What Is Ventilation In General?
Roofing ventilation refers to the system that allows air to circulate through the attic space of a home or building, typically achieved through vents installed at the soffit (intake) and ridge (exhaust) of the roof, enabling hot air and moisture to escape. This maintains a balanced temperature within the attic space. Essentially, it’s the process of providing fresh air flow through the roof to prevent excessive heat buildup and moisture issues. In 2015 the building code changed to specify a venting area, as well as specifying what the ratios needed to be (1 to 150 which we will go into greater detail on later). This was a major shift away from what was the status quo, and the majority of contractors didn’t bother to learn this change. Even 9 years later the majority of roofing contractors aren’t doing a ventilation calculation, which is a major issue for the homeowner. Ventilation might seem like a minor detail, but it is actually one of the most crucial aspects of your roofing system. Proper ventilation doesn’t just keep your attic space cool; it regulates airflow throughout your entire home. This airflow affects many things such as the manufacturing warranty, the temperature of your rooms, your energy bill, vulnerability to moisture damage where the attic/roofing system is concerned, and debatably the most important factor; your health with mold. It’s no exaggeration to say that proper ventilation is the key to maintaining a healthy home. Here is ventilation as it appears in South Carolina state residential building code.
R806.2 Minimum Vent Area
The minimum vent area requirement, R806.2, has undergone changes over the years to ensure adequate attic ventilation and prevent moisture buildup. Here’s a summary of the key revisions:
- 2012 International Residential Code (IRC): The minimum net free ventilation area was set at 1/150 of the area of the vented space. An exception allowed reducing the ratio to 1/300 in Climate Zones 6, 7, and 8, provided a Class I or II vapor retarder was installed on the warm-in-winter side of the ceiling.
- 2015 IRC: The exception for reducing the attic ventilation area based on cross ventilation was revised to require at least 40% and not more than 50% of the required venting area to be placed in the upper portion of the roof.
- 2018 IRC: The exception for reducing the attic ventilation area from 1/150 to 1/300 ratio was limited to cold-weather climates (Climate Zones 6, 7, and 8) only, and the vapor retarder requirement remained unchanged.
- 2021 IRC: The code clarifies that vents must be installed in accordance with the manufacturer’s installation instructions.
Key Takeaways
- The minimum net free ventilation area has remained at 1/150 of the area of the vented space, except for specific exceptions.
- In Climate Zones 6, 7, and 8, a Class I or II vapor retarder is required on the warm-in-winter side of the ceiling to qualify for the reduced 1/300 ratio.
- The percentage of upper ventilation required has increased from 50% to 50%-80% in the 2012 code, and then settled at 40%-50% in the 2015 and subsequent codes.
- Manufacturers’ installation instructions must be followed for proper vent installation.
What Is Proper Attic Ventilation?
Proper attic ventilation is all about free flowing air. You need the same amount of air coming into your house (the Intake) as going out of your house (the outtake). This creates an air current through your attic system that provides air to the attic space of the roof, your walls and will actually affect the entire home. This helps keep the temperature in its intended range and the constant airflow keeps these areas dry, making it difficult for moisture to rest. The scientific name for properly ventilating the attic space with free flowing air is called the Venturi Effect. The Venturi Effect refers to the phenomenon where wind passing over a roof with a constricted section, such as a narrow gap between two buildings or a specially designed roof shape, will speed up and create a lower pressure area.
An Example of the Venturi effect on a roof system. (Visually showing how ventilation works)
This is due to the principle of fluid dynamics. The faster the air moves through the narrow section, the lower the pressure becomes, effectively “sucking” air from beneath the roof. This effect can be utilized for natural ventilation, promoting airflow through the attic space to regulate temperature and moisture levels, or to counteract uplift forces during high winds. During high windstorms this will add to the structural integrity of the roofing system. As this process takes place it forces air that holds the heat and moisture out of the ridge vent (the outtake), allowing the attic to properly vent out the moisture and heat. Read this article by the Asphalt Roofing Manufacturers Association for a more in-depth look at ventilation, and how it is a vital aspect to your roofing project.
When a contractor is working on your roof, an essential calculation must be conducted to determine the correct amount of linear feet for ridge vents and soffit vents. This calculation is based on either the 1:150 ratio or, if that’s not feasible, the 1:300 ratio. Essentially, for every 150 (or 300) square feet of attic floor space (or uppermost floor space if there are vaulted ceilings), one square foot of Net-Free Air (NFA) is required. This square foot must be evenly divided between the intake and exhaust portions of the ventilation system. Proper positioning of intake and exhaust ventilators on the roof ensures that the 1:300 ratio is sufficient. However, while the 1:300 ratio is considered “adequate,” many manufacturers now recommend or require the 1:150 ratio for optimal performance. You must also convert units from feet to inches because the intake and exhaust products measure in inches not feet. It sounds complicated but let’s take a look at how proper NFVA is calculated.
How To Calculate NFVA For The 1:150 ventilation rule:
- Gather the attic floor space square footage by multiplying the length x the width of the attic floor or upper most floor space
- Existing attic floor space is __________________ square feet / 150 (1:150 ratio is code in South Carolina – For every 150 square feet of attic floor space, 1 square foot of NFA is required)
- _______ square feet of required NFA x 144 square inches = _______________ square inches of required NFA (All products in the US are rated in square inches for NFA, and therefore, we must convert the square feet to square inches.)
- Total square inches of required NFA / 2 = ____________ (You divide by 2 so that you have an equal intake and exhaust for even ventilation)
- Use the specific product’s NFA specifications to calculate how many linear feet are required for intakes/exhausts on the roof/soffits.
You can learn more about how to calculate these ratios by visiting our blog that touches on the subject of ventilation through the following link: Calculation Formula.
What Happens If You Have Improper Ventilation?
By now, you might be wondering how ventilation holds so much influence over your home. Proper ventilation regulates the temperature of your roof, attic space, and home in general, which is crucial for maintaining a healthy home. The materials used in your roof and walls are designed to operate within a specific temperature range. If these materials become too hot or too cold, they not only stop functioning effectively but can also begin to fail. Surprisingly, the ambient temperature of your attic space should be no more than 20 degrees above the outside temperature. Most attic spaces are known to be very hot, which indicates improper ventilation and can have severe consequences.
For instance, roof shingles are engineered to withstand the heat from the sun, with the top side designed to reflect and endure constant UV exposure. However, they are not designed to withstand excessive heat from the underside, which is the part facing the attic space. Without adequate ventilation, the shingles are heated from below within the attic, essentially baking them from both sides. This leads to premature failure of the shingles. The same principle applies to other materials, such as decking and sheathing, synthetic underlayment, caulk, sealant, and many more products. Ensuring proper ventilation is crucial to preventing issues and protecting the longevity of your home’s structure.
Manufacturers of these materials have built-in warranty exclusions. A “warranty exclusion” refers to specific conditions or situations explicitly listed in a warranty agreement that are not covered by the warranty. This means that if a product fails due to these listed reasons, the manufacturer or seller is not obligated to repair or replace it. Essentially, it outlines what the warranty does not cover. Manufacturers know that if there isn’t proper ventilation, the risk of their product failing prematurely is high. Improper ventilation causes a chain reaction, when shingles become hotter than designed, they begin to prematurely degranulate, exposing the shingle mat. Exposing the shingle mat will cause the attic space to become much hotter than intended, causing the sheathing and decking to expand and contract more violently, creating what is known as a nail pop. Nail pops create a pathway for water to enter your roof system, causing long-term leaks. Long term leaks cause structural integrity of material, as well as add moisture which then gives mold the environment to grow. This article by Trinity Exteriors Inc explains more about the effects of poor ventilation.
This is why every manufacturer of roofing materials of shingles, metal, or composite—stipulates that a properly ventilated attic space is necessary for the warranty to be valid. Without proper ventilation, these materials are prone to premature failure, voiding the warranty and causing potential damage to your home.
Here are some common attic ventilation mistakes to look out for:
- More than 1 type of exhaust vent
- Mixing too many different types of intake/exhaust ventilation products
- Unequal airflow at the soffits and exhausts
- Painting over intake vents at soffits, preventing sufficient airflow
- Over ventilating the attic space (More is not always better)
- Only having exhaust ventilation on the roof without any intake ventilation at the soffits or eaves
How Does Improper Ventilation In The Attic Space Cause Mold?
When your contractor is working on roofing your home, the biggest reason you as the homeowner want to ensure the attic space is ventilated is because you do not want mold. The purpose of properly ventilating your attic is to allow the heat and moisture to escape the attic space. Your attic regardless of what you do will have heat and moisture inside of it, and this is natural. What isn’t okay is if the attic space doesn’t have the properties to allow the heat and moisture to escape effectively. According to MoldManUSA, “Attics are one of the most common areas of mold growth in the home. Attics tend to have the perfect conditions for mold growth (hot, humid, and an abundant food source from the wood sheathing). Furthermore, most homeowners don’t ever go up in their attic, so the mold problem is allowed to grow undisturbed – sometimes for years!” The only way that you as the homeowner can mitigate the effects of mold growth in the attic is through proper ventilation. By properly ventilating the attic space, you wont have the environment where mold is capable of growing, as the free flowing air significantly hinders mold from propagating. The attic space isn’t the only area where mold growth occurs, it just tends to start there.
Here is an exurb from an article by North Carolina State University that talks about common places mold can hide. Hidden Mold: In some cases, indoor mold growth may not be obvious. It is possible that mold may be growing on hidden surfaces, such as the backside of dry wall, wallpaper, or paneling, the top of ceiling tiles, the underside of carpets and pads, etc. Possible locations of hidden mold can include pipe chases and utility tunnels (with leaking or condensing pipes), walls behind furniture (where condensation forms), condensate drain pans inside air handling units, porous thermal or acoustic liners inside ductwork, or roof materials above ceiling tiles (due to roof leaks or insufficient insulation).
Some building materials, such as dry wall with vinyl wallpaper covering it or wood paneling may act as vapor barriers, trapping moisture underneath their surfaces and thereby providing a moist environment where mold can grow. You may suspect hidden mold if a building smells musty, but you can’t see the source. You may suspect there is mold if you know there has been water damage and building occupants are reporting health problems. Investigating hidden mold problems may be difficult and will require caution as the investigation involves disturbing potential sites of mold growth, so be sure to use PPE. For example, removal of wallpaper can lead to a massive release of spores from mold growing on the underside of the paper, releasing it to spread easier. If you discover hidden mold, you should revise your remediation plan to account for the total area affected by mold growth. Full Article: Hidden Mold
What Are The Health Effects Of Mold?
Mold growth in homes can negatively affect health! This is especially true for individuals with respiratory conditions or allergies. Short term exposure to mold spores can trigger allergic reactions, such as sneezing, skin rashes, eye irritation, and worsen asthma symptoms like wheezing and coughing. The NIH or the National Institute of Environmental Health and Safety say that long-term exposure may lead to chronic respiratory problems, cancer and cognitive function losses. Other issues that arise from the mycotoxins by certain molds can cause fatigue, headaches, and throat irritation. Vulnerable groups, including children, the elderly, and those with weakened immune systems, are particularly at risk. Addressing water leaks, controlling humidity, and improving ventilation are crucial steps in mitigating these risks. For more information, see resources from John Hopkins, and NC State Extension. This next section by News Medical Life Sciences:
What Are The Health Effects Of Mold?
Humans tend to spend approximately 90% of their time indoors, therefore the high risk associated with the presence of fungal growth is especially pertinent. The presence of mold is associated with several health symptoms and can range from mild such as fatigue and reduction in concentration, escalating to cognitive impairment and adverse effects on the respiratory system. The most common of these effects include:
- Asthma
- Allergic rhinitis and sinusitis
- Allergic alveolitis
- Infections
- Sense of fatigue, reduced concentration, irritated nose eyes, and throat, rhinorrhea
- Rheumatic disease
(Full Article: News-medical.net)
Does Proper Ventilation Lower Energy Costs?
Absolutely, proper ventilation does indeed lower energy costs! It allows a constant flow of air, which helps regulate the temperature throughout your home. This free-flowing air helps retain warmth during the winter and keeps your home cool during the summer. By preventing your home from reaching temperature extremes, your HVAC system operates more efficiently, leading to significant energy savings.
This section of energy saving is from an article by Colony Roofers.
How Much Will The Proper Roof Ventilation Reduce Energy Costs?
The percentage by which proper roof ventilation is going to reduce your energy costs will vary depending on a number of factors, including whether there is any effective ventilation system currently in your home, the climate in your location, the orientation of the home, the temperature you set the thermostat to, and more.
That being said, if you live in a hot climate like Charleston and you don’t have proper ventilation in your roof, there will be a noticeable difference in both your cooling bills as well as the health of your A/C system if you install roof vents that help to move hot, humid air out of your home. This means that you will both be saving money on a monthly basis for energy, but that you will also save money on maintenance for your cooling system because it won’t be working nearly as hard. Beyond that, you won’t have to replace your A/C as soon because you won’t have to work so hard day in and day out to keep your home comfortable.
Here is a diagram of the Venturi effect on a roof system. (Visually showing how ventilation works)
By some estimates, having good ventilation on your roof can help reduce cooling costs by 30-40%. Considering that all A/C systems use electricity and the cost of electricity has been on the rise, this type of drop in cooling bills could make a big difference both from month to month and on an annual basis. Another study by the Florida Solar Energy Center found that the cooling energy consumption of a home could be reduced by 10% through proper attic ventilation. Beyond that, you have to consider the cost of maintaining, operating, and replacing your air conditioning unit. The harder your cooling system has to work to keep your house comfortable, the more wear and tear it is experiencing. Research conducted in Karns, Tennessee, found that a home’s temperature could be reduced by up to 30 degrees Fahrenheit when proper attic ventilation was in place, significantly reducing the cost of cooling a home.
The average new high-efficiency A/C system lasts for 15 to 20 years with proper maintenance and care. The cost of installing central AC units typically ranges from a little under $4,000 to nearly $8,000, averaging around $5,800. If it lasts a full 20 years, that’s not too bad of a deal, considering that it helps keep your home comfortable even during the hottest summer months. However, it’s important to understand that your AC system is experiencing wear and tear every single time it’s running. This means that a cooling system that is running 14 hours a day is going to wear out significantly faster than one that only runs for 5 hours every day. The cooler you can keep your house without using your AC unit, the longer your cooling system will last. This means you won’t have to incur the cost of replacing your system nearly as soon, you will lessen energy expenses, and you won’t have to shell out the big bucks for maintenance and repairs nearly as often. Here is the link for the full article.
Conclusion
In conclusion, attic ventilation is not just an optional feature for homes—it’s a necessity for maintaining structural integrity, optimizing energy efficiency, and safeguarding the health of the homeowners. Proper ventilation ensures balanced airflow, preventing excessive heat buildup and moisture retention in the attic space. By doing so, it protects roofing materials from premature failure, mitigates the risk of mold growth, and helps maintain a stable indoor temperature, reducing energy costs and extending the lifespan of HVAC systems. Without proper ventilation, homes are vulnerable to a host of problems, including damaged roofing materials, voided warranties, mold growth, and elevated energy bills. Furthermore, poor ventilation can have serious health consequences, as mold and other pollutants thrive in poorly ventilated spaces, causing respiratory issues and other serious health complications. Ultimately, investing in a professional contractor who wont skip out on providing a well-designed attic ventilation system is one of the most effective ways to ensure your home remains safe, efficient, and comfortable for years to come. It is a critical component of a home that no homeowner should overlook.
