Vented vs Solid Soffit: The Attic Ventilation Arithmetic (OBC 9.19, 1 in 300, Half at the Eave), How Many Feet Have to Be Vented, Where Solid Belongs, and the Bathroom Fan That Should Never End at the Soffit

The arithmetic, once

The code rule is 1:300. Take the attic floor area, divide by 300, and that is the total net free vent area in square feet. Half of it, at least, goes at the soffit; the rest goes high, at the ridge or in roof vents near the peak. Net free area is the open hole after the screen and the louvers are subtracted, not the size of the vent, and the panel maker prints it on the sheet.

Perforated aluminum soffit gives about 8 sq in. of net free area per linear foot at a 12 in. depth; vinyl vented panels are in the same range, a little more or less by brand. So the question “how many feet of vented soffit do I need” is three lines of arithmetic, and the table does it for four attics.

The 1,500 sq ft line is the one on the price schedule: 2.5 sq ft at the eave, 360 sq in., 45 lf of perforated soffit. On the hip-roof bungalow with 156 lf of eave that is less than a third of the run, so alternating one vented panel with two solid ones does it. Running it all vented does no harm and most crews do, because sorting two panel types on a ladder is slower than running one.

Why a solid eave starves the attic

Air moves through an attic because warm air rises. The ridge vent or the roof vents let it out at the top; the soffit lets replacement air in at the bottom. Take away the inlet and the outlet has nothing to pull, so the attic sits there with the same air all winter, warmed by the ceiling below it. A warm attic melts the snow on the roof from underneath, the melt runs down to the cold overhang and freezes, and that is an ice dam. I am not going to re-explain the physics here because the ice dam page already does, and the attic ventilation page covers the roof-vent half of the 1:300 rule and what a vent costs.

The other way to starve an attic is to vent the soffit and then bury the vents from the inside. Blown insulation pushed out to the eave covers the top of the soffit and the perforations might as well be paint. Every rafter bay at a vented eave needs a baffle, the foam or cardboard chute stapled to the underside of the roof sheathing that holds the insulation back and leaves a 2 in. channel from the soffit up to the attic air. A good soffit crew looks up through the opening before the panel goes on and tells you if the bays are plugged. A great one has baffles on the truck.

Where solid soffit belongs

1. Under a porch roof or a carport that has no attic above it. There is nothing to ventilate, and a vented panel there is a doorway for wasps.
2. Under a cathedral ceiling with spray foam against the roof deck, where the roof is an unvented assembly by design. Venting the soffit of an unvented roof just pumps cold air into the foam’s edge.
3. On the rake, the gable end, where the soffit is a narrow return and air coming in there does not reach the rafter bays anyway. Vented does no harm; solid looks tidier.
4. Over a deck or a patio door where a fully vented run would drip condensation from the attic onto the table in April. One or two solid panels there, vented either side.
5. Within 3 ft of a chimney or a gas appliance vent that terminates at the wall, so the attic does not breathe exhaust.

Everywhere else on an eave with an attic above it, vented, at least to the arithmetic. In the ICF houses we build the walls do not breathe, which is the point of them, so the roof has to, and every eave gets perforated aluminum from end to end with a baffle in every bay.

The bathroom fan that ends at the soffit

This is the commonest mistake I find on a 1990s house, and it is in the soffit, so it belongs on this page. The bathroom fan duct runs out to the eave and ends above a vented soffit panel, on the theory that the air will go out through the holes. Some does. The rest, warm and wet, gets drawn straight back up through the next perforated panel into the attic, which is the inlet, which is where the attic is pulling from. The attic sheathing above that bathroom goes black and the insulation under it goes damp, and someone sells the owner a mould remediation.

A fan has to exhaust to the outdoors through a hood with a damper. The honest options are a roof hood above the insulation line, a wall hood through the gable end, or a purpose-made soffit exhaust box with its own damper that is set into a solid panel with the vented ones kept a few feet away. The soffit crew sees the duct end when the old panel comes off, and the quote should carry a line for terminating it properly, which is small money next to the attic. The wall protection hub covers what to do if the sheathing is already stained.

What it costs, which is the easy part

Vented and solid are the same rate: $8 a foot for vinyl and $11 for aluminum up to 12 in. deep, plus $5 for each extra foot, or $25 a foot for the aluminum soffit-and-fascia package up to 24 in. The soffit and fascia cost page prices two houses through. Baffles are a few dollars a bay from the inside and an hour of a crawl in the attic; a crew that is already on the eave can often slip them in from outside before the panel goes up, and that is worth asking for in the quote.

If the attic is getting new insulation at the same time, the baffles go in first, then the insulation, then the soffit closes it up. Doing it in the other order means someone goes back into the attic on their stomach, and nobody prices that cheerfully.

Questions people ask

Sources and further reading

• Ontario’s Building Code: the official page with the current regulation; cladding is Part 9 Section 9.27, masonry veneer 9.20, attic ventilation 9.19.
• Keeping the Heat In: Natural Resources Canada: walls, exterior insulation at re-siding, air and vapour barriers, and what changes at the windows and soffits when the wall gets thicker.
• Canada Mortgage and Housing Corporation: the Canadian wall-assembly and moisture research behind rainscreens and the second plane of protection.