The Importance of Air Intake Ventilation

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 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.

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.

Minimum Required Net Free Area Intake Ventilation

When using any type of powered ventilation product for an attic application, all passive attic vents will become sources for air intake.  Unlike passive ventilation where air flow must travel from the lowest point in the attic to the highest, powered ventilation uses air intake from all areas of the attic both high and low.  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.

Estimated Passive Attic Ventilation Based on National Building Code

How Much Net Free Area Does a Vent Provide?

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.  As a general rule, screen mesh sizes of ¼" mesh and larger have a negligible effect on airflow, so the net free area for ventilation provided by the vent is essentially the same as the total area of the vent itself.  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.

Using this table, you can calculate the required amount of any of the listed soffit types needed for your home.  For example, let's assume you have a 2400 sqft home and wish to install an Attic Breeze 25 watt fan.  From the tables above, we find that a 25 watt fan will require a minimum of 5.2 sqft net free area for proper operation.  We also find that if built to code, at 2400 sqft of attic space your attic should already provide 8.0 sqft net free area of passive ventilation.  This should be an adequate amount of passive intake ventilation to support your Attic Breeze fan.  However, there may be a situation where you need to replace some of the soffits or possibly add additional air intake ventilation.  If so, here's the calculation:

Total area of soffit needed = [required net free area] x [sizing factor for soffit type]

If we will be using for example a continuous soffit with 1/4" holes, then from the table above 1.89 would be the sizing factor.  Therefore for this example, you would calculate:

Total area of continuous soffit = [5.2 sqft net free area] x [1.89] = 9.8 sqft total

If the continuous soffit you are using is roughly 6 inches wide (0.5 feet), you would need:

Total length of continuous soffit = [9.8 sqft] ÷ [6 in]/12 = 19.6 feet

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.

Additional Passive Ventilation

My Home has Ridge Vents

When installing Attic Breeze products in conjunction with ridge vents, the solar attic fan unit should be installed a minimum of 5-7 feet away from the ridge vent to allow for balanced airflow through the attic.  Experience shows that this distance will typically allow enough air diffusion space for proper balancing of air intake ventilation from both the ridge vent and other passive roof vents installed on your home.  In some cases, however, due to spacing restrictions, it may be necessary to install your Attic Breeze solar attic fan a little closer to the ridge.  In these situations, we recommend blocking off a portion of the ridge vent above the attic fan (roughly 5-7 feet on either side of the fan).  Please note that the ridge vent does not need to be removed, it just needs to be blocked off from inside the attic to restrict airflow.

In many cases, having an existing ridge vent on your home in conjunction with an Attic Breeze is actually a good thing.  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 airflow through your attic and ultimately better cooling performance.  When your solar attic fan is not operational (at night, during rainstorms, etc...), the ridge vents will function as normal providing your attic with passive ventilation.

What about Attic Depressurization?

Attic depressurization is a phenomena that can occur when the air pressure inside your attic becomes lower than the outside ambient barometric pressure.  An example of this occurring naturally is during thunderstorms where the sudden cooling of your roof due to rain results in a reduction of air pressure inside your attic.  When this occurs, you might notice air rushing into your home when you open a door or window.  The problem with attic depressurization is that any air being pulled into your home from outside is displacing the cooler air-conditioned air already there, resulting in higher cooling bills.  So in general, it's good practice to minimize attic depressurization when possible.

In addition to naturally occurring attic depressurization, this phenomena can also occur when using active ventilation products such as wind turbines (whirlybird vents) or powered attic fans.  Attic depressurization can result when there is not enough air intake ventilation to support the volume of air being removed from the attic by the active ventilation product.  Because of this, some professionals in the building industry have taken a negative view of active ventilation products in general, assuming that all of these products are the same.

Although by appearance they may seem similar to some in the industry, AC powered attic fans and Attic Breeze solar attic fans are distinctly different.  Attic Breeze fans by design cannot cause attic depressurization and we have the Breeze Mate solar ventilation controller which monitors the air pressure inside your attic to prove it.  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.

By contrast, Attic Breeze solar attic fans only have a limited source of solar power and use a variable speed DC motor.  Given the same situation, our solar attic fans are designed to reduce fan speed since additional power from the solar panel is simply not available.  Because our fans can automatically operate at lower speeds, and with our proprietary UltraFlo® design technology, attic depressurization never occurs.  But don't take our word for it, all you need is one of our Breeze Mate® solar ventilation controllers and you can see for yourself.  The Breeze Mate controller actively monitors attic air pressure and allows you to test for depressurization.  With the Breeze Mate control system, you can directly observe the performance of your Attic Breeze solar attic fan and never need to be concerned about attic depressurization again.