Passive venting solution needed.

How do you manage the air flow to properly ventilate an attic when one soffit is low and the other somewhere in between low and high?  I have a cape with an almost full length dust pan on the rear.  Original venting from back in 79 included 2 small (12" x 12") gable vents and 4 each front and read rectangular (12" x 3") vents in the soffits.  Ice dams have always been a problem so now the old shoe maker is finally going to fix the problem.  You know, the shoe maker's children have no shoes, well energy auditors suffer from a similar affliction:).

Now that I'm slowly making improvements I find myself halfway between the old design and a new design, which was going to be lots of soffit venting and a full ridge vent, and I would probably keep some gable venting, TBD.  The new siding went on the front a year ago with extended rafter tails with a full 10" perforated vinyl soffit, about 30 times the previous vent area, take that Mr. ice dam. 

Currently I've extended the rear rafter tails and have the fascia and soffits removed, but the roof with its ridge vent will be delayed until next year.  Then I started thinking, are those little gable vents going to be adequate for this winter with all of this additional soffit venting?  A quick trip up onto the rear staging with some smoke confirmed my fears.  The rear soffits are exhausting warm air, Arghh!  That is definitely not what I want for ice season.

Note, I have some known heat loss and air leakage issues inside I can address which will certainly help.

My problem is two part.  One, I need a temporary solution to get me through this winter, and two, what is the best design for a final solution, and hopefully incorporate as much of the temporary work into the final product, I'm cheap?

Let's review the problem.  With my rear soffit vents just a couple of feet below my undersized gable vents, the new excessive ventilation area in my low front soffits (9' below my gable vents) has shifted my neutral pressure plane (NPP) below those rear vents.  Remember, as you increase a leakage area, or vent area, the NPP moves towards the increased opening.  So, following Dr. Joseph Lstiburek's advice is not working in my situation.

"3) Put more vents down low than up high
This is where the code tends to have it wrong. You want more entry points at the perimeter than exit points at the top."

It's not that Dr. Joe's advice is wrong, it is just that it doesn't apply to mixed height soffit vents and I'm assuming the problem gets even more complex when you add in multiple height exhaust vents such as ridge, gable, or other.

Here's my proposed solution, which I can patch in for temporary and incorporate into my completed project.  This isn't cast in concrete, that's why I'm here.   The now oversized front soffit vents enter mostly into two accessible small front attics, separated by a set of stairs.  I can add an upper vent to the gable ends of each of these small attic spaces and then a restrictor in each rafter bay to limit the air flow up through the slope to the top attic.  By limiting the air flow, I will shift the NPP upwards.  These slopes are well insulated and have minimal leakage areas so a reduced air flow should be fine.  I will also replace the two existing small gable vents with large triangular gables vents, trying to maximize the high vent area, again, shifting the NPP upwards.  I will then do another smoke test to confirm whether or not I have achieved my goal of cold air flow into those rear soffits.  Comes spring and a new roof, I would like to use a single sided ridge vent to minimize rain issues and further shift the NPP towards the top adding some margin to my venting strategy. Basically, I'm being forced to put the majority of my high vent area well above my low vent area and restrict my lowest of my low vents.

The searching/reading I have done has provided no guidance for how to balance the venting when multiple intake and exhaust heights are involved, beyond isolating attic areas so each area can function by itself.  But, even that solution doesn't deal well with a cape such as mine or the typical salt boxes out there.

Any suggestions or comments?


Tags: attic, venting

Views: 2566

Replies to This Discussion

The 12" aluminum externally braced whirlybirds go for about $50 each, not unreasonable at all. They don't rust and come with high quality bearings. Rarely do they leak, the spinning action of the turbine sheds water off the vanes. If a few drops of rain do get in they normally end up on the low side of the duct and drip out on top of the shingles. This is why no sealant is applied to the lower side of the flashing. Figure one per 500sqft of attic floor area, although it's common practice to install 2 regardless of house size.

"Ventilation towers" were common in older buildings, they were designed as miniature  steeples. Gable style vents were then applied to the sides of the miniature steeples.

If homeowners aren't willing to have whirlybirds installed, large "turtle vents" can be an effective solution. They are about the size of a typical attic fan, but have no motor or blade. Using several of them can prove an effective "high point" solution. Large dormer or gable vents also can be effective when combined with oversized soffit venting. Oversizing soffit venting also helps when there is any wind, the hot air will EXIT out of the downwind soffit vents.

I got to disagree somewhat with "air cannot" - air seeks the path of least resistance & yes it will exit via ridge vents (some better than others, of course). Are they my favorite item, no but they can & do work in many situations. You got to remember the purpose of venting is simply to allow excess heat & moisture to exit the attic assembly

Yeah I do like whirlybirds & yes Bud it is a "passive" system system that works whether or not there is wind or not - if there is wind it does work a lot better. Now would I use these up in an area with lots of snow - no, probably not

Fair enough. Air will exit ridge vents, but at an insignificant rate. There is simply not enough pressure difference to force much air through the filter. On windy days an external baffle may help the ridge vent pull air through the filter.

Bob, I want to continue this and I also want to hear from anyone in regards to the original question, how do we deal with multiple heights when balancing our static attic air flow.

Bob, your last comment < There is simply not enough pressure difference to force much air through the filter.> I'm not sure how these filters are designed, but they are rated as near 100% air flow. A NFA of 18 in² per foot for a 3/4" high opening says it is essentially totally open. Therefore, at just one pascal of air pressure a full ridge vent would provide 18 CFM per foot or 720 CFM over 40'. Correction to my calculations is welcome.

Since I'm in cold country and mother nature has opened her test lab, I can now drive around and observe the ridge vents with a roof covered in snow. Observations are mixed and I cannot see what is happening on the backside, but most ridge vents are clearly open and functioning. A few homes with both ridge and gable vents showed the ridge fully covered with snow. A few well insulated homes also showed (from the front) snow over the ridge vent. I may try to get some IR pictures of a few homes to see if it provides more details. I will also use the IR on some multiple height homes to see which way the air is flowing.


We're talking about the surface same area as two 20x20 HVAC filters. 360CFM through a 20x20 HVAC filter will give a higher static pressure than 1 Pascal. Ridge vent filters aren't made of some magical material, light passes through them at about the same rate as a standard HVAC filter.

The filter may be open when new, but flow 720CFM through a 720sqin air filter and it will quickly become dirty. Just think of a HVAC unit with continuous fan for 90 days. Now you expose this to dusty attic air. Have you ever seen the screens clog on soffit vents? IMHO if the dust can clog those relatively coarse vents it will clog a HVAC filter that much quicker.

Net free area claimed by the manufacture does not take into account the increased static pressure created by "equivalent duct length" of the sharp turn the air has to make at the ridge and the baffles.

However all that being said I don't have a thermal imaging gun, I'd like to see the results. Shot a couple of wind turbines for me while you're at it ;) Maybe even throw and electric attic fan in just for good measure. I'd love to find an unbiased test of all the attic ventilation methods commonly used.

Here's an article on ridge vents from "ask the builder"

I'm not trying to stir the flames, but it is quicker if I just spit these out.
Bob <360CFM through a 20x20 HVAC filter will give a higher static pressure than 1 Pascal.> One pascal looks about right by my math. What equations are you using?

I've never seen one filled with dirt and I have never read about any concerns that should be addressed. Your source?

The attic air is only dusty after we disturb its contents. Shortly thereafter it is simply outside air and in my end of the country, very little dust.

From your link "Anyone who paid attention in their high-school physics class knows that hot air rises and absolutely doesn't want to go down"
LOL, you know that is one of my hot buttons and Tim needs to join us and learn about why hot air goes up, or down. In the case of the ridge vent, there is no magic, there is simply a bit more pressure on the inside than on the outside. Up or down doesn't matter to hot air it will just go the direction the pressures or gravity are pushing it. Yes there would be some resistance to the changes in direction, but only a fraction of the moving force. The moving force continues in the exit direction. The resistance (▲p) of the twists and turns is always less than the moving force.

I didn't spot any turbines or recognize any powered vents, but most would be on the backside anyway. There are a few neighborhoods where I can see the backs so I will try. I ran into some concerns from my local law when I notified them I would be out IR snooping several years ago. They didn't say no, but cautioned me that home owners might take exception. Maybe I need a "get away driver" so I can sit in the back seat.

The bottom line is that static venting works, despite the extremely low pressures. I'm sure wind helps but can only judge from my region where the wind always seems to be moving.


About 250 pascals in 1" of water column. HVAC filter operation at a typical 0.1" @ 125FPM So we're at 25 pascals, not 1. Huge difference when talking about relying on air to simply "float out" of a ridge vent. I have a hard time believing if you were to put 40' of ridge vent against a blower moving 720CFM that the static pressure would only be 1 pascal or 1/250 of an inch. 0.1" sounds much more realistic, but it will never happen on natural convention alone.

Here's another video:|-Cooling-a-hot-attic-space.html

Yes there are cobwebs near the ridge, telling me at least some air has been going through the vent at some point since it's installation. Could have been on a windy day when the filter was clean. A typical attic fan pulls 1400CFM, 720CFM passively floating out of an attic through an air filter I don't see happening. If the ridge vent were that open you would see some air being pulled through it in the video.

Agreed, Tim does have some learning to do about neutral pressure planes. However I don't doubt his smoke test. IMHO wind is the primary mover of air through the attic, which in Oklahoma we have no shortage of. Wind will even come OUT of the soffit vents on the "back side" of the building. The ideology of air coming evenly through the soffits and gently floating out the top of the attic is another thing I've questioned. IMHO the wind is so much stronger of an influence of how the air actually flows in an attic since the pressures are much greater.

Then there is the question of which brand/type of ridge vents we're talking about. To read it from manufacturers tests they all claim theirs works the best. Independent testing? Good luck finding it ;) Also notice for powered ventilators, plain static vents and roof turbines none of them tend to perform tests against other brands. It's like they are OK with their products published specs, and one brand doesn't deliver more than another. The only possible exception are the solar attic fans which claim ridiculous CFM ratings out of tiny motors. Is there a particular brand/style of ridge vent you are considering?

I will try to be short again. That link did not come up for me.
Agreed and that is one of my interests. If I can show that static ventilation works at a very minimal level, everything else is better.
Tim's test did not give us the ▲t. Yes, hot out, but probably hot in the attic as well. A 30 degree delta t and a 6' attic would give us 1.26 pascals which then gets divided high and low resulting in just 0.63 pascals trying to push out that ridge vent. Not entirely the vents fault, no pressure. He also didn't describe the low venting if memory serves me.

I haven't selected a ridge vent as yet and as a temporary fix over the winter I installed a single roof vent (61 in² NFA) right in my trouble spot. That with a couple of other fixes should get rid of most of my ice problems. Comes spring time I will look to see what they are installing and decide. On-line I looked at DCI primarily because they have a single sided option and I have a low slope (3.5 x 12) on one side.

Now for the math. I'll copy and paste without further comment for now.

I went looking for a simple equation for air flow and found this:

The equation I came up with from a HVAC type forum somewhere was:
Q = 2610 x A x (square root of the pressure, ▲p)
Q = air flow in CFM
A = area in ft²
▲p = the pressure across the opening in inches water column


When I converted this to inches and pascals I got (roughly) this:

Q = A x √(▲p)
The constant became almost one
Q = is in CFM
A = is in²
▲p = is in pascals

If I use their equation with a 1 ft² opening I get:
Q = 2610 x (1) x √▲p
Solving for ▲p I get:
▲p = [125/ (2610 x 1)]²
▲p = 0.0023" wc, or multiply by 250 and get 0.57 pascals.

Corrections welcome

.57 pascals is a low pressure, not enough to overcome the resistance of the filter at any significant airflow rate. Oddly the .57 pascals is enough to operate a wind turbine, even with no wind. A wind turbine must have an extremely low resistance for this to work.

Seeing as I live in a climate where cooling is the primary reason for attic venting, I will say it takes LOTS of airflow to significantly reduce the high attic temperatures caused by solar gain. Passive solutions simply aren't effective, you need something that uses electrical power or can take advantage of the wind. In winter ice dams are primarily caused by heat leaking from the structure, not solar gain, not much airflow required. 50CFM or so provided by 40' of ridge vent may be enough to do the job. You're just trying to prevent ice dams, not reduce summer heat gain, a simple static vent the size of an electric attic fan will do the same job. Happens all the time, fan doesn't run in winter, yet no ice dams form.

Short circuit myth

Ridge vent and attic fan

I've seen the first link on the "short circuit myth" and just read the other, I don't really need the video. The real shame here is that both conclusions are correct, but based upon poor deduction. But why haven't the experts in this field stepped in and added the corrections?

Maybe another thread sometime and we can go over the real reasons both conclusions are correct.

In my cold country I have to deal with a wide range of attics. Some have little or no insulation and leak like mad, while others are super insulated and well sealed. One of the interesting areas involves the older homes that have never had ventilation or insulation for that matter. Yet one cannot run in and just convert to well insulated and sealed. See the roof structure was never designed to hold snow, they just melted it off as fast as it came down. now the buildings are antiques and energy costs 100 time what they were when these were built. Hard to save them.

Got to call it a night.


Since you are primarily interested in reducing ice dams, a new article you might want to see:

Ice Dams


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