Air Vent is committed to helping roofing professionals everywhere by collecting and sharing best practices and solutions in residential attic ventilation. Air Vent University is an extension of this commitment. Here you’ll find useful information to help you in the field now and as a reference for the future. We’ll add new “lesson plans” regularly. We’ll also mix in some fun with pop quizzes and occasional prizes. And check out our podcast, too. Welcome. For additional learning opportunities, see our Education page.
Air Vent University
This document, published by The Asphalt Roofing Manufacturers Association (ARMA), takes a deeper dive into the considerations when evaluating an attic’s ventilation needs. Key takeaways from Considerations in Attic Ventilation System Selection:
- Attic ventilation fights heat, moisture, and ice dams
- To work properly, attic ventilation should be balanced with intake and exhaust airflow
- Do not mix different types of attic exhaust vents on the same roof above a common attic.
There’s also a review of the various attic design configurations in the field: insulation at the underside of the deck, insulation at the floor of the attic, and conditioned attics. Access the document here: Considerations in Attic Ventilation System Selection
Airflow slot width for ridge pole construction. When there is a center board (ridge board, ridge pole) involved at the peak of the roof, cut a ¾-inch wide airflow slot on each side of the center board after clearing the width of the center board itself. This will result in an overall airflow slot that is approximately 3 inches wide. However, 1.5 inches of the slot is obstructed by the center board. That’s OK because the ridge vent will still have an airflow slot that is 1.5 inches wide clear and free.
Airflow slot width for truss construction. For truss construction there isn’t a center board involved at the peak of the roof. That means there’s no center board to clear when cutting. Instead, the airflow slot is cut 1.5 inches wide grand total.
Slot WIDTH aligns with ridge vent Net Free Area. The slot width is designed to match the amount of Net Free Area (airflow capacity) provided by the ridge vent itself. For example, ShingleVent® II ridge vent has airflow openings on the vent that allow 18 square inches of Net Free Area per linear foot. Well, over the span of 1 linear foot, a 1.5-inch-wide airflow slot cut into the peak of the roof will also allow 18 square inches of Net Free Area (which then passes through the ridge vent itself.). Likewise, a ¾-inch wide slot on both sides of a center board (ridge board, ridge pole) over the span of 1 linear foot equals 18 square inches of Net Free Area per linear foot (which then passes through the ridge vent itself).
Wider is NOT better. Cutting the ridge vent slot wider than the Installation Instructions require DOES NOT increase the ridge vent’s Net Free Area capacity. That’s because the actual airflow through the vent (once the ridge vent airflow slot in the roof deck is cut correctly) is restricted by the airflow openings on the vent itself. The ridge vent only allows so much air to pass through – in the case of ShingleVent II, it’s 18 square inches NFA per linear foot. While it may seem logical that cutting an extra wide slot into the peak of roof deck will then allow more air to pass through the ridge vent, the fact is the manufacturer designed the vent with a specific amount of airflow openings. Those airflow openings in the ridge vent determine the amount of air that passes through the vent regardless of how extra wide the slot is cut into the roof deck. WORD OF CAUTION: There is the potential for weather infiltration if the airflow slot is cut too wide. The ridge vent must fully cover the airflow slot cut in the roof deck without that slot getting close to the vent’s airflow openings; which could then allow weather entry. Bottom line = following the Installation Instructions.
Airflow slot length. Cut the slot length along the ridgeline 6 inches from the end wall or hip intersection. Cut the slot length 12 inches from a chimney or an intersecting ridgeline. COSMETIC TIP: For maximum curbside appeal that homeowners will love, run the actual ridge vent to the very end of the horizontal peak of the roof (even though the airflow slot stops short of the end of the roof peak). This results in a clean, continuous, uniform line or profile at the top of the roof instead of an uneven line/profile when the ridge vent stops short of the end of the roof.
Where is the attic? The “attic” for a cathedral or vaulted ceiling is the gap between the underside of the roof deck and insulation. Many building codes and manufacturers call for a minimum 1 ½” air space.
Ridge vent is most practical solution. For exhaust ventilation, a ridge vent is the most practical solution because it will allow continuous airflow along the peak across all rafter bays. Otherwise, individual vents will be needed to access each rafter cavity.
Caution with insulation baffles. When using insulation baffles do not butt them end-to-end or overlap them. Leave a space to allow hot air and moisture to pass through. If a space is not included between consecutive pieces, the insulation baffles could act as a vapor retarder on the wrong side of the insulation.
Skylights. When a skylight is installed in a rafter bay, it basically blocks the rafter bay. To unblock it, drill (See Note Below) six 1” holes horizontally in the rafter above and below the skylight or other rafter bay obstruction, such as a chimney.
Valleys. When a cathedral ceiling ends in a valley you can’t access the soffit vents for intake ventilation. One solution is to drill (See Note Below) six 1” holes in the rafter that intersects with the valley. These openings will allow intake ventilation.
Drilling Holes Note: Do not drill holes in 2x4 rafters. On 2x6 and larger rafters it is important that the holes are in the center of the wood thickness, the holes are not placed in the middle 1/3 of the total length of the rafter, and the holes are no larger than 1” in diameter.
We’ll define roof width as the distance from eave to ridge to eave in the plan view. Roof pitch does not matter; just a straight line across the eave to the ridge to the opposite eave.
A combination ridge vent/roof-top intake or soffit vent system will be effective on a roof that is 75 feet wide or less, assuming the ridge vent provides 18 square inches of Net Free Area per linear foot and is balanced with sufficient intake venting. This is based on establishing absolute minimum code requirements of 1 square foot of Net Free Area for every 300 square feet of attic floor space balanced with half intake and half exhaust airflow. To stay within the 1/150 building code requirement for Net Free Area balanced with intake and exhaust airflow, the total width would have to 37½ feet.
These numbers are based on the building code requirements and the net free area specifications of the ridge vent. They have nothing to do with the ridge vent’s performance.
Another common question we’re asked is, “What’s the maximum rafter run on which you can use a ridge vent?” If it’s a traditional attic with a flat floor in which you can walk into the attic, crawl around inside the attic, or store items inside the attic, there is no limit on the rafter run (but be mindful of the roof’s overall width as explained above). However, if it’s a vaulted/cathedral ceiling style attic or otherwise compact roof system in which the “attic” is literally the airflow gap between the roof deck and the insulation, here is some guidance.
An airflow gap of 1” to 1½” between the roof deck and the insulation is sufficient for rafter lengths up to 35 feet.
Rafter runs longer than 35 feet should have at least a 2” to 3” airflow space to allow for an easier path for the air to travel along the extra distance.
Using The Edge™ Vent with Gutter Protection Devices
Using gutter protection devices with The Edge Vent (our roof-top installed intake vent) is possible depending on the installation requirements of the gutter protection device. In this Lesson Plan we examine when the two are compatible and when they are not. Below we take a glance at the installation scenarios but please see the full document for complete details.
For Full Details: Using The Edge Vent with Gutter Protection Devices
Attic Ventilation Educational Video: 2nd Installment
(Shortened version of the “Ask the Expert” Seminar)
This video is extracted from the annual Air Vent Seminar “Attic Ventilation: Ask the Expert™” held in-person and virtually for roofing professionals across North America every 1st quarter. The seminar is an overview of residential attic ventilation best practices and solutions. This is the 2nd video installment. See the April 2020 Lesson Plan for the 1st installment.
Attic Ventilation for a Knee Wall
Trying to move intake air from low on the roof to the exhaust vents high on the roof is no piece of cake when knee walls are involved, sometimes found in Cape Cod or Bungalow style homes. The knee wall is a short wall – usually 3 feet in height – used to support the rafters.
“The closed off attic areas that they create make it especially challenging to be properly ventilated,”
says Jeff Barnett, vice president of sales, Barnett Roofing, Canton, MI.
Jeff Nunham, owner, Precision Roofing Services, Lansing, MI, agrees that getting air from the eaves to the ridge is not easy when knee walls are in play. “In particular, getting the air to flow through the cathedral part of the roof becomes difficult without sacrificing insulation R-Value (the insulation’s capability),” Nunham says. “Often behind the knee wall it isn’t properly insulated or air sealed, which can lead to cold air entering the living space. Depending on what’s behind the knee wall, it may not be possible to get fresh air in at the eaves and have it make its way to the ridge. In those cases, we’ll use gable vents to achieve cross flow behind the knee walls. If the homeowner and the structure of the roof allows, we will make an air passage along the ceiling rafters to the ridge, but it’s always a pain,” says Nunham.
Considerations:
When the Roof Deck Will be Replaced. In situations where roof deck replacement is required, try placing insulation baffles in the rafter bays (See Illustration A below) before putting the decking down. The insulation baffles will provide the air space required for the air to flow from the roof-top installed intake vent The Edge Vent or Vented Drip Edge in the knee wall area (See Illustration B below showing Vented Drip Edge example) to the ridge vent in the vaulted area. Be sure that the baffles are placed in every rafter bay and at the full length of the cathedral portion so that the entire roof area can be ventilated.
When the Roof Deck Will NOT be Replaced. Make sure there is a path for the intake air to travel above the vaulted ceiling to the ridge vent. In many Cape Cod style homes with knee walls, the vaulted portion above the second floor room is densely packed with insulation. To open this area for airflow, you can slide wood slats up the underside of the roof deck (for short spans only) and then flip them on edge to force down the insulation. This process is easiest to achieve if the fascia can be pulled down to allow access to the underside of the roof deck from outside the house. Someone can feed the wood slats from outside while someone in the knee wall area guides the slats toward the ridge (See Illustration C). Once the slats have been positioned it may be possible to slide insulation baffles up in the space that has now been created, similar to snaking wire. The best time to insert the slats is when all the roofing nails are removed prior to papering and shingling. How many slats to use? Use enough to try to get the air space even across the bay, regardless of width. Aim for an overall open area equal to a ¾” gap across the entire bay.
Use Gable Vents. If the vaulted ceiling portion is too tightly packed and/or you cannot use either The Edge Vent or Vented Drip Edge for intake, consider cross ventilating the knee wall space using gable vents in the ends of the knee wall. Similarly, vent the upper attic portion (vaulted ceiling) between the two vented areas (knee wall and attic) – which is not ideal – but it is better than no ventilation at all. It will help get rid of moisture getting introduced to the vented spaces, and it may also help with some of the moisture in the vaulted area.
Attic Ventilation for a Mansard Roof
In a previous Lesson Plan we reviewed how to vent a gambrel roof. Another tricky roof design is a mansard roof. It's a four-sided gambrel-style hip roof that has two slopes on each of its sides. The lower slope often has dormer windows and is steeper than the upper slope.
"When the dormers are close together it's nearly impossible to obtain enough horizontal intake ventilation because there's just not enough linear feet available," says Patrick Readyhough, president, Pond Roofing Company, Fairfax, VA. "We'll sometimes install a vented drip edge. If the back of the roof has no dormers, we'll install roof-top intake like The Edge Vent."
One of the challenges with a mansard roof is at the point where the roof pitch changes from the steep eave area to the shallow top. This "elbow" must be open so that there is a path for the air to move from the intake vents under the steep slope through the break into the upper shallow slope.
Considerations:
In situations where there isn't a path through the "elbow," can roof-top intake be installed at the elbow (assuming that it can be installed so that louvers will be horizontal, and the vent can be properly installed to prevent weather intrusion)?
Are gable louvers/vents an option? If the roof line has gables or if the roof has gable vents designed into the peak of the roof “ similar to dormers except in line with the ridge “ consider gable vents. It may be possible to vent the upper shallow slope portion through the gable vents and use undereave vents to ventilate the smaller steep slope areas.
Attic Ventilation for a Gambrel Roof
Air in (intake vents) and air out (exhaust vents) is attic ventilation in a nutshell. But it’s not always that simple depending on the complexity of the roof’s configuration. We asked residential roofing contractors which roof designs are trickiest for attic ventilation. Gambrel roofs made the list. A gambrel roof is usually a symmetrical two-sided roof with two slopes on each side. The upper slope is positioned at a shallow angle, while the lower slope is steep. How do you address the attic ventilation when it needs a new roof?
“With a gambrel roof, most of the time the drywall is tight to the rafters, which are jammed full of insulation without an airflow gap,” says Ron Bastian, owner and president of Bastian Roofing, Richfield, WI. “There is no easy way to exhaust the steep part and add intake ventilation for the upper part. We’re only able to ventilate the upper section of the roof with intake vents at the transition; making sure there are insulation baffles in place to allow the air to flow to the ridge vent at the peak.”
Considerations:
- Is it possible to install intake vents at the bottom of the lower, steeper roof?
- If so, confirm that there is an airflow channel from the lower roof all the way to the peak of the upper roof clearing the transition point from steeper roof to shallower roof.
- If there isn’t an airflow channel for the entire roof, can it be created?
- If an airflow channel cannot be created for the entire roof, can the upper roof attic at least be properly vented?
VenturiVent Plus Ridge Vent Installation Animation Video
In this short animated video you’ll see how our four-foot stick ridge vent VenturiVent Plus (VVP) installs – from cutting the correct size slot to fastening the product. VVP is the unfiltered sister ridge vent to ShingleVent II. We hope you find the information helpful.
Insulation Mistakes that Affect Attic Ventilation
Although attic insulation plays an important role in home energy efficiency and comfort for the occupants of the house, there are a few common mistakes that can negatively impact the attic ventilation. They can offset any gains the insulation originally provided and cause costly damage. Listen to the Christmas nightmare Ron Bastian witnessed, Bastian Roofing, Richfield, Wisconsin, 18 years after he originally installed the roof and all was fine at the time.
Do You Know Where Your Bathroom Fan is Taking the Moisture?
The ductwork from our bathroom fans is supposed to take the moisture to the outdoors through the roof vertically or out the side gable wall. But as our podcast interview with Barry Sigler of Barry Sigler Inspections, Canton, OH, revealed, that is not always the case. And it can be very problematic. Listen to what Barry has witnessed here:
Ice Dams vs. Roofs
It is true Mother Nature often has the final word when it comes to protecting the roof and house from the elements, including ice dams. But there are key steps roofing contractors are taking to put up a good fight on behalf of homeowners. We interviewed roofing professionals across North America for their tips.
Read the article here: Ice Dams vs. Roofs
What Separates You from Your Competition?
Taking a look in the mirror to self-evaluate what separates one roofing company from another, residential roofing pros responded to our survey with very interesting observations. Our one-on-one follow-up interview with Stan Robinson, owner of Pacific West Roofing, Hubbard, Oregon, revealed it often boils down to forming a relationship with each potential customer.
Tips for New Owners and Operations Managers
Owning a roofing business or managing the operations is a big responsibility. We collected tips from seasoned veterans for new roofing companies as well as first-time operations managers. Listen to our podcast interview with Trevor Atwell, owner of Atwell Exterior Services, Greenville, North Carolina. Atwell shares his perspective from 30 years in the industry and comments on the tips from his peers.
Handling Negative Reviews from Customers
WARNING to Homeowners: DO NOT use this roofing company. We are very unhappy with our new roof they installed. They left a mess, it took much longer to complete than they promised, and we’re not too crazy about how it looks either.
At some point, your company might receive a negative review from a customer, hopefully not as negative as the example above. HOW and WHEN you respond just might turn it into a positive. Listen to our one-on-one podcast interview with Tom Picha, vice president, All in 1 Home Improvements, Oswego, IL, about how to handle negative reviews from customers.
Tips for Venting Hot Attics
“There’s no doubt proper attic ventilation can impact the effectiveness of the air conditioning system. But here are some common mistakes roofing contractors see in the field as a result of someone trying to help the A/C. In some cases, these mistakes are more problematic than the struggling air conditioning.”
That statement is extracted from a recent article by Air Vent published by Western States Roofing Contractors Association.
Read the full story here: Vent Those Hot Attics Carefully
Homeowner’s Energy Bill Cut 25% Thanks to Attic Ventilation
“As I was ready to pull the attic access door open the homeowner warned me, ‘Prepare yourself.’ As soon as I opened the attic door it felt like a sauna…I told the homeowner the attic ventilation correction that I’m proposing is going to be huge based on what I’m seeing.”
The statement above is from our one-on-one podcast interview with Khaled Kazmi, roof specialist, Precision Roof Crafters, Houston, TX. Listen to the entire episode in which Kazmi details the red flags he found inside the attic, how his team fixed the problems, and the reaction from the very pleased homeowner.
Improper Attic Ventilation Can Reduce Shingle Life 24%
“When we arrive at a home and begin to climb the ladder and we are greeted with excessive granules in the gutter, we know we have a premature shingle deterioration situation. The homeowners often report that the roofing system is new and that they were sold a lifetime roofing system. Without even having to look, we know the answer: They were sold a ‘lifetime roofing system,’ but sadly one without ‘lifetime ventilation.’ ”
The statement above is from one of the 15 roofing professionals who shared their first-hand experience with premature shingle deterioration due to incorrect or zero attic ventilation. Read the full story here: The Impact of Attic Ventilation on Shingle Life.
Read the full article here: The Impact of Attic Ventilation on Shingle Life
Listen to our Podcast episode “Improper Attic Ventilation Reduces Shingle Life 24%”
Getting Insurance to Pay to Bring Attic Ventilation Up to Code
“If you’re working with a municipality that does not even enforce the building code…send the insurance company specs from the shingle manufacturer as it pertains to attic ventilation.”
This statement from an episode of our podcast Airing it Out with Air Vent was said by Kyle Pyatt, Gen 3 Roofing Corporation, Centennial, CO. The episode f" cuses on the various ways to get homeowner’s insurance to pay for proper attic ventilation during a storm-damaged roof replacement claim. One of the clearest paths to the insurance company agreeing to pay is the insurance policy itself. But there are other ways, too.
Listen to our Podcast episode “Getting Insurance to Pay to Bring the Attic Ventilation Up to Code”
Learn More: Handy Checklist to Get Insurance to Pay for Proper Attic Ventilation
Attic Ventilation Per Building Code: IRC 2021
Every three years the International Residential building Code is reviewed for updates and then released. Residential attic ventilation is Section R806. In the 2021 IRC
You can read the code here: 2021 International Residential Code for One- and Two-Family Dwellings
Invest in Yourself and Your Business with Continuous Education
“If you want to be best in this industry
This statement taken from an episode of our podcast Airing it Out with Air Vent was said by Kevin Marcano
Listen to our Podcast episode “Invest in Yourself and Your Business with Continuous Education”
Attic Ventilation is NOT Just a Summertime Concern
Balanced attic ventilation is important and provides benefits year-round. Battling heat in the summer is commonly known. But equally important is the fight against wintertime moisture.
Listen to our Podcast episode Fighting Wintertime Moisture featuring
Oregon roofing contractor
Edge Vent Passes 30-Day Ice Dam Test
Installing a roof-top intake vent at the edge of the roof for houses in climates battered with snow and ice may cause you to wonder: What happens if an ice dam occurs? This test answers the question.
Read the Test Summary: Edge Vent Passes 30-Day Ice Dam Test Highlights
{modal modal youtube="7_51ANy-bsE" width="880" height="500" title="Airing it Out with Air Vent - Episode 8 - Ice Dams vs. Roofs" id="ep8"} Listen to our Podcast episode Ice Dams vs. Roofs featuring
an Ohio roofing contractor sharing her experiences fighting Mother Nature.
Fact or Fiction? Mixing Exhaust Vent Types is Problematic
Never mind what the Installation Instructions
Read the article: Fact or Fiction? Mixing Exhaust Vent Types is Problematic
How Does Proper Attic Ventilation Protect My Roof? (ARMA Video)
Every home has a roof
Video courtesy of Asphalt Roofing Manufacturers Association (ARMA)
Lean on Air Vent to Help Educate the Homeowner
A roofing contractor interviews Air Vent “Ask the Expert” seminar host Paul Scelsi in two quick minutes to boil down the “How’s and Why’s” every homeowner should factor in when it’s time for a new roof. NOTE: If you’d like your own customized short video with Paul
Video interview courtesy of Drew Cope
Sizing NON-MOTORIZED Attic Vents for the Attic
To help you determine how many NON-MOTORIZED attic exhaust vents are needed for an attic (as well as the intake vents required for those exhaust vents to perform properly)
1.Use the Air Vent App for iOS and Android devices. It’s free, easy to use and has many additional features beyond a built-in calculator.
2.Use the Air Vent Online Calculator in either its web-based version or the jazzed up spreadsheet that is customizable with your company colors and personalized text message to clients. It’s free as well.
3. Calculate it yourself by hand.You never know when a client may ask you,“Can you walk me through the math used to calculate that?”
• Step One:Determine the attic square footage(measured length x width, floor of the attic). If you cannot measure the attic floor, an acceptable Plan B measurement is the footprint of the house aerial view.
• Step Two:Determine the Net Free Area (NFA) needed in total for the entire attic.(Note:Non-motorized vents are specified in terms of Net Free Area – the clear, unobstructed area a vent has through which air can move.) To do that, divide the attic square footage by 150 (This is based on the 1/150 ratio of attic ventilation; which is 1 square foot of NFA for every 150 square feet of attic floor space. If you want to use the 1/300 ratio, divide attic square footage by 300.) The resulting number is the total amount of NFA insquare feet neededfor the attic.
• Step Three:Balance the attic ventilation by determining the NFA needed for EXHAUST VENTS and for the INTAKE VENTS.Using the results from Step Two, divide by 2 because proper attic ventilation is balanced between EXHAUST VENTS (which will be installed at or near the peak of the roof) and INTAKE VENTS (which will be installed in the soffit/undereave or low on the roof). The resulting number is the total amount of NFA in square feet needed from the EXHAUST VENTS and the total amount of NFA in square feet needed from the INTAKE VENTS. Next, multiply by 144 (the number ofsquare inchesper square foot) because non-motorized vents are specified in square inches not square feet. You now know the NFA needed in square inches for EXHAUST as well as for INTAKE.
• Step Four:Select the EXHAUST and INTAKE Vents Desired and Determine the Quantity.It’s time to pick both the EXHAUST VENT and the INTAKE VENT suitable for the size and design of the roof/attic. Once that is done, learn the vents’ NFA as rated by the manufacturer. Use that NFA number to determine the quantity by dividing the “NFA Needed” by the actual “NFA the vent provides.”
Here’s an example of the math start to finish.
Step One: (measure the attic) 40 feet length x 50 feet width = 2,000 square foot attic.
Step Two:(determine the NFA needed for theentire attic) 2,000 ÷ 150 = 13.3 square feet of NFA in total needed for the entire attic.
Step Three: (balance the systemwith EXHAUST and INTAKE vents).
‣ 13.3 square feet of NFA ÷ 2 = 6.7 square feet of EXHAUST ventilation needed as well as 6.7 square feet of INTAKE ventilation needed.
‣ (convert to square inches by multiplying by 144) 6.7 x 144 = 965 square inches of EXHAUST net free area needed, and 965 square inches of INTAKE net free area needed.
Step Four: (select the EXHAUST and INTAKE vents desired; determine quantity).
‣ Select an INTAKE vent desirable for the project, determine its net free area rating in square inches per unit/linear foot.
✓ For example, an 8 x 16 rectangular undereave vent allows 56 square inches of net free area per unit. Thus, 965 ÷ 56 = 17 rectangular undereave vents needed.
‣ Select an EXHAUST vent desirable for the project, determine its net free area rating in square inches per unit/linear foot.
✓ For example, ShingleVent II ridge vent is 18 square inches of NFA per linear foot. Thus, 965 ÷ 18 = 54 linear feet of ridge vent needed.
What Size Power Fan Does the Attic Need?
To help you size a roof-mount or gable-mount power attic fan (whether traditional electric or solar powered) for an attic
1.Use the Air Vent App for iOS and Android devices. It’s free, easy to use and has many additional features beyond a built-in calculator.
2.Use the Air Vent Online Calculator in either its web-based version or the jazzed up spreadsheet that is customizable with your company colors and personalized text message to clients. It’s free as well.
3. Calculate it yourself by hand.You never know when a client may ask you, “Can you walk me through the math used to calculate that?”
• Step One:Determine the attic square footage (measured length x width, floor of the attic). If you cannot measure the attic floor, an acceptable Plan B measurement is the footprint of the house aerial view.
• Step Two:Determine the CFM needed from the power attic fan to service the attic. Multiply the attic square footage by a factor of 0.7 (this produces 10-12 air exchanges per hour from the power attic fan). The resulting number is the CFM (Cubic Feet of air moved per Minute) needed from the power attic fan to properly exhaust the attic. Find a power attic fan with the required CFM or close to it. NOTE: It’s OK to have more CFM than needed as long as the fan(s) are given the necessary amount of intake ventilation (at the soffit/eave or low on the roof near its edge). That’s the next step.
• Step Three:Give the power attic fan proper intake ventilation. The amount of intake ventilation a power attic fan needs is based on its CFM. If the attic requires more than one power attic fan, this formula applies to each fan used or installed for the project. Here’s the formula:
‣ CFM of the power attic fan ÷ 300 = Square feet of intake net free area needed.
‣ Since intake vents are rated/specified in square inches (not square feet), it’s necessary to convert from square feet to square inches. That is easily accomplished by multiplying by 144 (the number of square inches per square foot).
‣ Once you know the intake ventilation net free area needed in square inches, find an intake vent best suited for the project, identify its net free area in square inches (each manufacturer should publish that number) and determine how many units/feet of intake vent are needed.
Here’s an example of the math start to finish.
Step One:(measure the attic) 40 feet length x 50 feet width = 2,000 square foot attic.
Step Two:(determine CFM of the fan needed) 2,000 x 0.7 = 1,400 CFM power attic fan needed.
Step Three:(give the power attic fan proper intake ventilation).
‣ 1,400 ÷ 300 = 4.7 square feet of intake ventilation net free area needed.
‣ (convert to square inches by multiplying by 144) 4.7 x 144 = 677 square inches of intake ventilation net free area needed.
‣ Select an intake vent desirable for the project, determine its net free area rating in square inches per unit/linear foot.
‣ For example, an 8" x 16" rectangular undereave vent allows 56 square inches of net free area per unit. Thus, 677 ÷ 56 = 12 rectangular undereave vents needed.
What Good Are Attic Exhaust Vents? They’re Not Without Proper Intake Vents
Overlooking intake ventilation is the #1 mistake in residential attic ventilation.
Read why these roofing contractors work hard to avoid it.
Read the article: What good are Attic Exhaust Vents? They're Not Without Proper Intake Vents
ShingleVent II Installation Animation
In this quick video you’ll see how our four-foot stick ridge vent ShingleVent II installs
Attic Ventilation Educational Video
(Shortened version of the “Ask the Expert” Seminar)
This video is extracted from the annual Air Vent Seminar “Attic Ventilation: Ask the Expert™” held in-person for roofing professionals across North America every 1st quarter. The seminar is an overview of residential attic ventilation best practices.
Take the 5-Question Quiz
Ask the Expert Seminar: Early Notification
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Click here for additional information about the “Attic Ventilation: Ask the Expert™” seminars.
