Attic Ventilation Basics: Soffits, Baffles and Ridge Vents

Attic ventilation to prevent mold works by bringing outdoor air in low at the soffits and letting it out high at the ridge or roof vents, so moisture that reaches the attic is carried away before the wood stays damp. It only works as a system. Open soffits, baffles that keep the insulation off them and a matched exhaust vent all have to be in place.

How attic ventilation works to prevent mold

A vented attic is meant to be a cold, dry space that tracks the outdoors. Wind pushes air into the soffit vents on one side of the house and pulls it out the other side and out the top. Warm air rising inside the attic adds a gentle upward draw. That steady exchange does two jobs. In winter it removes small amounts of moisture before they build up on the sheathing. In summer it lets heat escape.

The air has to enter low and leave high to sweep the whole underside of the roof. Where air cannot get in, pockets of still, damp air form, and that is where mold starts. The lower few feet of sheathing near blocked eaves are the classic spot.

The three parts: soffits, baffles and ridge vents

• Soffit vents are the intake. They are the perforated panels or individual grilles under the roof overhang. They are the most important part and the most often blocked.
• Baffles, also called rafter vents, are foam or cardboard chutes stapled between the rafters at the eave. They hold the insulation back and keep a clear channel from the soffit up into the attic. Without them, blown insulation slumps into the eave and seals the intake.
• Ridge vents are the exhaust. A continuous slot is cut along the peak and capped with a vent that sheds rain. Square box vents near the peak do the same job on roofs without a ridge vent. Gable vents and spinning turbines are other exhaust types found on older homes.

How much ventilation an attic needs

The common rule of thumb in Ontario is one sq ft of open vent area for every 300 sq ft of insulated ceiling, split between intake and exhaust. The number that counts is net free area, which is the actual open space after screens and louvres are subtracted. It is stamped on the vent or listed by the manufacturer, and it is much smaller than the outside dimensions of the vent.

Balance matters as much as the total. Intake should be at least equal to exhaust. If there is plenty of exhaust and very little intake, the roof vents pull their makeup air from the easiest source, which is the house below, through gaps in the ceiling. That brings in exactly the humid air you are trying to keep out.

Common ventilation mistakes that lead to mold

• Insulation over the soffits. A top-up without baffles is the usual cause.
• Vented aluminum soffit installed over solid wood. On many older houses the original plywood or board soffit was left in place with no holes cut. It looks vented from the ground and moves no air.
• Mixing exhaust types. A ridge vent combined with gable vents or box vents can short-circuit. Air enters one high vent and leaves another, and the lower roof never gets swept.
• Powered attic fans. They can pull more air than the soffits can supply and draw house air up through the ceiling.
• Too few exhaust vents for a complicated roof. Hips and valleys divide the attic into sections, and each needs a way out.

How to check your own ventilation

1. On a bright day, open the hatch with the attic light off. Look toward the eaves. You should see slivers of daylight at the bottom of each rafter bay, or at least at regular intervals.
2. Look for baffles. There should be a chute in the rafter bays above the outside walls, with insulation stopping below its top edge.
3. From outside, look at the soffits. Count the vented panels or grilles and check that they are not painted shut or clogged with dust and nests.
4. Count the exhaust vents on the roof and note the type.

Houses with no soffits

Many older London homes, including a lot of storey-and-a-half houses in Old North, Wortley Village and Old East, were built with short or closed eaves and no intake vents at all. Roofers can add intake in other ways, such as edge vents that sit under the first rows of shingles or low-mounted roof vents on each slope. Sloped ceilings on a half storey are harder, because the insulation fills the rafter space and leaves no air channel. Those need a plan from someone who knows that style of house.

Keep in mind what vents cannot do. Ventilation removes modest amounts of moisture. It cannot keep up with a bathroom fan blowing into the attic or a ceiling full of air leaks. Outdoor air in January is cold and can carry very little water away. So vents are half the answer, and stopping house air at the ceiling is the other half.

Start with the daylight check at the hatch. If you cannot see light at the eaves, you have found the first thing to fix, and it costs nothing to look.

Common questions