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: 2075

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Got it - http://www.icc-es.org/Criteria/pdf_files/ac132.pdf

The mesh doesn't count against the NFA so .75 x 24 linear inches = 18 for your example

For those with plastic inserts, etc... they just need to eliminate the leading edge to get the numbers which is why the numbers vary based on manufacturer & type



That looks like the specifications, just one thing missing, the plain language summary, ARGhhh!  I was only able to skim the report and will go through it slowly when time allows, but it still doesn't seem correct to be able to claim 100% NFA on any product other than an open hole.  Here is the product showing 18 NFA per linear foot.  Maybe?



You bring up a good subject. I think the manufacturers recommendations are best.  As a building inspector for many years I saw many of these installations where the ridge opening was 3/4" or less.  The manufacturers state that the opening of the ridge sheathing should be 1 1/2" wide per side of the ridge.  To me the most important thing with ridge vents beside weatherproofing is the allowable opening in the sheathing.

I'm sure you have seen a few with NO opening :(. 

I'm still concerned as to what the performance will be.  I'm trying to maximize my upper vent area with both ridge ans gable vents to reverse the air flow, currently exiting my soffits on a mid height dust pan, see other thread.  But the large triangular gable vents I looked at are showing 12% NFA and now I'm having trouble trusting the ridge vent specifications.  Sean's link is right on target, we'll have to promote him to scrounge for digging up good information :), and I'll have to read it in depth to elevate my confidence level.


Sure Bud, no problem my hourly rates are... : )

Of course I can't say that my confidence levels on that being a true opening are relieved any though I would definetly prefer this type for its ability to block snow or rain

Eric, I would be careful on relying on just what one manufacturer's state... some are 2" to 2.5" on each side but I hear you - it would be nice to see at least an 1 1/2" on some of these places 

Good point. It depends on the product.

Ridge vents are a good idea in theory, but they simply don't work. Air cannot go up, go through a filter mesh similar to a home air filter, then make a 120 degree turn (depends on roof pitch), flow DOWN, then make another 120 degree turn before going back up. If there was any airflow the filter material would quickly clog up, just like it does in a home HVAC system. 


I will admit I have never dissected an older ridge vent to see how bad it might be clogged with dust, but I have never seen that on the list of issues we should check for.  On the other hand, there are untold numbers of ridge vents out there that seem to be functioning as intended despite the twists and turns.

Static venting is a low pressure slow process and it may be the nature of slow moving air that deposits that dust at the attic floor rather than carrying it up to the ridge vent.  But I'm speculating.  In any case ridge vents, and others, do work.

What I'm seeking in this thread is a fair representation of the effective vent area for ridge vents.  The link I provided above implies 100% NFA, which just seems unrealistic.


Exactly, the VERY slow nature of air moving though the vents means they don't clog. How many CFM do you speculate is going through a typical ridge vent? How much is the real net free area? How many air changes per hour for a typical installation? How many feet per minute of air across the "filter"? Static pressure drop for a typical "rock catcher"  HVAC air filter at specified feet per minute?

Run the numbers, you will see it's physically impossible for ridge vents to work.

Bob, if you are going to make statements like this, I have to ask for something more than your opinion.

<Run the numbers, you will see it's physically impossible for ridge vents to work.>

You are saying the number one choice for high attic vents don't work.  Without some strong supporting material that is hard to accept.

Got some links or research I can read?


That's just the problem, finding independent research is difficult to find. All the "Research" I've been able to find showing ridge vents work has all been provided by the manufacturers of the vents. This is why I posted the YouTube link show how ridge vents don't work, it's the only "research" I've been able to find that isn't sponsored by a manufacturer.

Builders and roofers love them because they are easy to install. Consumers love them because they look good. Typical homes have AC units that are WAY oversized, so the homeowner doesn't notice the lack of performance in their ridge vent. Most homeowners don't go into their attic during hot summer afternoons so they just don't realize how hot it gets up there and how long the heat stays in the attic after the sun goes down.

Most ridge vents use some sort of filter material that's very similar to HVAC air filters. With any significant airflow they will have significant static pressure drop, the same way HVAC filters do. Lets take some theoretical numbers, using standard building practices: 1/300 ratio for attic ventilation is code in many areas, with 1/2 up high and 1/2 down low so I will use that.

1: Attic floor area 1,000 sqft

2: 1000/300 = 3.33sqft required = 1.67sqft of total ridge vent area required. About a 16 x 16 HVAC air filter.

3: Attic air change 5 per hour.

4: Roof height 8ft, standard gable roof.

5: Attic volume 4ft x 1000 sqft = 4,000 cuft

6: Airflow required 20,000 cuft per hour/60 = 333CFM.

7: 333CFM through a 1.67sqft 16x16 air filter will create a face velocity of about 200 feet per minute.

8: 200 feet per minute through a fiberglass mesh filter will create a static pressure drop of about 0.05 inches.

9: 0.05" static pressure isn't going to happen unless there is a 50 degree difference.

10: The above calculations are for the CLEAN mesh filter, if 200FPM of air was actually going through the filter it would be clogged within one summer.

We haven't even discussed the additional static pressures created by the 120 degree turn and the internal baffles..

It's just like the solar attic fans that don't work as advertised. They are promoted by manufacture's as being able defy physics. Outrageous claims such as 800CFM out of a 12W motor are being promoted. Oddly a conventional attic fan is requiring a 400W hardwired AC motor with larger blades at significantly higher RPM's to perform the same 800CFM. One is BS, and the other uses real numbers, that's the difference...

Yet Solar Attic Fans are promoted as the latest and greatest and continue to be sold as such. Good luck finding independent research on real CFM numbers...


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