Attic ventilation may take a backseat to the more glamorous aspects of home improvement, but that doesn't mean that it's not important. A dry, well-ventilated attic prevents mold from developing both inside the attic and in the living areas below. Attic ventilation can cool down attics during the warm season and it can minimize ice dams in the cold season.
All code-compliant houses with attics will already have some attic ventilation, but it's always a good idea to expand upon minimum venting requirements to keep your house in top condition for the long term.
What Attic Ventilation Does
Attic ventilation both brings air into the attic and allows air to escape from the attic. Because roofs face the sun, they build up considerable heat in the warm season. Attic ventilation provides an outlet for this heat gain, and it allows fresh air to enter the attic.
Attic ventilation also prevents moisture buildup in the cold season. Colder air isn't able to carry moisture as well as warm air. So, cold attic air will condense the airborne moisture and deposit it on insulation, rafters, joists, and the bottom of the roof deck.
With attic ventilation, ice dams along the edges of the roof are kept to a minimum, as well.
What Is an Ice Dam?
An ice dam is a large mass of ice that builds on the edge of a roof. When the attic or roof heats up, the ice or snow on the roof melts. This water is trapped above the ice dam. The water can work its way under the shingles and leak into the house.
Related building materials last longer and work better because of attic ventilation. Since shingles and roof decks curl, buckle, warp, and split when the attic gets too hot, attic ventilation keeps roof temperatures at sustainable levels.
Ceiling drywall, joists, insulation, and rafters benefit from attic ventilation because they stay dry.
Passive vs. Mechanical Attic Ventilation
Passive Attic Ventilation
No cost to run
Vents even in power outage
Less air intake
Less air outflow
Cannot be controlled
Passive or natural attic ventilation comes in the form of holes or slots in the roof, gable, or the eaves, soffits, or facia. These holes or slots are screened to prevent birds or vermin (such as bats) from entering the attic. Types of passive attic ventilation include roof ridges, screened gable vents, roof turbines, or screened vents in eaves, soffits, and fascia.
Passive attic ventilation requires no electricity and is easy to maintain, because there are no moving parts or motors. Yet air movement is limited and often depends on climatic conditions.
Mechanical Attic Ventilation
Consistent air circulation
Moves greater quantity of air
Often requires professional installation
Mechanical or active attic ventilation comes in the form of electrically operated fans built into the roof or gable. Types of mechanical attic ventilation include powered roof fans and gable fans. Electricity is supplied by the household's electrical system or by solar panels that capture and store energy in batteries.
Mechanical attic ventilation provides consistent airflow that isn't dependent on wind or temperatures to operate. It can also move greater quantities of air than passive venting. Yet mechanical attic ventilation typically requires professional installation. Mechanical venting systems are frequently in operation, so they will break down over time and need maintenance and repair.
Well-Ventilated vs. Poorly Ventilated Attic
Roof deck in good shape
Clean attic insulation
Rafters, joists, and underside of roof deck clean and free of mold
Dry throughout the attic; no condensation
Fewer or no insects
A/C running at normal capacity
Fewer or no ice dams
Reasonable summer energy bills
Buckled, curling, warped shingles
Delaminating roof deck plywood
Mold on attic insulation
Mold on attic structural pieces (rafters, joists, etc.)
Condensation on rafters, joists, and roof deck
Greater number of insects
A/C working too much in hot months
Ice dams building up near eaves
High summer energy bills
Attic Ventilation Code Requirements
Building code requires attics to be vented. International Residential Code (IRC) R806.1 specifies minimums only:
- Cross ventilation is required in enclosed attics and other enclosed rafter spaces where there is a ceiling under the rafters.
- If the attic is separated or segmented (as if with a wall), each separate section should be ventilated.
- Cross ventilation openings should be no less than 1/16 inch and no more than 1/4 inch.
- In the event an opening is more than 1/4 inch, it should be covered with wire cloth, perforated vinyl, or other screening material. The openings in the material should be no less than 1/16 inch and no more than 1/4 inch.
- Ventilation openings should open directly to the outside air.
- Roof vents need to conform to International Residential Code (IRC) R802.7, which specifies the size and type of the openings.
How to Improve Attic Ventilation
Vents in Eaves, Soffit, Fascia
All homes should already have passive air vents in the eaves, soffit, or fascia. Improving attic ventilation starts here. Access the attic to make sure that no insulation is covering up these vents. Check for vermin droppings or nests that might be blocking the vents. Confirm that the vent screens are solidly attached. Nail into place any loose vent screens.
Ridge vents are passive attic ventilation systems built into the top ridge of the roof. A ridge vent is a long plastic or metal vent running along the length of the roof ridge (peak). This vent is covered with cap shingles to help it blend in with the rest of the roof. Heat rising from the attic can escape through the ridge vent. Wind blowing from either direction can flow into the attic.
Gable vents are passive ventilation devices installed on the upper portion of the house's gable—the triangular section. Gable vents can be as simple as large holes or slots covered in wire screen. Or they can be metal or plastic vents or louvers that are 1- to 2-feet square in size. Gable vents allow air to escape on its own from the attic, preventing moisture build-up.
Roof turbines are large spherical-shaped rotating metal vents. While classified as passive attic ventilation devices, they come with a boost—the wind. When wind catches the vanes on the turbine, the turbine rotates on ball bearings and draws air upward, out of the attic.
Actively spinning roof turbines can have a noticeable effect on attic ventilation. Even turbines at rest will expel some air from the attic, though not as much as when the turbines are rotating.
Gable Attic Fans
Gable fans are mechanical attic ventilation devices installed behind gable vents, on the inside of the attic. Gable fans run off of the household power. Once they are initially turned on, they will automatically turn on or off as needed, according to temperature needs.
Gable attic fans can move a tremendous amount of air from the attic—up to 5,000 cubic feet per minute (CFM). Most gable fans are rated in the 800 to 1,600 CFM range, though.
Roof Attic Fans
Roof attic fans are mechanical attic ventilation devices that draw 1,000 to 1,500 CFM of air from the attic.
Roof fans use household 120V power or solar power. Generally about two feet in diameter, most roof fans hug the roof at less than 10 inches high. Since some turbine fans can crest 24 inches high, roof attic fans, with their saucer-shaped hoods, are far less noticeable. They can even be installed on a pitched roof.
Hiring an Expert to Improve Attic Ventilation
Professionals can help you calculate how much ventilation you need, based on the size of the attic, then assess the necessary type of attic ventilation. They'll also have experienced technicians who can implement a full suite of attic ventilation devices.
A general contractor can do this work, but first check their qualifications for experience with energy- or ventilation-related projects.
A full-service ventilation and insulation company is often your best bet. These companies address many projects, ranging from crawlspaces and attic insulation to air ducts, vapor barriers, attic fans, gable vents and fans, and turbines. As with the general contractor, ask questions and study the qualifications of the ventilation/insulation company. Not all companies deal with the entire range of projects.
To install a roof ridge vent only, call a roofing company.