You want a beautiful roof with quality shingles in a color that matches your home, neat detail work, and straight, clean lines. We can do that. However, what’s going on underneath your roof, in the attic space, is just as important.
The movement and temperature of air under your roof can affect the lifespan of your roof, and can dramatically change your heating and cooling bills. So let’s learn a little bit about what happens in your attic.
What is the attic?
Under the decking of your roof is a space of open air or insulation, this is called the attic. Even if there isn’t a walkable space, every home has this area for ventilation.
How does my attic temperature affect my home?
Because heat rises, this space under the roof can trap hot air rising through your home and create a stuffy, stifling pocket of air. Signs of this warm air getting trapped can vary, but you may have noticed this the last time you went looking in the attic for that old box of baseball cards.
How does roof and attic ventilation work?
Airflow in the attic
Air is able to flow through the attic with a two part system: intake vents and exhaust vents. This constant pull and release of air keeps your attic space cooler by pulling in fresh air and venting stale air.
There are two principles to keep in mind when dealing with attic ventilation.
- Warm air rises
- As the dwelling below the attic is cooled, the warm air finds its way up into the attic space. Combined with the heat coming from the sun (conductive heat coming from the rooftop), this leads to dangerously high attic temperatures in cases of inadequate ventilation. In warm months, temperatures in the attic can be in excess of 130 degrees fahrenheit.
- Warm air absorbs more moisture than cold air
- During cooler months, the warm air from the dwelling rises into the attic space. This warm air is laden with moisture. When this warm air meets the cool attic air, condensation happens as it cools. Condensation collects on the underside of the roof, and stays there to wreak havoc (rot, mildew, etc…) if no ventilation exists to carry it out of the attic.
Intake Vents vs Exhaust Vents
Intake vents are located under the eaves of your roof or at the lowest part of your roof. This open area allows fresh air to enter the attic space. The intake vents are most commonly a part of soffits. Because these are situated at a lower point in the attic than the exhaust vents, the cooler air enters here.
From there, air is circulated through the attic. The hot, moist air continues to rise and is cycled out through the exhaust vent located at the peak or end of the attic.
How much ventilation does an attic need?
Official attic ventilation recommendations exist. The U.S. Federal housing authority recommends: For every 300 square feet of attic floor space, there should be a minimum of at least 1 square foot of attic ventilation (evenly split between intake and exhaust). GAF has a calculator to determine the proper amount of attic ventilation based on your attic dimensions.
What are the different types of attic ventilation?
Six main types of attic exhausts.
- Ridge Vents – A ridge vent is the best type of exhaust vent for a typical steep roof application where there is an open attic and proper intake at the soffits. There are many types of ridge vents on the market, many of which are inadequate or even downright counter-productive. Proper ridge vent allows air to circulate throughout the entire attic evenly, as opposed to fans and louvers which only exhaust the air closest to where they’re installed, which can leave pockets of air uncirculated.
- Power Fans (solar or electric) – Power fans come in useful where there is insufficient ridge space for a ridge vent. Most often, this is the case on houses with hip-style roofs where the ridge length is short. If a power fan is necessary, make sure you get one that has a humidistat included. A humidistat will turn the fan on if the humidity in the attic air gets too high. This can prevent mold and other issues which come with high humidity.
- Wind Turbines (aka Whirlybird Ventilation) – Turbines and roof louvers are the least effective form of attic ventilation, but are better than nothing!
- Gable Louvers – Gable louvers should not be used for attic ventilation unless all other forms cannot be used. This is because gable vents are situated at least a few feet (typically) below the uppermost portion of the roof, which allows hot/moist air to hang out up there while the air below it gets circulated through the gable louver.
- Roof Louvers (aka Box Vents)
- Cupola Vents
Four types of intake systems.
- Soffit Vents
- Gable Vents
- Over Fascia Vents
- Drip Edge Vents
What type of attic ventilation is best?
In most cases, installing a ridge vent and soffit vents are best case scenario. However, the type and shape of your roof can greatly impact which type of attic exhaust is most effective. For example, if your home doesn’t have a long enough ridge, such as with hipped roofs, other ventilation systems are recommended.
If you have a flat or low slope roof and have an attic space just below it, attic fans will be best for ventilation. Lack of air intake can be mitigated with a wall louver or a pop vent.
In most cases, an attic fan is only ventilating a section of the attic space vs. a consistent flow across the entire roof. This can cause some areas of the attic to still store heat and moisture which can lead to heat/cooling issues, as well as mold is more severe cases. There are also mechanics that can fail with an electric attic fan, where a ridge vent is able to pull heat and moisture out of your enclosed attic space without any electricity year round.
Attic ventilation systems to avoid
It is never a good idea to mix various types of attic exhaust. If you have sufficient ridge length, a ridge vent should be installed. Oftentimes, contractors ignorant of ventilation practices will install a ridge vent and a roof fan. The more, the better, right? In a case like this, the ridge vent, which should be exhausting the attic air, turns into an intake whenever the fan is running. The fan forces air to be pulled in at the ridge, and gets blown out through the fan. What you get is a ridge vent that’s sucking in not only air, but also things like snow, bugs, and debris. Furthermore, the air below the fan does not circulate properly, causing the same issues you’d see if there was no ventilation.
Is Attic Ventilation Required?
As of 1948, all building codes require attic ventilation. The International Residential Code (IRC) revamps its requirements every three years, so the 2018 edition is the most recent version available. Even then, attic ventilation regulations haven’t changed much. The highlights from Section 806, Roof Ventilation of Chapter 8, Roof-Ceiling Construction. The following is taken from the IRC:
R806.1 Ventilation required.
Enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain or snow. Ventilation openings shall have a least dimension of 1/16 inch (1.6 mm) minimum and 1/4 inch (6.4 mm) maximum. Ventilation openings having a least dimension larger than 1/4 inch (6.4 mm) shall be provided with corrosion-resistant wire cloth screening, hardware cloth, perforated vinyl, or similar material with openings having a least dimension of 1/16 inch (1.6 mm) minimum and 1/4 inch (6.4 mm) maximum. Openings in roof framing members shall conform to the requirements of Section R802.7. Required ventilation openings shall open directly to the outside air and shall be protected to prevent the entry of birds, rodents, snakes, and other similar creatures.
R806.2 Minimum vent area.
The minimum net free ventilating area shall be 1/150 of the area of the vented space.
Exception: The minimum net free ventilation area shall be 1/300 of the vented space, provided both of the following conditions are met:
- In Climate Zones 6, 7, and 8, a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling.
- Not less than 40 percent and not more than 50 percent of the required ventilating area is provided by ventilators located in the upper portion of the attic or rafter space. Upper ventilators shall be located not more than 3 feet (914 mm) below the ridge or highest point of the space, measured vertically. The balance of the required ventilation provided shall be located in the bottom one-third of the attic space. Where the location of wall or roof framing members conflicts with the installation of upper ventilators, installation more than 3 feet (914 mm) below the ridge or highest point of the space shall be permitted.
Even though the primary code requirement is the 1:150 ratio, a 1:300 ratio is commonly used. In order to use 1:300, one must meet the two conditions in the exception. The first condition requires a Class I or II vapor retarder for buildings located in Climate Zones 6 through 8. An example for a Class I vapor retarder would be a polyethylene sheet, and it should be installed on the warm side of the attic insulation. A Class II vapor retarder could be kraft-faced fiberglass batt insulation installed at the attic floor with the kraft paper side facing down.
Figure 3 – Vent location and amount requirements for the second exception to Section R806.2 – Minimum vent area.The second condition requires specifics on vent location in order to achieve a balanced system, as illustrated in Figure 3.
A noteworthy takeaway from this “exception” is that it is acceptable to have as much as 10% more than 50% of the ventilating area at the intake level. Additionally, it can be interpreted that it is not desirable to have more than 50% ventilating area at the exhaust level. Intake area should always be equal to or more than exhaust area, or intake air may be pulled from the interior of the building/residence through openings in the ceiling (e.g., can lights, attic access doors).
As previously mentioned, this article only addresses the 2018 IRC. It is important to verify with the building code official where the building is located which code has been adopted. Also keep in mind that state and local jurisdictions may add or delete portions of the model code or have local amendments.
Attic ventilation affects your roof warranty and integrity
All manufacturers of roofing shingles have stipulations in their warranties regarding attic ventilation. They know that if the attic gets too hot or damp, the integrity of their products will be compromised. Shingle manufacturers require that proper attic ventilation is used. The last thing anybody wants is to file a warranty claim, only to find out it’s ineligible because the roofer didn’t put attic ventilation in!
Results of Improper Attic Ventilation
Effects of heat buildup in the attic
In the Mid-Atlantic region where we are, attics absorb a significant amount of heat during the summer months. As the house below the attic is cooled, the heat transfers up through the ceiling and into the attic. The heat from sunlight on the rooftop pushes more warmth into the attic space. Often, air temperatures in the attic can be in excess of 140 degrees Fahrenheit. This increases your cooling bill as that raises the temperature in the rooms below. Furthermore, it shortens the lifespan of roofing materials significantly as that heat buildup causes them to deteriorate.
Effects of moisture in the attic
In cool months of the year, moisture becomes the problem. Everyday household activities like cooking, showering, breathing, etc… all contribute to a rise in humidity. This moisture-laden air rises through the ceilings and into the cool attic space. As soon as it hits the underside of the roof, which is cold, the air releases it’s moisture in the form of condensation. This condensation becomes a threat to the integrity of the roof itself (think wood rot), and to the inhabitants of the structure as mold can form, causing possible health risks. Furthermore, the condensation drips down onto your attic insulation, reducing its insulative properties and affecting the heating quality of your home.
How proper attic ventilation works to keep your home healthy
Having proper ventilation in the attic helps to mitigate these problems. The airflow provided by intake and exhaust takes with it the heat and humidity. This greatly reduces the risk of premature shingle failure, rotten roof sheathing, mold growth, and compromised insulation. The energy needed to heat and cool the space below the attic will be reduced. Who doesn’t want that?!
Fixing Attic Ventilation
Unless there is a way for the hot air to escape, it will continue to build up and be trapped. That’s why most building codes require some type of roof venting. With a ridge vent installed, the hot air can rise out of the attic space.
Many types of venting exist such as box vents and wind vents, but we recommend ridge venting for almost all homes. Ridge vent runs the length of the building’s peak, providing a way for air to escape, while keeping the ridge watertight.
Talk to a Roofing Professional
Whether you’re installing a new roof or improving your old one, talking to a professional roofer about your ventilation needs is a must. Make sure your roof installer carries high quality venting materials and is prepared to provide excellent ventilation for your roof.
Remember, your roof is about more than what you see on the surface!
Editor’s Note: This post was originally published in June 2017 and has been revamped and updated for accuracy and comprehensiveness in June 2021.