Let’s clarify the concept. Proper ventilation provides multiple benefits:
■ Helps protect the life of the roof
■ Minimizes moisture and helps prevent mold
growth
■ Helps prevent ice dams in cold climates
■ Helps keep attic and roof temperature regulated
■ Promotes energy efficiency
■ May be a requirement for extended warranty
coverage of a roof covering.
Now, let’s examine how proper ventilation provides these benefits.
Roof sheathing is organic material that provides the perfect environment and food source for mold when the conditions are optimum, which include high levels of moisture. Proper ventilation helps exhaust moisture-laden air from the attic, which will reduce the likelihood of mold growth. Without mold feasting on the sheathing, it will last much longer.
During the winter, heat from the conditioned space below the attic leaks through the ceiling and rises through the ceiling insulation. The warm air will rise to the highest points of the roof, eventually melting any accumulated snow. When the snow melts, the water runs down the roof where it encounters areas on the lower roof and eaves that are still cold enough to refreeze
the water, causing an ice dam. When additional water continues to run down the roof, it will be impeded by the ice dam and
eventually back up under the shingles. By providing proper ventilation, the roof deck temperatures will be fairly even across the
entire deck, eliminating the opportunity for hot air to melt the snow on the roof. Ensuring proper insulation R-value will also help reduce ice dams by impeding the flow of warm air into the attic.
Proper attic and roof ventilation provide a constant flow of air through the attic, which helps keep the attic temperature better regulated. In warm, humid climates, lack of proper ventilation results in hot air building up in the attic. This creates a sizeable difference in the attic temperature versus the living-area temperature, which leads to the cool conditioned air from the living space leaking into the attic at a high rate. Additionally, since the HVAC systems in most homes located in Florida are in the attic, it reduces the efficiency of the system. For that reason, proper ventilation promotes energy efficiency by providing a cooler environment for the HVAC equipment.
Check the manufacturer’s warranty information for all components being installed on the roof. Some warranties require proper roof ventilation.
Now that the benefits of proper ventilation have been discussed, let’s jump into how proper ventilation is provided. For that topic, we follow a very simple A-B-C approach.
A. Ample Amount – Most building codes require a minimum of one square foot of net free vent area (NFVA) for every 150 square feet of attic/roof area (1:150 ratio). There are exceptions in most codes that allow the NFVA to be reduced to a 1:300 ratio.
Balancing the ventilation between intake vents (typically at the eaves) and exhaust vents (ridge or off-ridge vents) is one of the primary requirements for reducing the ratio to 1:300.
B. Balanced – Provide equal amounts of intake and exhaust ventilation. Acceptable limits allow up to 60 percent intake and as little as 40 percent exhaust. The most important thing to remember? Avoid installing more exhaust than intake ventilation. Providing greater intake ventilation helps to create a positive pressure in the attic or roof space that will help force air out of the exhaust vents. Owens Corning recommends balanced ventilation be installed regardless of the ratio. Our research has shown that even if you install the adequate NFVA using roof vents, either exclusively at the ridge or at the soffits, there is little improvement
over not installing vents at all.
C. Control Air Flow – Properly air sealing and installing adequate insulation in the attic/roof space can help prevent conditioned air from leaking into the attic, creating a more comfortable home.
■ Ventilation is the FIRST thing that you should look at when assessing a roofing project.
■ Prior to commencing a re-roofing or roofing recover project, the deck should be inspected, including the underside of the deck, for signs of poor ventilation and increased moisture. After all, 50 percent of the roof is on the underside, so it should be inspected. It doesn’t make much sense to install new products on a roof deck that may only last another five years. Look for evidence of mold, bath and range vents terminated in the attic, insulation blocking soffit vents, etc.
■ Balanced ventilation is crucial.
■ Avoid installing more exhaust ventilation than intake.
■ Don’t just cut a vent slot from end to end of the ridge and slap the ridge vent over it. Calculate the correct NFVA requirements for intake and exhaust vents, then cut the ridge vent slot only as long as is required to maintain 50 percent or less exhaust ventilation. For aesthetics, it is recommended that you install the ridge vent product the full length of the ridge.
■ Don’t assume that the existing ventilation on a re-roofing or recover job is adequate. Inspect the vent components and openings carefully and perform the required calculations to determine adequacy.
Most roofing and ventilation product manufacturers have handy ventilation calculators on their websites that can be used to determine the proper NFVA and products for your job. Take advantage of those tools and if you have questions, reach out to the manufacturer to get answers.
Greg Keeler is the Technical Services Leader for Owens Corning Roofing and Asphalt, LLC. Greg has been with Owens Corning for 9 years, has 36 years of experience in architecture and spent 24 years as a building official. He serves on and chairs several task groups within ASTM D08, Committee on Roofing and Waterproofing and the Asphalt Roofing Manufacturer’s Association. He also serves on the UL Standards Technical Panels for UL 790, UL 2218, and UL 580/1897 and on FRSA’s Codes Committee and Codes Subcommittee.
Next Article