Soffit Vents

Soffit vents are the intake half of an attic ventilation system. They’re intake components installed on the underside of roof eaves to allow cool air into the attic, which is necessary to prevent condensation, mold, and ice dams. That last word matters here. In Montana winters, blocked attic airflow is how ice dams form at the eaves — and a soffit vent is your first line of defense.

Here’s the catch most people miss. A soffit vent does nothing on its own. It must be paired with exhaust vents, such as ridge or gable vents, to function. Air comes in low at the eaves, rises, and exits high at the ridge. Break that loop and you’ve got dead air, trapped moisture, and a roof deck that stays wet. Always maintain a balanced 50/50 ratio between intake and exhaust to ensure effective ventilation.

So choosing a soffit vent isn’t really about the vent. It’s about matching Net Free Area to your eave construction and to the exhaust side of the system.

Types and How to Choose

Three formats cover almost every job. Pick based on whether you’re building new, retrofitting, or chasing a clean look.

Continuous strip vents run the full length of the eave. They offer the highest Net Free Area and most uniform airflow. These are the gold standard for new construction or full soffit replacements. The Cor-A-Vent S-400 is the benchmark here — the original continuous soffit venting solution for overhangs and eaves, with 10 sq. inches NFVA per foot, providing intake ventilation for any detail from zero overhang to open rafter tails. Its 1” x 1-1/2” x 4’ cross section fits narrow spaces where other vents can’t, like zero overhangs, and it’s crush resistant so you can install it with a power nail gun.

Got a tight eave? The PS-400 is a 1” wide by 3/4” high strip that matches the thickness of a 1x soffit board, providing the same 10 square inches of Net Free Vent Area per foot as the S-400 but fitting into tighter places where the S-400 can’t. Installation is easy — just cut an inch off the soffit board and nail PS-400 to the end, creating a pre-vented soffit panel.

Perforated panels replace standard soffit material with integrated holes, providing a streamlined, clean appearance.

Individual plug vents are the retrofit answer. Such as 4-inch round or 4x16-inch rectangular units, these are best for retrofitting existing solid soffits where you cannot replace the entire panel. These come in a range of sizes and finishes — 4x16, 6x16, and 8x16-inch rectangular vents in brown, white, and galvanized, plus damper versions on the larger sizes.

Net Free Area Is the Spec That Matters

Don’t buy on the size of the hole. Buy on Net Free Area. Always verify the manufacturer’s NFA rating rather than the gross size, as screens and louvers significantly reduce actual airflow. A big-looking vent with a fine screen can move less air than a smaller one rated honestly.

NFA is also how you size to code. The general requirement is a ratio of 1:300 — 1 square foot of ventilation for every 300 square feet of attic area, with 50 percent of the ventilating area at the eaves and 50 percent at the ridge. But interpretation of these codes may vary, so for exact information consult your local building code or building official.

Here’s how that math plays out on a real roof. In a straight gable roof application, a continuous V-600 series vent with continuous strip vents at both soffits provides 40 square inches of ventilation per linear foot, which is capable of adequately venting an attic up to 75 feet wide at a ratio of 1:300. Buildings 40 feet or wider may require additional soffit venting depending on local code, and on installations of that size you’d want to contact the manufacturer’s technical support.

Product	Format	Cross Section	Net Free Area
Cor-A-Vent S-400	Continuous strip	1” x 1-1/2” x 4’	10 sq in / lin ft
Cor-A-Vent PS-400	Low-profile strip	1” x 3/4” x 4’	10 sq in / lin ft
4x16 / 6x16 / 8x16 plug vents	Individual rectangular	Rectangular	Varies by size
ClarkDietrich stucco vents	Stucco/EIFS integrated	1” to 6” wide	4 to 18 sq in / LF

For stucco and EIFS work, ClarkDietrich runs a different family. Their architectural soffit vent is a one-piece vent with a reinforcing rib to key into stucco, with 3/4” elongated vent holes. Vent area on those ranges from 4 sq in per LF on the 1” models up to 18 sq in per LF on the 6” models, depending on width.

Material and Corrosion

Material decides durability and where a vent can live. Aluminum is durable and resists corrosion, vinyl offers consistent color, and fiber cement provides high strength.

The Cor-A-Vent strip vents take a different route. They’re made of durable corrosion-free high-density polypropylene with no motors, no energy use, and no maintenance from moving parts — suitable for all common roofing materials and conditions. The S-400 specifically is made from heat resistant polypropylene and comes in White, Black, or Tan.

For high-corrosion environments, there’s an upgrade. Any of the soffit vents can be special ordered with type 304 stainless steel staples for areas with high corrosion or rusting concerns.

Installation Basics

Two mistakes kill more ventilation systems than bad product: insulation blocking the airflow path, and unsealed frames letting weather and pests in.

Fix the first with baffles. Place insulation baffles (vent chutes) between rafters to keep insulation from blocking the airflow. Without them, blown insulation packs into the eave and chokes off the intake you just paid for.

For the vents themselves, use a template to mark precise cuts, then secure vents using exterior-grade screws. The Cor-A-Vent strip vents are power nailable. Then seal the perimeter — apply a bead of silicone caulk around the frame to create a weather-tight seal and prevent pest entry.

One system rule worth burning into memory: ridge and soffit vent systems should not be used in conjunction with other roof-top vents or gable-end vents. Mixing exhaust types short-circuits the airflow. Pick one exhaust strategy and pair it with your soffit intake.

A note for the snow country in this part of the world. On the exhaust side, ridge vents with an “E” in the name have an enhanced snow screen — worth specifying when you’re pairing continuous soffit intake against heavy snow loads and wind-driven powder.

Frequently Asked Questions

Do I really need soffit vents if I already have ridge vents? Yes. Ridge vents are exhaust only. Soffit vents must be paired with exhaust vents to work, maintaining a balanced 50/50 intake/exhaust ratio. A ridge vent with no intake below it pulls air from the path of least resistance — which often means it sucks conditioned air or moisture back through the attic instead of drawing fresh air up from the eaves.

How much soffit venting do I need? Start with the 1:300 rule. That’s 1 square foot of ventilation per 300 square feet of attic area, with half at the eaves and half at the ridge. Size the eave half using each vent’s Net Free Area rating, not its gross dimensions. For exact requirements, consult your local building code or building official.

What’s the difference between the S-400 and PS-400? Height. The PS-400 is 3/4” high and fits the thickness of a 1x soffit board, providing the same 10 square inches of Net Free Vent Area per foot as the S-400 but fitting tighter places where the taller S-400 won’t. Same airflow, lower profile.

Which vent works for a roof with no overhang? A continuous strip vent. The S-400 fits narrow spaces where other vents can’t, like zero overhangs and open rafter tails.

Why does Net Free Area matter more than vent size? Because the visible opening isn’t the working opening. Screens and louvers significantly reduce actual airflow, so always verify the manufacturer’s NFA rating rather than the gross size. Two vents the same physical size can move very different amounts of air.