Roof Ventilation and Attic Moisture: Why Intake and Exhaust Have to Balance

1. What Ventilation Does in Summer and Winter

Summer. CertainTeed's installation manual gives typical numbers for a home with no attic ventilation on a sunny 90 degree day: up to 170°F at the roof sheathing and up to 140°F at the attic floor. That heat can carry into the rooms below overnight, and over time excess attic heat can contribute to early failure in some shingles. A steady flow of air along the underside of the deck carries the heat out.

Winter. Cooking, showers, laundry and dishwashing put water vapor into the air; CertainTeed's manual puts a family of four at 2 to 4 gallons a day. Warm, moist air works its way up into the attic through ceilings and around light fixtures and pipes. When it reaches the cold underside of the roof deck and the nail tips, it condenses as water or frost. That moisture can swell and buckle the deck, rot it, and drip onto the insulation. Ventilation dilutes and carries that moist air out before it does damage.

Ice dams. When attic heat melts snow on the upper roof, the water runs down and refreezes at the colder eaves. CertainTeed's manual is clear that no single fix stops ice dams: it takes ventilation, insulation and an ice barrier at the eaves working together.

2. Intake and Exhaust: How the Air Moves

Warm air rises, so exhaust vents go at or near the ridge and intake vents go at the lowest part of the attic. Wind moving over the house adds to the pull. The common types:

3. Why the Two Sides Have to Balance

An attic can only exhaust as much air as it can take in. CertainTeed's manual uses the example of a system with 75 percent of its vent area high and 25 percent low: the airflow is limited to what the low vents can supply. That is why its rule is that intake should be equal to or greater than exhaust, with the vents spread evenly so air washes the whole underside of the deck instead of leaving hot spots.

4. What Ohio Code Requires

Section 806 of the 2019 Residential Code of Ohio requires enclosed attics to be cross-ventilated (the code has separate rules for sealed, unvented attics; this guide covers vented ones). The minimum net free vent area is 1/150 of the area of the vented space. It drops to 1/300 when 40 to 50 percent of the required vent area is in the upper part of the attic, no more than 3 feet below the ridge, and the rest is in the bottom third. The code also requires at least a 1 inch air space between the insulation and the roof deck at the eave vents.

Here is the arithmetic for a 1,200 square foot attic floor. At 1/150 it needs 8 square feet of net free area, which is 1,152 square inches: 576 in, 576 out. With balanced high and low venting at 1/300, it needs 576 square inches, 288 in and 288 out. Every vent product is rated for its net free area; CertainTeed's manual lists about 50 square inches for a typical box vent and 56 for a 16 by 8 inch soffit grille. Use the actual rating of the product being installed.

CertainTeed's shingle warranty has ventilation terms of its own, and its manual says coverage can be reduced for shingle problems caused by inadequate ventilation.

5. Common Ventilation Mistakes

• Insulation over the soffit vents. Blown-in insulation pushed into the eaves can close off the intake entirely, even when the soffit grilles look fine from outside. Baffles keep a channel open.
• Mixing exhaust types. CertainTeed's manual says never to mix two types of exhaust on the same attic. When a ridge vent shares an attic with box vents, gable louvers, turbines or a powered fan, the other openings short-circuit the flow from soffit to ridge and can let weather in.
• Adding exhaust without intake. More vents at the top do little when the bottom is starved. Over a cathedral ceiling, CertainTeed warns that a ridge vent without balanced intake can pull moist air up from the living space.
• Bath and kitchen fans venting into the attic. Ohio code requires exhaust fans to discharge outdoors, not into an attic, soffit or ridge vent.

6. Signs Worth a Closer Look

• Frost or water drops on the nail tips or the underside of the deck on a cold morning
• Dark staining or mold on the attic side of the sheathing, or damp, matted insulation
• A wavy or sagging deck you can see from the street
• Heavy icicles and ice dams along the eaves after snow
• Upstairs rooms that stay hot long after sundown

A roof replacement is the best time to correct ventilation, because the deck is open and the ridge and eaves are being rebuilt anyway. Before we write a replacement scope, we look in the attic where it is accessible, count the existing intake and exhaust, and check whether the soffits are actually open. See What Goes Into a Complete Roof System for how ventilation fits with everything else.