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Attic Ventilation

Attic Ventilation Done Right

The reason a ten-year-old roof in Wisconsin can look twenty. Intake and exhaust, balanced and actually open — roof ventilation for Watertown’s housing stock, with the code section that sizes it.

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The Answer

Attic Ventilation In Wisconsin: Intake, Exhaust And The Code That Sizes It

Attic ventilation in Wisconsin is intake at the soffits and exhaust at the ridge, sized to the ceiling area — at least 1/300 under SPS 322.39 — with insulation kept off the soffit vents. The most common fault is no intake: a ridge vent over buried soffits pulls air from the house and cooks the shingles.

Every shingle manufacturer conditions its warranty on adequate ventilation, so a blocked soffit is a voided warranty as well as a short roof. The section below is the code’s own arithmetic; the rest is what it looks like in an attic.

Why It Matters

Attic Intake Versus Ridge Exhaust, In Plain Language

An unventilated attic cooks in summer and sweats in winter. The heat bakes the asphalt out of the shingles from underneath, and the moisture condenses on the sheathing until it delaminates. Both shorten the roof, and neither is visible from the street.

The most common fault we find is not too little exhaust — it is no intake. A ridge vent with the soffits blocked by insulation pulls its air from the house instead of from outside, which makes the problem worse and costs you heat. It is also the fault that voids shingle warranties: every major manufacturer conditions its cover on adequate ventilation, and an attic without intake does not have it.

Inspection Points

What An Attic Ventilation Check Covers

Soffit intake

Open, baffled, and not buried under blown insulation. This is the half everyone forgets, and the half the code is explicit about.

Ridge exhaust

Continuous, with an external baffle so wind does not drive rain or snow through it. Ridge vent installation is best done at a replacement but retrofits in a day.

The balance

Roughly equal intake and exhaust area. Mismatched, the attic short-circuits and half of it gets no airflow at all.

Bath and kitchen fans

Vented outside, not into the attic. A bathroom fan discharging into the roof space is a lot of water every day, and frost on the underside of the sheathing in January is how it shows.

Consequences

How Bad Ventilation Shortens Shingle Life And Voids Warranties

Summer

An attic can run well above outdoor temperature on a July afternoon. The shingles above it are heated from both sides, the asphalt oxidizes early, and the roof curls and cracks years before its rated life.

Winter

Warm, moist house air reaches the cold sheathing and condenses. The plywood delaminates, nails rust, and the snow on the roof melts from below into an ice dam at the eave.

The warranty

Manufacturers condition their shingle cover on adequate ventilation, and an inspector who finds blocked soffits has a reason to decline a claim on the material. The warranty page explains what the two warranties each cover.

The heating bill

A ridge vent with no intake pulls conditioned air out of the house through every ceiling gap. You are paying to heat the attic and then venting it.

Straight Answers

Ventilation Questions

Usually not. Powered fans can depressurise the attic and pull conditioned air out of the house through ceiling gaps, which costs you money and adds moisture — the opposite of what you wanted. Fix the passive intake and exhaust first: with the soffits open and a continuous ridge vent, the attic moves its own air, and the 1/300 sizing in SPS 322.39 is met without a motor.

Both, for different reasons. Summer, it takes heat out so the shingles are not cooking from below. Winter, it keeps the deck cold so snow does not melt and refreeze at the eaves as an ice dam, and it carries out the moisture that would otherwise condense on the sheathing. A cold, dry attic in January is the sign of a roof that will last its rated life; the ice dam page explains what the alternative looks like.

Often, yes. Clearing the soffit intake and installing baffles at every rafter bay is an attic-side job — SPS 322.39(2)(c) is explicit that insulation shall not block the free flow of air, and blown-in insulation at the eaves is exactly what does. So is redirecting a bathroom fan that discharges into the attic. A ridge vent is best cut in at a replacement, but it can be retrofitted in a day.

Two quick checks from the ground and one from inside. After a snowfall, a roof that stays evenly white is a cold attic; a bare stripe along the ridge with icicles at the gutters is a warm one. In the attic, on a cold morning, look for frost or moisture on the underside of the sheathing, and look at the eaves for daylight through the soffit vents — if the insulation is pushed to the roof deck there is no intake. The free inspection includes the attic where we can reach it, and the FAQ covers what happens next.

Related

Ice Dams And Roof Replacement: The Pages This Ties To

All roofing services

Replacement, repair, storm work, inspections, ice dams and ventilation, and how they fit together.

Roofing services

The questions we get asked

Cost, timing, permits, insurance and warranties, answered plainly on one page.

Roofing FAQ

Ice dam removal

The winter symptom of the problem on this page, and the order the fixes go in.

Ice dams

Roof replacement

Balanced ventilation is part of the scope of every roof we put on, and the reason the new one lasts longer than the old.

Replacement

Free Inspection

Roof Aging Faster Than It Should?

Free inspection, attic included where we can reach it. We will tell you whether the soffits are open and what the fix costs before any shingle is touched.

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