I'm confused.  If a ridge vent is advertising 18 in² of Net Free Vent Area (NFA) per linear foot, yet is only 3/4" thick, what is the real NFA.  18 in² would be the exposed edge of the vent (9 in² each side) and doesn't seem to be accounting for the internal construction of the vent, which varies, which prevents snow and rain from blowing in.

If we are to calculate high and low vent areas, I have no problem doing this, how can we calculate the effective upper vent area without the "net" number? 

As an example, I looked at a large triangular gable vent and its NFA was only 12% of its outside dimensions, a terribly low number, but at least it sounds like what I can expect.

What are you using for vent areas, NFA, on your ridge vents?


Tags: NFA, ridge, vents

Views: 1413

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LOL, I now see the difference in our perspectives, hot climate vs cold climate.  The solar gain you folks deal with is extreme compared to the tiny heat flow we see here in the north country.  0.1 air changes per hour would probably meet our static needs, just guessing. 

I lived in NJ for awhile and installed a whirlybird on my first house and I was impressed at how well it performed.  A friend installed a huge whole house fan and it could cool the attic in 30 minutes once the sun went down.

Then I get into this business and hear nothing but negative talk about fans, thus I wanted to know if static was doing its job.  In the north, it looks like it can, to a point.  In a hot climate, they will have a hot attic.  But again, the recommended solution is to insulate and air seal. 

I agree that solar powered attic fans are a marketing bonanza, hype that sells.  But, can we justify the energy used for a powered fan solution?  That's one of the questions I hope to answer for myself.  Also, motors and fans have improved greatly in efficiency, so the cost gap is narrowing.

I watched the video, but it lacked the details to judge what they were doing.  I did go to the home page and had to laugh that they are still clinging onto the phrase "Hot Air Rises" in many of their explanations.  Even the old video about the "short circuit myth" doesn't really understand how air is moved through our homes and attics.  But that will be a discussion for another thread.


I don't think powered fans make sense for Northern climates. Even in southern climates they should only be used when a good passive solution would be difficult to implement. Fans with 1125RPM PSC motors should be used, their power consumption is reasonable for the amount of airflow.  When powered fans are used, setpoint should be relatively high, around 120F on, 110F off. Of course this of no value to those in a northern climate, it would never run...

Now, if we could convince homeowners in southern climates NOT to install BLACK shingles, but that's also a discussion for another thread...

Another problem with NFA is is doesn't calculate the static pressure loss created by the length of the air path which should include the 120degree turn, internal baffles, and filter material. This significantly increase static pressures per inch of net free area. Static pressure and NFA are what determine airflow levels.

The old school "whirlybird" type vents have a very low static pressure in relation to their NFA. Low "skin effect" losses since the "duct" has a low amount of surface area in relation to the typical 12" cross section. One 30 degree turn in a 12" "duct" doesn't create much static pressure loss. The turbine head will typically spin as air flows out of it, minimizing pressure losses at the outlet.

BUT the whirlybird vents are considered unattractive, and the bearings go bad on the low quality turbines. Galvanized turbines rust over the life of the roof. When they don't' work, homeowners are aware of it since they don't turn or become noisy. When a ridge vent doesn't work, the homeowner has no idea. When an electric attic fan fails the homeowner rarely notices unless it fails in such a way the bearings make a lot of noise or the motor windings stink when it burns up.

So for homeowners and roofers, Ridge Vent = Good, Whirlybird = Bad. Who cares if they actually WORK or not.


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