What You Need to Know About Attic Ventilation

Attic Ventilation

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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

Pros
  • No cost to run

  • Less maintenance

  • Vents even in power outage

Cons
  • 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

Pros
  • Controllable

  • Consistent air circulation

  • Moves greater quantity of air

Cons
  • Uses electricity

  • Requires maintenance

  • 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

Well-Ventilated Attic
  • Flat shingles

  • 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

Poorly Ventilated Attic
  • 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

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.

Tip

The best time to install a roof ridge vent is when you are re-roofing the house (or during initial construction), but a ridge vent can still be installed retroactively.

Gable Vents

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

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.

Tip

To rotate properly, roof turbines need to be upright. In order to accommodate roof pitches, most roof turbines are adjustable to allow for installation on an angle. After installation, the upper fan portion is twisted until it is upright.

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.

Tip

A whole house fan is not the same thing as an attic fan and does not have attic venting capabilities. A whole house fan draws air from the house into the attic. In fact, use of a whole house fan necessitates more attic ventilation.

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.

With all mechanical ventilation devices, an electrician must install electric power in the attic. An electric permit will be required, too.

To install a roof ridge vent only, call a roofing company.

Article Sources
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  1. R806.1 Attic Ventilation / International Residential Code (IRC)

  2. R802.7 / International Residential Code (IRC)