Ventilation may not be at the top of your list of things to worry about when installing a residential roof, but maybe it should be. After all, air flow, or the lack of it, has a lot to do with energy efficiency, the formation of ice dams and even that deadly black mold from hell we’ve been hearing so much about.

Do the Math

According to the Principles of Attic Ventilation, published by air Vent Inc., a CertainTeed company, “An effective venting system provides year-round benefits. During warmer months, ventilation helps keeps attics cool. During cooler months, ventilation reduces moisture to help keep attics dry. It also helps prevent ice dams.” Other benefits of effective ventilation include providing added comfort, protecting against damage to materials and structure, and helping to reduce energy consumption – year round.

Attics may be ventilated with powered ventilators or a system of fixed static vents, including exhaust vents (ridge vents, roof vents, gable vents and turbines) and intake vents (undereave, cornice or soffit vents). These vents must be balanced, as Principles of Attic Ventilation explains, “In general, the net-free area of intake venting should equal the net-free area of exhaust venting … Net-free area means the total unobstructed area (usually measured in square inches) through which air can enter or exhaust a static ventilation component.”

To meet minimum code requirements, it is agreed that you need 1 square foot of ventilator net free area for every 150 square feet of attic floor space. For homes that have a vapor barrier, the minimum is 1 square foot for every 300 square feet of attic floor space.

Square feet of attic floor space/300* = square feet of net free area needed.

*this number will be 150 for homes without a vapor barrier.

In addition, because most vents are marked in square inches of net-free area, you should multiply the above number by 144. The total number of static vents you install should equal that number in square inches.

There’s an Association for Everything

So if ventilation can be figured out with accepted formulas, isn’t that the end of the story? Not exactly. Believe it or not, there is an entire organization devoted to the concept. A division of the Air Movement and Control Association, the Home Ventilating Institute was founded in 1955. It began with manufacturers of bathroom fans who recognized the need to market fair and accurate information. Today, in addition to bathroom fan manufacturers, the organization represents a wide range of home ventilating products, including kitchen range hoods, downdraft kitchen fans, inline fans, heat/energy recovery ventilators, single- and multi-port exhaust fans, exterior mounted fans, balanced ventilators, whole-house cooling fans, powered-attic ventilators, passive fresh-air inlets and static ventilation devices for attics and crawl spaces.

HVI offers its manufacturer members many services, such as test standards, certification programs and market support. “HVI is a manufacturers association by charter,” explains Dale Rammien, the organization’s director. “Our programs are developed through a voluntary, cooperative effort on behalf of manufacturers in any product group.” HVI certification has been recognized by many agencies, including the U.S. Department of Housing and Urban Development, the U.S. Department of Energy — Bonneville Power Administration, National Building Code of Canada, Washington State Building Code, Minnesota State Building code, BOCA, ICBO and the National Electrical Manufacturers Association.

Static ventilation products, of the most direct interest to roofing contractors, became part of the HVI in the mid-1980s. According to Rammien, about eight to 10 manufacturers of static ventilation products got together to adopt a standard for measuring the net free area of their products. The result was a measuring procedure similar to that found in the Department of Housing and Development’s Minimum Property Standards.

“The basic measurement procedure continues to provide a uniform and fair means of determining net free area, and it is now known as the HVI 922 Static Product Measurement Procedure. It specifies exactly how a product shall be measured to determine the governing net free area of a product,” Rammien explains. “The governing net free area of a product is the ‘gross area of the smallest plane reduced by the percentage of physical obstruction to the plane.’”

The original group of manufacturers dropped down over the years, and now, only the following companies have received HVI certification for their products: Owens Corning, Trimline, Leigh and Headrick Building Products. Of those that dropped out, Rammien comments, “They felt that they were at a marketing disadvantage, having to report numbers lower than the competition.” He adds, “It is fairly commonly known that net free areas have been exaggerated. There have been claims of 150 percent or more over the actual numbers. Some of that comes from a company using measurement techniques that are beneficial to its particular product.”

“Roofing is an uncertified industry,” says Charlie Headrick, owner of Headrick Building Products, Cumming, Ga., noting that neither roofers, nor their products, need to be certified. Recent literature from Headrick Building Products points out that, “If manufacturer claims turn out to be errant, the result could be what appears to be a properly vented house that develops unexplained mold, ice dams and heat build up problems.”

What You Should Know

Rammien wants roofing contractors to know, “First, that the long-term tried and proven ventilation requirements (1 square foot per 150 feet of horizontal floor space, as explained above) work.” In addition, though minimum code requirements say to install 50 percent of the vents at the ridge and 50 percent at the eave, HVI recommends a 60/40 ratio, with 60 percent being located at the eave (the net free area at the eaves should also be balanced). Having more intake vents creates positive pressure in the attic, so exhaust will continue to function. “Small variances are not critical,” Rammien adds, “but HVI recommends that intake net free area always exceed the exhaust net free area.”

Rammien continues, “Once you have established that number (of net free area needed), you need to be assured that the product you are using actually provides what you need. Without standards, you are never really certain what you are getting. In addition, some products, such as ridge vents, are more easily crushed, further diminishing net free area. Other products can clog at the opening.”

“We have found that those in our program have more accurate numbers,” says Rammien. “Our program requires that an independent engineering agency or lab make the measurement in accordance with our procedure.”

The HVI is currently the only organization that certifies net free area. “We have a verification testing program and a challenge testing program,” Rammien explains. “The verification program allows HVI to conduct random testing. We do this by obtaining products from the marketplace and comparing them to them to their certified ratings. So companies have to continuously produce good products. Our challenge program allows those who participate to have the ability to challenge anyone else’s numbers. If the product fails, then the manufacturer has the option to lower the rating, upgrade the product, or withdraw the product.”

In closing, Rammien emphasizes that the purpose of a certification program is to provide uniform, comparable and impartial performance testing in order to allow fair and accurate comparison from independent, reliable sources. “The value arises from broad representation in the development of the program, uniform testing/measuring requirements, and reliable and consistent reporting of data,” Rammien says. “This enables contractors, builders, and others to accurately compare and confirm product performances and ratings.”