Attic ventilation: soffit intake to ridge exhaust

Most of the attics we put a ladder into have vents. Fewer than half of them have ventilation. Those are not the same thing, and the difference explains a great many Ontario roofs that frost up in January, grow mould by March and shed their shingles ten years early.

A vent is a hole. Ventilation is air actually moving from one hole to another. If the air cannot complete the trip, the holes are decoration.

The system is a pair, not a product

Cold outside air has to come in low, travel up the underside of the roof, and leave high. Intake at the soffits, exhaust at the ridge. That is the whole mechanism, and it runs on nothing but the fact that warm air rises.

Remove either half and the other half stops working. A ridge vent with buried soffits has nothing to pull from, so it sits there doing almost nothing. Open soffits with no exhaust path fill the attic with cold air that has nowhere to go.

We get called about the exhaust side perhaps nine times out of ten. The problem is almost always the intake side.

How much you need

Ontario’s building code asks for unobstructed vent area of at least one square foot for every 300 square feet of insulated ceiling, and twice that on shallow slopes and in roof joist spaces with no attic above them.

Run it on a real house. A 1,500 square foot ceiling needs about five square feet of net free area, which is 720 square inches. Split it roughly in half: around 360 square inches coming in at the eaves, 360 going out at the ridge.

On the exhaust side that is easy. Most continuous ridge vents give somewhere in the region of 12 to 18 square inches per linear foot, so 25 to 30 feet of ridge covers it.

The intake side is where houses fail, because the number that matters is net free area, not how much soffit you have. A properly perforated aluminum or vinyl soffit gives you real area along its whole length. What a lot of Ontario houses actually have is long runs of solid soffit panel with a short vented section every few feet, installed for looks, and the total free area works out at a fraction of what the roof needs.

Aim for slightly more intake than exhaust, never the reverse. An attic with more exhaust than intake starts pulling its make-up air from the easiest available source, which is the heated house below, straight through every ceiling light and gap.

Why your intake is blocked

In the order we find them:

Insulation in the eaves. Someone topped up the attic, the blown cellulose or loose fibreglass drifted down the slope, and it packed solid into the space where intake air is supposed to enter. This is the single most common fault in the 1960s to 1980s housing stock around Barrie and across Simcoe County, where attics are shallow and the eave space is tight to begin with. The upgrade made the insulation better and the ventilation worse.

No baffles. Rigid channels stapled between the rafters at the eave hold that path open. They cost very little. On houses built before they became routine, there is nothing stopping insulation from closing the gap.

Soffit that was never vented. Older wood soffits were often simply wrapped in aluminum during a siding job, and the new panel was solid. From the ground it looks finished. From inside the attic there is no daylight at the eave at all.

Paint. Three coats of exterior latex over a perforated soffit closes the perforations. We see it on older homes through Orillia, Collingwood and the Grey Bruce towns where the soffits have been painted repeatedly.

Birds, wasps and mice. A nest at the eave can block several feet of intake, and the insulation behind it will be matted and useless.

A ridge vent over an uncut ridge. This one is worth its own warning. A shingle over ridge vent only works if the installer cut a slot through the boards along the peak first. We have climbed onto roofs with 40 feet of ridge vent sitting on a solid, uncut ridge. The homeowner paid for ventilation and got a plastic strip.

Exhaust: what to use and what to avoid

Continuous ridge venting is the best residential exhaust there is. It pulls along the entire peak, it has no moving parts, and it draws evenly, which is exactly what you want feeding from continuous soffit intake.

Box vents work. They are a reasonable answer where the ridge is short or broken up by dormers and hips, which describes a lot of cottage rooflines through Muskoka and the Kawarthas. Several, placed high and spread across the slope, are better than two placed wherever the old ones were.

Turbines spin, which people find reassuring. They are a bearing and a moving part added to a roof that did not need one, and in a cold snap they can seize. We would not put one on.

Powered attic fans are worse than nothing on most Ontario houses. A fan moves far more air than the soffits can supply, so the shortfall comes out of your heated or air-conditioned house through the ceiling. You end up paying to pull conditioned air into the attic, which adds moisture in winter and load in summer. If a contractor suggests a powered fan to cure a moisture problem, they have diagnosed the wrong thing.

Gable vents combined with ridge venting short-circuit each other. The ridge vent takes the easy air from the gable opening a few feet away instead of drawing from the eaves, and the lower half of the roof stops ventilating. Pick one path. If you are going ridge and soffit, block the gables.

Where ventilation is not the answer

Two honest exceptions.

Cathedral ceilings and finished attic rooms. There is no attic to ventilate. Each rafter bay is its own narrow channel and has to be vented individually from eave to ridge, which many were not. Some of these assemblies are better handled as a sealed, fully insulated roof rather than a vented one. That is a design decision, not a vent you can add later.

Unheated outbuildings. A drive shed or an unheated garage with no moisture source inside does not need the same treatment as a house. Do not let anyone sell you an attic ventilation package for a building that produces no moisture.

And a third thing that is not an exception but gets treated like one: ventilation does not fix a leaking air barrier. If your bathroom fan discharges into the attic, or your pot lights are open to it, ventilation is trying to carry away moisture that should never have arrived. Air seal the ceiling first. We go through the whole moisture picture in metal roof condensation and ventilation.

What changes when the roof goes metal

Three things, and one of them is us talking you out of an upgrade.

First, the ridge detail changes but the principle does not. Under a metal ridge cap we use a vented closure that lets air out while keeping snow and rain from driving back in. A standing seam roof vents at the peak as well as any shingle roof does, provided the slot is cut.

Second, a strapped assembly adds a ventilated cavity above the boards, between the roof and the panel. Air moving behind the metal keeps the panel closer to outdoor temperature, which helps with ice damming at the eaves. That is a real benefit and it is why the strapped system is what we put on most Ontario homes.

Third, and this is the part worth reading twice: the cavity above the boards is not a substitute for the ventilation below them. If your soffits are packed with insulation, a strapped metal roof will not solve it. You would be buying the more expensive assembly to address a problem that a morning’s work with baffles and a shop vac would fix properly. We would rather tell you that in the driveway. Clear the intake, then choose the roof assembly on its own merits.

The same caution applies to going over existing shingles. Strapping over a shingle roof is a sound method, but it has to leave the eave open. Sealing the soffit line under new strapping builds a moisture trap, which is one of the checks we walk through in going metal over shingles.

Check yours in twenty minutes

On a cold morning, with a torch:

• Stand in the attic at the eaves and look toward the soffit. You should see daylight. No daylight means no intake, whatever the vents look like from the ground.
• Check whether insulation has drifted into the eave space, and whether there are baffles holding it back.
• Look at the nail points coming through the boards. Frost or beads on them means moist air is reaching a cold surface.
• Look along the peak. Is there a cut slot above the ridge vent, or solid wood?
• Count your exhaust types. If you have gable vents and a ridge vent, you have a short circuit.
• Follow every bathroom and kitchen fan duct. Each one must leave the building.

Photograph whatever you find. It saves time for whoever quotes the work, and it tells you quickly whether you have a roofing problem or a ventilation problem.

The short version

Intake low, exhaust high, a clear path between them, and no second exhaust type stealing the draft. Get that right and the roof above it lasts longer, whatever it is made of. Get it wrong and no covering will save you.

We look at the attic before we quote the roof, on every job, from Barrie and Simcoe County through Muskoka, Grey Bruce, the Kawarthas, Collingwood and down into York Region. Inspections are also something we do on their own, without a roof attached, and you can see the rest of what we handle on our services page.

If you have frost in the attic, ice at the eaves or soffits you suspect have never worked, call 705-816-5282 or get in touch.