Two jobs, not one
Attic ventilation gets explained badly because people try to give it a single purpose. It has two, and they matter in different seasons.
In winter the job is moisture. A household puts several litres of water into the air every day through cooking, showers, laundry, plants and breathing. Warm air holds that moisture; cold air cannot. When indoor air works its way into the attic and meets sheathing sitting at twenty below, the moisture comes out of it as frost. A good frost layer on the underside of a roof deck can be a centimetre thick. Come the first thaw it melts all at once and rains on the insulation, and the homeowner calls a roofer about a leak that is not a leak.
In summer the job is temperature. An unvented attic under dark shingles in a Prairie July runs extremely hot, which cooks the asphalt, bakes the sheathing adhesives and pushes heat down through the ceiling into the bedrooms. Moving air through the space takes the edge off.
Winter is the one that does structural damage in this country, so if the two goals ever conflict, moisture wins.
Intake and exhaust: a loop, not a hole
Ventilation works because air enters low and leaves high. Warm air in the attic rises to the ridge and exits; that departure pulls replacement air in through the soffits at the eaves. Wind helps. The whole thing is a loop, and a loop with one end blocked does not run.
This is where most attics fail. The exhaust side is visible from the street and gets attention; the intake side is tucked under the overhang and gets forgotten. Three failures account for nearly all of it:
- Insulation pushed into the soffit. Whoever blew the attic in filled the perimeter wedge solid, and now the vents open into a wall of cellulose.
- Soffit panels that are decorative. Continuous vented aluminum soffit looks perforated along its whole length, but if the plywood behind it was never cut open, it does nothing. Someone has to look.
- Paint. Older wood soffits with small round vents, repainted four times, end up with the holes bridged over.
The fix is a pair of parts: cut or clear the openings, then install baffles, sometimes sold as rafter vents, which are rigid channels stapled to the underside of the sheathing. They hold a 40 to 50 mm air gap open from the soffit up past the insulation line so the insulation can be full depth at the edge without choking the airflow.
Intake should be at least equal to exhaust, and splitting the total roughly half and half, low and high, is the standard target. An attic with generous ridge venting and sealed soffits is worse off than one with modest venting at both ends, because the exhaust will draw its make-up air from the house.
How much is enough
The convention used across Canadian practice is a ratio of net free vent area to insulated attic floor area, commonly expressed as 1 to 300 where there is a vapour barrier and balanced high and low venting. Net free area is the real open area after the louvres and insect screen, not the size of the hole in the roof, and it is printed on the vent packaging for that reason. Low-slope roofs and certain cathedral assemblies have their own requirements. The number that governs your house is whatever your province has adopted from the National Building Code, so confirm it locally rather than trusting a rule of thumb from a forum.
The exhaust options
| Type | Works well when | Watch for |
|---|---|---|
| Ridge vent | There is a long, continuous ridge and matching soffit intake. | Needs an external baffle to resist wind-driven snow. Short ridges give little area. |
| Box or static vents | Hips, dormers and cut-up roofs with no usable ridge. | Must be set near the top of the slope. Too few is the usual error. |
| Turbine | Exposed, windy sites. | Moving parts, bearings, noise, and nothing happening on a still day. |
| Powered fan | Rarely the right answer on a house. | Can depressurize the attic and pull heated indoor air, and moisture, up through the ceiling. |
| Gable louvres | Older roofs without soffits, as a secondary path. | Fighting with a ridge vent for the same air. Pick one strategy. |
That last point deserves emphasis: mixing exhaust types on one attic usually makes things worse. A ridge vent paired with open gable louvres tends to short circuit, drawing air in the gable and straight out the ridge while the far corners of the attic stay dead. One continuous exhaust strategy, fed by soffits, is the arrangement that behaves predictably.
Things that quietly ruin a good system
- A bathroom fan ducted into the attic instead of through the roof or wall. It is the single largest moisture source you can install.
- An uninsulated fan duct running a long way across a cold attic, which condenses inside and drips back down through the ceiling fan.
- A dryer vented into the attic. Never acceptable.
- Pot lights, attic hatches and plumbing stacks left unsealed, which make the attic an extension of the house.
- Spray foam or stored boxes packed against the sheathing at the eaves.
Cathedral ceilings and finished attics
Where there is no attic, only a rafter cavity between the drywall and the deck, the same loop has to run inside each individual bay: intake at the eave, a kept-open channel against the sheathing, exhaust at the ridge, and nothing blocking the route. A single bay stuffed with batts after a renovation will show up as a dark line of staining on the ceiling a winter or two later. The alternative is an unvented assembly designed as such from the start, with the insulation applied tight to the underside of the deck so that no air space exists to condense in. Both approaches work. A half-finished mixture of the two does not.
Reading the signs
Go up with a flashlight on a cold day. Frost or dampness on the nails and sheathing means indoor air is arriving. Rusted nail shanks, dark mould blooming on the north slope, matted or discoloured insulation, and sheathing that feels soft when you press it all point the same way. In summer, an attic that is not merely hot but stifling and still, with no perceptible movement at the soffits, says the intake is closed.
None of this requires special equipment, and a twenty-minute look once each season tells you more about the health of a roof than its age does.