Section 9.19 of the Ontario Building Code is short — barely two pages — but it is the reason attic sheathing on an old Toronto bungalow stays dry through a 40 °C swing from August humidity to February cold. The whole rule set collapses to one idea: a ventilated attic has to breathe faster than the interior of the house can push moisture into it. Size the vent area from the insulated ceiling, split it 50/50 between low and high, give the air a 63 mm channel to run in, and the physics handles the rest.
Two problems stack up in an unvented attic. The first is summer heat — unvented roof sheathing hits 80 °C on a sunny July afternoon and bakes the shingles from below, shortening their service life by a factor of two. A vented attic strips that heat off and drops sheathing temperature by 15 to 25 °C. The second, bigger, problem is winter condensation. Warm interior air carries water vapour at roughly 8 grams per cubic metre at 22 °C and 50 percent relative humidity. That same air, cooled to -10 °C in the attic, can only hold 2 grams per cubic metre — the other 6 grams have to come out of solution, and the coldest surface is the underside of the roof sheathing. Without ventilation, that 6 grams condenses every cubic metre of air that leaks up from the house below, soaks into the OSB, freezes into a plate of ice, and thaws each sunny day onto the insulation. A year of that cycle rots the sheathing from the back. The vent area from 9.19.1.2 exists to flush that moist air out before it condenses.
OBC 9.19.1.2.(1) gives you 1:300 as the default ratio — one square foot of net free vent area per 300 square feet of insulated ceiling. 9.19.1.2.(2) doubles the requirement to 1:150 in two cases: either the roof slope is less than 1:6 (any low-slope or flat roof) or the construction is a roof-joist assembly, meaning a cathedral ceiling where the insulation sits between the rafters instead of on an attic floor. In field practice there is a third case every seasoned carpenter applies even if the code text is loose about it: 1:150 when the ceiling assembly has no warm-side vapour barrier. The reason is physical — without poly or a vapour-retarder primer between the living space and the insulation, interior moisture diffuses freely into the attic, and the attic needs twice the exhaust capacity to carry it back out. A new build in Ontario will almost always have 6-mil poly, so 1:300 is the common case. A 1950s renovation where the old plaster ceiling is staying in place probably does not qualify, and sizing at 1:150 is the safer call.
9.19.1.2.(3) requires vents distributed uniformly on opposite sides of the building, with at least 25 percent at the top and at least 25 percent at the bottom. Every AHJ enforces this as a straight 50/50 split — half the NFA at the eave, half at the ridge or gable — because the attic needs both an intake and an exhaust to complete the convective loop. Soffit-only venting is the most common failure in older stock: builders put perforated aluminum soffit on a 1970s bungalow, never cut a ridge slot, and the total vent area looks fine on paper — but the air has no way to leave. The result is a stagnant attic where warm moist air piles up against the cold sheathing and condenses. Ridge-only has the opposite problem: no intake, so the ridge vent pulls conditioned air up from the living space through every ceiling penetration and depressurizes the house. Gable-end vents alone can work mathematically but give a weak loop that ignores the centre of the space — most builders treat gable vents as a supplement to soffit intake, not a substitute.
Ice dams are the visible symptom of a failed attic ventilation package. The loop starts in the middle of the roof: heat leaks from the ceiling into the attic, warms the attic air, and melts the snow sitting on the sheathing above the heated footprint of the house. Meltwater runs down the roof and hits the eave — which overhangs the exterior wall and therefore has no heat under it — freezes, and builds an ice dam. Subsequent meltwater backs up behind the dam, finds any nail hole or shingle lap, and drips into the soffit or the top plate. A vented attic breaks the chain by keeping the whole roof surface at ambient temperature: cold air enters at the soffit, washes up under the sheathing, and exits at the ridge, so the middle of the roof never gets warmer than the eave. The 63 mm clearance in 9.19.1.3.(1) exists so that convective path does not choke off against the sheathing.
Cathedral ceilings have two legal paths. The vented path — 9.19.1.2 with 1:150 NFA and 9.19.1.3 with a continuous 63 mm air channel above the insulation — works if every rafter bay is either separately vented (continuous soffit + continuous ridge) or interconnected at the top with 38 by 38 mm purlins per 9.19.1.2.(4). The channel is usually a rigid foam baffle installed in every bay before insulation goes in; the 25 mm baffle minimum at the eave in 9.19.1.3.(2) steps up to 63 mm through the rest of the run. The unvented path — 9.25.2.5 — fills the rafter bay with closed-cell spray polyurethane foam up to the underside of the sheathing, no air gap. The foam is the vapour barrier, the air barrier, and part of the insulation in one layer. Thickness is a dew-point calculation for the climate zone, not a guess — a typical Zone 5 spec lands around R-31 (about 5 inches of closed-cell). Unvented cathedrals are the default on architect-designed homes with dormers and valleys where maintaining a continuous air channel is impractical.
Three failures drive most attic ventilation problems. First, insulation blocking the soffit. Blown cellulose installers let the fill drop into the eave, which chokes the soffit airway and cuts the NFA to nearly zero even though the soffit looks fine from outside. 9.19.1.3.(2) requires a preformed baffle at every rafter bay before insulation goes in — rigid foam, cardboard, or moulded plastic. A missing baffle is a 10-minute fix from above. Second, an unsealed attic access hatch. A 550 × 900 mm hatch with no gasket leaks interior air upward whenever the house is heated, dumping moisture directly into the attic. 9.19.2.1.(3) requires the hatch to have a door or cover; 9.25.3 requires it to be part of the continuous air barrier. Weatherstrip it like an exterior door and insulate the cover to match the ceiling. Third, a bathroom fan ducted into the attic instead of to the outdoors. Saturated air blown straight at the roof sheathing condenses and runs back down the duct. 9.32.3.7 requires the bathroom fan to duct to the outdoors through a wall cap or a dedicated roof cap — never terminate into the attic, never daisy-chain two baths on one duct.