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). This formula is traditionally used for static roof vents which are rated for Net Free Area in terms of square inches.
NRCA recommends designers of steep-slope roof assemblies provide attic ventilation by using static, balanced ventilation systems with a minimum amount of 1 square foot (0.09 m²) of net free ventilating area for every 150 square feet (28 m²) (1-150 ventilation ratio) of attic space measured at the attic floor level.
What Is the 1:300 Rule? The US Federal Housing Authority recommends a minimum of 1 square foot of attic ventilation for every 300 square feet of attic floor space, evenly split between intake and exhaust.
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.
For most homes, a rule-of-thumb is to just multiply the total square footage of the attic by 0.7 to find the minimum ventilation rate in cubic feet per minute (CFM). For homes that have dark shingles and/or steep roofs, the multiplier is increased by 15%, to 0.805.
It is absolutely possible the ventilation system on your roof is excessive. Maybe your home is older and prior ventilation was installed by a homeowner, or incorrect vent sizes were used in an older roof layout. Too much ventilation can negatively influence the attic's and home's insulation as well.
Usually, if you have an 1800 SF attic floor area you /2 = 900 SQ IN for intake and 900 SQ IN for exhaust ventilation, that goes for about up to 6/12 pitch from there you add about 20% for up to 10/12 pitch anything after that you add about 30%.
Attic Ventilation
area shall be no less than 1/150 of the attic floor area if no vapor retarders are used, or 1/300 of the attic floor area when a vapor retarder having a permeance of one perm or less is used.
Rafter vents should be placed in your attic ceiling in between the rafters at the point where your attic ceiling meets your attic floor. Once they are in place, you can then place the batts or blankets, or blow insulation, right out to the very edge of the attic floor.
Flat roof ventilation ratio: 1/150
These venting ratios suggest that for every 150 ft2 or 300 ft2 of insulated ceiling surface area within the attic, 1 ft2 of ventilation area shall be applied to the roof.
You can use a thermometer and a hygrometer to measure the temperature and humidity in your attic. Ideally, the temperature and humidity in your attic should be close to the outside air. If your attic is too hot or too humid, it means that your attic is not properly ventilated and needs more airflow.
Ridge Vents are popular. They work well because they provide even ventilation across the whole attic. They are installed along the roof's peak, allowing hot air to escape naturally without the need for power.
The 3-degree rule: Ventilate a hygroscopic cargo if the temperature of the outside air is at least 3°C below that of the cargo temperature (taken at loading). The dew point rule: Ventilate when the dew point of the outside air is lower than the dew point of the air in the hold.
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). This formula is traditionally used for static roof vents which are rated for Net Free Area in terms of square inches.
MYTH: Considering the problems caused by insufficient insulation, it's best to have as much air flowing through the attic as possible. Truth: Ventilation needs vary. Too much ventilation can cause leaks, encourage pest infestations, and leave your attic vulnerable to storm damage.
Maintaining adequate attic ventilation is essential for the overall health of you and your home. Without proper ventilation, you risk mold mildew growth, general deterioration of your roofing system, and even increased respiratory problems due to this moisture accumulation.
When calculating how much attic ventilation you need, your roofer will either use the 1/150 or 1/300 rule. The one they use determines how much Net Free Ventilation Area (open area for air to pass through) you'll need for every 150 or 300 square feet of attic space.
Incorporate Under-Eave Vents
Also known as soffit vents, under-eave vents are crucial for drawing in fresh air and setting up the cyclical air flow necessary for optimal attic ventilation. They work hand in hand with ridge vents. As fresh air enters through the soffit, it pushes out the hot air via the ridge vents.
If you are installing insulation with a vapor barrier, make sure it faces the interior of the house. Another option for a vapor barrier is to take sheets of plastic and lay them between the ceiling joists. Then, using a staple gun, tack them to the sides of the joists.
Most local building codes require compliance with either the 1/150 method or the 1/300 method exception (refer to local code). These methods can dictate that one (1) square foot of ventilation is provided for every 150 or 300 square feet of attic floor space.
The short answer is no, you can't install too many soffit vents. The air is cleaner and there is a nice breeze when the house is properly ventilated. This same practice applies to soffit vents, and it's much better to overventilate than to have air sit in the attic without movement.
How much ventilation does an attic need? The IRC requires 1 square foot of net free ventilation area to every 150 square foot of attic floor space (1/150), but does not specify the location of intake vents or exhaust vents, nor does it specify the ratio of intake to exhaust.