The NFA is the open area that exists for air to pass through. This means that for each 150 square feet of attic floor space, 1 square foot of Net Free Area is required for ventilation. Click to see full answer.
How much net free area does an attic vent provide?
If built to national building code standards, your home will have a minimum of one square foot of net free area passive ventilation for every 300 square feet of attic space. Total net free area of passive attic ventilation based national building code standards are shown in the table below. How Much Net Free Area Does a Vent Provide?
What is net free area (NFA) ventilation?
When discussing ventilation, the term "Net Free Area" or NFA is used to describe the amount of space available in any type of vent that can be used to freely pass airflow. This is essentially the empty spaces you see in the installed screens and louvers on most passive vents.
How much net free area is available from a passive vent?
The amount of net free area available from any given passive vent depends on the type of vent used and the size of its screen, vent holes, or louvers.
What is proper attic ventilation?
Proper attic ventilation consists of a balance between air intake (at your eaves, soffits or fascias) and air exhaust (at or near your roof ridge). The U.S. Federal Housing authority recommends a minimum of at least 1 square foot of attic ventilation (evenly split between intake and exhaust) for every 300 square feet...
What does net free ventilation mean?
Related Definitions Net Free Ventilation Area means the combined product of unobstructed clear opening dimensions that permit the free flow of required ventilation air.
What is the best method of venting an attic space?
The best way to ventilate a finished attic is rafter venting. Rafter vents, or insulation baffles, are installed in an attic's rafter space and create narrow gaps that direct fresh air from the soffit vents to the roof's peak.
How is net free attic venting calculated?
Attic square footage ÷ 2 = square inches of EXHAUST and square inches of INTAKE Net Free Area (NFA) needed. (NFA is the unobstructed area through which air can pass through a vent, usually measured in square inches.
What are the three kinds of attic ventilation?
Choosing Exhaust Vents for Your Attic Ventilation System. Exhaust ventilation is most efficient when it's installed at or near the highest point of your roof where hot, humid air can easily escape. Exhaust vents are divided into three common categories: static, powered, and mechanical.
How do you tell if your attic is venting properly?
How to determine whether you need better attic ventilationLook at your eaves and roof. ... Touch your ceiling on a warm, sunny day. ... Thick ridges of ice on your eaves in winter are a sign of poor attic ventilation. ... Warm air that escapes living space also carries moisture that will condense on rafters or roof sheathing.
Do attic vents really work?
Proper insulation and air sealing also keeps attics cold in winter by blocking the entry of heat and moist air from below. In the summer, natural air flow in a well-vented attic moves super-heated air out of the attic, protecting roof shingles and removing moisture.
How much airflow should an attic have?
Your attic needs at least 1 square foot of ventilation for every 300 square feet of attic space. That means the typical ratio of attic area to attic ventilation is 300-to-1. Half of your ventilation should be exhaust vents, and half should be intake vents.
How much airflow do I need in my attic?
Most codes use the 1/300 rule for minimum residential attic ventilation recommendations. This means that for every 300 square feet of enclosed attic space, 1 square foot of ventilation is required – with half at the upper portion (exhaust vents) and half in the lower portion (intake vents).
What type of roof vents are best?
BEST OVERALL: LOMANCO 12 in. ... BEST BANG FOR THE BUCK: Broan-NuTone 3 in. ... BEST RIDGE VENT: GAF Cobra Ridge Vent.BEST SOLAR VENT: Master Flow 500 CFM Solar Powered Roof Mount Exhaust.BEST HARD-WIRED POWERED: Master Flow 1000 CFM Mill Power Roof Mount Attic Fan.More items...
What are the 4 main types of roof vents for attic ventilation?
The 4 common types of active roof vents: turbine, power, ridge, and solar-powered vents.
Can a roof have too much ventilation?
The two main dangers associated with too much ventilation involve roof damage and increased utility bills. If you have too much air circulating, your roof will collect moisture causing damages that will weaken spots and then cause leaking.
What is better ridge vent or roof vent?
Roofers prefer installing ridge vents because they're easier than cutting numerous holes in the roof and then having to cut shingles around them. The other benefit to ridge vents is that small animals like squirrels can't chew holes and get into your attic like they can with box vents.
What Does The Code Require?
The majority of the requirements related to attic ventilation have not changed much from the previous editions.
How many static vents are needed for a gable roof?
When looking at residential roofs, it is not unusual to see a gable roof with only two or three static exhaust vents. Ventilation calculations show that most of the time, that is not nearly enough. To illustrate this, let’s look at the NFVA value for the above example. The calculations indicate that the building needs 240 in. 2 for exhaust. A typical static exhaust vent provides 50 to 60 in. 2 per vent. Three static vents with an NFVA of 60 in. 2 would only provide 180 in. 2 of venting.
What are the requirements for cross ventilation?
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.
How much ventilation area is needed for an intake?
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).
Why vent the attic?
Why Attic Ventilation. Attic ventilation serves two main functions: to lower attic temperatures and to remove excess moisture. Lowering the temperature in the attic can help reduce air-conditioning energy costs and roof deck temperature, optimize the service life of a roof covering, and minimize ice damming.
Why is attic venting important?
Attic venting serves an important purpose and is key to a well-performing roof system. Anyone involved with residential roofing should become familiar with the purpose of attic ventilation and understand how to achieve an effective ventilation system while meeting building code requirements.
How much airflow per square foot for a power vent?
According to the National Roofing Contractors Association (NRCA), a power vent with an airflow rate of 1.0 cubic foot per minute per square foot of attic space measured at the attic floor is generally considered to be equivalent to a 1:150 ventilation ratio.
What happens if you have more exhaust than intake?
However, if the attic has more exhaust than intake it potentially can cause the extra exhaust to pull its missing intake from itself (if it’s a ridge vent) or from another nearby exhaust vent (from one wind turbine to another or one roof louver to another), which means possible weather ingestion. 3.
Why does my intake vent work on the leeward side of the house?
As a result, the intake vents on the leeward side of the house will work “with” the exhaust vents to release air.
Why is ventilation important in the attic?
The goal of an effective attic ventilation system is to help fight heat buildup inside the attic during the warmer months and moisture buildup in the colder months. Additionally, in climates where snow and ice are common, attic ventilation can help fight the formation of ice dams.
How much does a roof pitch increase ventilation?
7:12 to 10:12 roof pitches increase the amount of ventilation by 20 percent.
Do you have to be good with numbers to calculate attic ventilation?
Good news, roofing contractors: You do not have to be good with numbers nor do you have to enjoy math to be able to quickly—and accurately—calculate the amount of attic ventilation needed for residential attics. Here it is, a handy shortcut for quick calculations:
Is there a longer formula for attic ventilation?
There is a longer “official” formula based on building code you can reference or point your clients to for reassurance that you know what you’re talking about. Most attic ventilation manufacturers list the longer formula on their websites and inside key product brochures. But the shortcut is just as good and faster!
How Much Intake Ventilation Do I Need?
When discussing ventilation, the term "Net Free Area" or NFA is used to describe the amount of space available in any type of vent that can be used to freely pass airflow. This is essentially the empty spaces you see in the installed screens and louvers on most passive vents. As you may imagine, airflow becomes more difficult through a vent when the screen or louver size becomes smaller, even though the vent itself may be quite large. Therefore to be able compare different vent types and sizes, the idea of Net Free Area was developed to standardized ventilation.
How are attic fans different from solar fans?
AC powered attic fans have a very large source of grid power from which to use, as well as a constant speed AC motor. When an AC powered fan does not have enough air intake ventilation to support the fan's set operating speed, the fan simply uses more power in order to maintain its motor speed, thus pulling more air from any air intake source available and depressurizing the attic.
How does screen mesh affect airflow?
As screen mesh size decreases , the net free area of the passive vent decreases as well, only providing a fraction of the total area of the vent as usable airflow space. The table below shows the effects of screen, vent hole, and louver size on net free area for various types of soffit vent designs.
How much air intake ventilation is required for an attic breeze?
The national building code recommends a minimum of one square foot net free area intake ventilation for every 300 CFM of powered ventilation airflow. Since Attic Breeze solar powered fans are variable speed in terms of airflow, the calculation for required air intake ventilation must be made at the fan's highest rating to ensure adequate ventilation under the full range of operation. The table below may be used to estimate the minimum required amount of air intake ventilation (NFA) for all Attic Breeze solar powered ventilation products.
What is ridge vent?
Ridge vents are great at providing ventilation for "pocketed" areas of your attic and are especially effective on homes with multiple ridge lines and elevations. While your Attic Breeze solar attic fan is operating, the ridge vents on your home will all work in reverse acting as air intake vents. This will allow for better balanced ...
Why is air intake important?
Good air intake ventilation is very important for achieving optimum performance results from your Attic Breeze solar attic fan. Attic Breeze fans will operate at peak performance when supplied with the proper amount of ventilation. However, failure to supply adequate air intake ventilation will result in decreased air flow and reduced cooling performance from your product. We recommend becoming familiar with your attic ventilation design to ensure that your attic has the required amount of air intake ventilation before installing your new Attic Breeze ventilation system.
How many watts of solar attic fan?
So from the tables and calculations, 19.6 feet of 6 inch wide continuous soffit with 1/4" holes would be adequate to support one 25 watt Attic Breeze solar attic fan with proper air intake ventilation.
Why is Roof Ventilation Required?
Common activities around the house such as cooking, bathing, laundering, and even human breathing and perspiration can produce a large amount of water vapor that can make its way through the building envelope and into the attic.
Why vent the attic?
To prevent this, the space within the attic must be vented to not allow the water to collect on the building components. Ventilation openings allow air to flow through the attic to remove any moisture and possibly minimize condensation problems within the attic and enclosed rafter spaces.
How much insulation should be held back from the vent openings?
To ensure that vent openings are not blocked by insulation, the code requires the insulation to be held back from the vent openings a minimum of 1 inch.
What are the two scenarios that require attic ventilation?
Based on this code section there are two scenarios that will require attic ventilation: Enclosed Attics and Enclosed Rafter Spaces.
What is the IRC code for roof ventilation?
Section R806 of the 2018 International Residential Code (IRC) outlines when roof ventilation is required and how to calculate it.
Where is a Class I vapor retarder located?
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.
What is the minimum net free ventilation area?
To properly calculate the required roof ventilation, Section R806.2 specifies that the minimum net free ventilation area shall be 1/150 of the area being vented.
Who set up some test equipment to try to measure whether the calculation methods used by ridge vent manufacturers and soff?
George,#N#According to building scientist Bill Rose , who set up some test equipment to try to measure whether the calculation methods used by ridge vent manufacturers and soffit vent manufacturers was accurate, the method used by manufacturers hardly matters -- because it isn't accurate.
Who set up the test equipment to measure the accuracy of ridge vents?
According to building scientist Bill Rose, who set up some test equipment to try to measure whether the calculation methods used by ridge vent manufacturers and soffit vent manufacturers was accurate, the method used by manufacturers hardly matters -- because it isn't accurate. Rose never got around to publishing his research;
