The Science of Soffit Ventilation

Research shows “most homes don’t meet basic ventilation requirements and are in violation of residential building codes” and, “approximately half of all U.S. homes have evidence of improper ventilation.” The Lawrence Berkley Laboratory states “poorly ventilated homes have been reported to be one of the leading causes of moisture build up, mold, mildew and rot in attics; which has contributed to alarming rise of many illnesses, including; asthma, allergies and other respiratory issues.”

In order to prevent attic air from overheating, pressurizing and drawing into the home, there must be equal amounts (or more) air entering through soffits than exiting through roof vent systems. The U.S. Department of Energy recommends using a continuous roof ventilation system at the rooftop, and a continuous vented soffit system that provides more intake or net free area (NFA) at the eave edge than what is offered through the roof vent system. The problem is that most soffit panels today don’t offer enough NFA to work with the amount of roof ventilation required for the majority of homes.

Proper ventilation (more in-take through soffits than exhaust through roof vents) according to the U.S. Department of Energy “can lower the attic temperature by as much as 50°f, which significantly reduces the home’s heating and cooling, demands.” Just because a soffit product has vents doesn’t mean it works! Typical one foot soffit overhangs should have at least an 11 NFA, so when used on both sides of the home will have a combined NFA of 22 to draw the air flow needed to work with a ridge vent system.

Residential Building Code requires proper ventilation through soffit systems that offers more net free area (NFA) for air flow, creating the updraft required to get air to exit through the ridge vents. Disher Engineering has estimated energy savings on a 2,000 square foot attic to be $292 annually – $4,380 over a 15 year mortgage. When evaluating the costs of ventilation systems, be careful not to be enticed by products that cost a little less up front and end up costing significantly more in both the short and long term!

According to the EPA, “Indoor pollution sources that release gases or particles into the air are the primary cause of indoor air quality problems in homes. Inadequate ventilation can increase indoor pollutant levels by not bringing in enough outdoor air to dilute emissions from indoor sources and by not carrying indoor air pollutants out of the home. Dampness and mold in homes are estimated to account for 21 percent of current asthma cases in the United States” Based on the analysis of 49,130 indoor air quality tests conducted in homes across North America, Air Advice shows that, “people in 9 out of 10 American homes are breathing unhealthy indoor air. Over 91% of homes tested showed elevated particle allergen levels.”

Improperly vented and overheated attics are a primary cause of premature aging of roofing materials which can be costly. The Asphalt Roofing Manufacturers Association states that “Recent research has reinforced the theory that prolonged exposure to extreme heat accelerates the aging of asphalt roofing products. By properly ventilating the underside of the roof deck, heat buildup and its related problems can be reduced.” It’s one of the main reasons shingle manufacturer warranties are voided when proper ventilation is not present. In financial terms, if a roof’s life is 15 years instead of 30 due to improper ventilation, and the average roof cost being $9,360, it would produce an average cost of $312 annually, or $4,680 over 15 years for shingle replacement.