by exactly this line, and the $99 review will say which one has it.
Sources and further reading
- Ontario’s Building Code, Section 9.27 Cladding: the first and second planes of protection, sheathing membrane, flashing at openings and transitions, and the clearances from grade and roofs that every siding and stucco job has to meet.
- James Hardie installation instructions: the clearances, fastening, flashing and primed-cut rules a fibre-cement warranty depends on; the installer is held to this sheet.
- Canada Mortgage and Housing Corporation: the Canadian wall-assembly and moisture research behind rainscreens and the second plane of protection.
