Look at the roof of any house in your neighborhood and you'll see one or two plastic or metal pipes jutting up about a foot above the roof line. These are plumbing vents, which are essential for proper drainage of the plumbing system. The house probably also has attic vents, but you might not see them, because they often installed under the soffits and incorporated into the ridge. Some houses do have highly visible have gable vents, which are grids placed into the siding near the top of the gable wall, or turbines, which are much larger than plumbing vents. The purpose of these vents is to keep air circulating in the attic, which helps control ice dams in winter, prevents overheating in summer, and controls moisture all year long.
Plumbing Vents Keep Drains Flowing
The idea of venting drains revolutionized plumbing back in the 1800s, and today, every house has a network of vent pipes that connect to the main soil stack, which is the largest pipe you see jutting through the roof. The purpose of the vents is to allow air into the system and prevent flowing water from creating a suction that slows it down and can draw water out of P-traps. If the roof vent on your house gets blocked, you'll know it. Your toilet won't flush properly, you may notice sewer smells, and your drains may even back up.
The position of the main stack helps you locate the main bathroom in the house, because the convention is to put the main vent directly over that bathroom. The roof vent is connected to a pipe that extends all the way to the sewer, and the toilet in the main bathroom is usually less than 5 feet from this pipe. Large homes may have two or more soil stacks venting through the roof, one for each major bathroom. The configuration of the network of vents that connects to the main stack is different for every house, but it's always designed to vent every drain in the house. Where necessary, smaller branch vents that can't easily be connected to a main soil stack may also run directly up through the roof.
Attic Vents Control Moisture and Temperature
When the architectural design allows for it, a typical attic ventilation system consists of intake vents located under the soffits and output vents located on the roof or in the gables. In a passive design, which is the most common, the cooler air entering through the soffits warms and rises inside the attic and passes out through the roof or gable vents. The resulting convection current keeps air in the attic moving. In some attics, air circulation has to be maintained by providing fans or other positive ventilation methods. Some attic ventilation fans are solar-powered.
Circulating the air in the attic has a number of beneficial effects on the roof and the house in general. In winter, the circulation allows heat to escape, thus keeping the roof colder and preventing ice dams from forming. In summer, attic ventilation does just the opposite. It keeps the attic cooler and reduces the amount of energy you need to devote to air conditioning. There's one more benefit that is probably more important than either of these: the vents keep the attic dry.
When air stagnates in the attic, moisture tends to condense out and settle on the framing and insulation. This provides a perfect environment for mold and rot to take hold. Mold loves to grow in damp attic insulation, and damp insulation has a tendency to clump, which reduces its effectiveness. Rot affects the framing and the roof decking, and the end result can be a sagging roof. Sagging roofs leak, which worsens the moisture problem in the attic and creates big problems in living areas below.
Proper Attic Ventilation
The rule of thumb adopted by the building code is that the amount of ventilation in the attic should be equal to or exceed 1/150 of the total square footage of the attic. The ventilation should be divided equally between soffit vents and roof or gable vents. When a vapor retardant has been applied to the building during construction, the attic ventilation can be reduced to 1/300 of the total area of the attic floor.
If your attic floor has an area of 1,500 square feet, for example, you need 10 square feet of ventilation space, unless your house has a vapor retarding liner, in which case you need 5 square feet. The area of soffit and roof vents is typically measured in square inches. Use the factor 1 square foot = 144 square inches to convert.
Soffit vents, which are grids that fit over holes you cut in the soffits, come in different sizes. You can mix and match these as desired, as long as the total venting area is sufficient. On some aluminum or vinyl soffit systems, the vents are integrated into the soffit panels. Since the roof line and soffit form a corner in the attic, you usually have to fit baffles over the soffit vents, running up along the roof sheathing beneath the insulation. These are cardboard or plastic air passageways that you fasten to the underside of the roof decking. They prevent insulation from blocking the vents and keep air flowing freely.
Ridge vents are the most economical roof vents. A ridge vent is a continuous vent that runs along the peak of a sloped roof; it is usually covered with shingles to match the rest of the roof. You can also choose any number of vent styles that open directly onto the roof deck. Care should be taken, though, to make sure that the vent openings aren't covered by snow in the winter, which will hinder their function. If your house accommodates them, you can also satisfy the roof vent area requirement by installing gable vents that penetrate the siding and open into the unheated attic space. You have to exercise some care when designing a ventilation system to ensure uniform circulation throughout the attic, especially in corners and behind obstacles.
Chris Deziel is a contractor, builder and general fix-it pro who has been active in the construction trades for 40 years. He has degrees in science and humanities and years of teaching experience. An avid craftsman and musician, Deziel began writing on home improvement topics in 2010. He worked as an expert consultant with eHow Now and Pro Referral -- a Home Depot site. A DIYer by nature, Deziel regularly shares tips and tricks for a better home and garden at Hunker.com.