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."

http://www.greenbuildingadvisor.com/blogs/dept/building-science/lst...

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?

Bud

Tags: attic, venting

Views: 1643

Replies to This Discussion

This is a diagram of Bud's current issue (as I recall 3.5/12 on back & 9/12 up front)

I am kinda of surprised no one else jumped in, so here is my .02 from a separate convo I had with Bud.

The cheapest quick fix to help ensure air flow out through the gables & eliminate ice dam issues (seeing it is wintertime) is to place a cheap box fan in the attic & direct it towards a gable vent. For thermostat control you could consider using one for heat tapes - just make sure the thermostat is located outside & not inside the attic where it may not kick on properly. 

While I know Bud falls under the "it doesn't matter crowd" he should go with only a ridge vent or enlarge the gable vents & possibly adding a cupola assuming he wants a ventilated attic. The optimal fix for this (based on I never want air flowing out through a mid) is a hot roof system.

Of course the one needed fix (no matter which system) is air sealing all the penetrations between the floors & attic spaces.

First off, most ridge vents are practically worthless. 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. Whirlybird style vents will more the most air per sqin of free area than any other passive solution. You can FEEL the air blowing up through the vent, there is only one 30 degree turn (depends on roof pitch) and the spinning vent impeding airflow. While standing in the attic you can see light coming through the vent, which means air can actually get out. Wind is the biggest determining factor of airflow direction, not heat rising. If the wind is over 10 MPH air will come OUT of the soffit vents on the side of the house the wind isn't blowing into.

http://www.youtube.com/watch?v=SyIw2GTiKvQ

Open up the soffits and put a few whirlybird vents up there, you will have PLENTY of airflow.

Maybe it's just me, but I don't consider "Whirlybird style vents" as being passive and neither is wind driven ventilation.  Passive venting is the bottom of the barrel, the least effective method of moving air in and out of our attics.  But if we design it correctly it provides us with a base value of ventilation, beyond which everything else is better.

Using both gable and ridge vents, it is my intention to increase the upper vent area to a point that will shift the neutral pressure plane to a position above the rear soffits and thus allow cold air to flow into both front and rear soffits.  My reason for posting is to see if others have considered the issues of mid-height vent areas and how they balance (or unbalance) the vent areas.

Bud

Ridge Vents don't actually allow any air to go through, so the neutral pressure point is wrong anyways. If you are thinking of the low pressures involved in the hot air rising theory, is there really enough to overcome the pressure drop on he HVAC style filter? How many inches of water column does the hot air floating up produce?

Again you are presenting your opinion, where's the support?

The static pressure in an attic is calculated in the same way as stack effect in a house.  The reduced equation (close enough for discussion) is:

p (pa) = ▲t (F°) x ▲h (ft)

This is the total available pressure which sets up as a positive pressure at the upper vent and a negative pressure at the lower vent with a NPP somewhere in the middle.

I can estimate the air flow, but what we lack is how much is necessary.  The old 1/150 number seems to be unsupported, especially with today's tighter homes. 

I accept that you aren't pleased with ridge vents, so what are you proposing?  Not every home is going to install a whirlybird or two.

Besides the above problem, one of the reasons I started digging into attic venting was to exclude or support the use of power venting.  I haven't formed a final opinion/s, but I do realize now just how low the static pressures are.  Uniquely, even with these tiny pressures, the venting works.

Again, what do you propose all homes should be using for attic venting?

Bud

The tiny pressures aren't enough for the ridge venting to work, it simply doesn't. The only way a ridge vent has a possibility of doing anything is by the wind going across the top of the roof pulling a vacuum, and that's only if the ridge vent is the externally baffled type and doesn't have too much internal resistance. Of course the ridge vent needs to be perpendicular to the dominant wind direct for that to happen.

I will definitely be running a smoke test on my ridge vent when installed, but that will wait for spring.  I agree it will have minimal flow, but it is a path and it does have a pressure across it.

One observation that supports that ridge vents do work is the problems that surface when the roofer leaves them covered.  Kind of a backwards confirmation that air gets out somehow, be it wind or static pressures.  So it isn't a case that ridge vents don't work, but that their static contribution may be less than advertised or anticipated.

Glad someone else is questioning the standard practices that we all too often blindly follow.

Bud

If you must have the ridge vents installed, take a CLOSE look on how it's constructed. The airflow between designs varies, some are able to take advantage of the wind more than others. None of the ridge vents will provide significant airflow based on neutral pressure plane theory alone..

Note, I forgot the constant in the pressure equation.  Should be:

p (pa) = 0.007 x ▲t (F°) x ▲h (ft)

Bud

According to your formula, how hot does it have to be in a standard 8' tall attic in order to develop a static pressure of .05" across a ridge vent?

How much static pressure does the filter, 120 degree turn, and internal baffling of a typical ridge vent create and an what airflow rate?

Point is, if the ridge vent is working the filter will get dirty. Once the filter is dirty, the ridge vent is rendered worthless. It's a gradual process the homeowner won't notice in most cases. the filter must be either cleaned (wont happen in most homes) or eliminated for ridge vents to be a long term viable solution.

If my conversion to pascals is correct, that would be a static pressure of 20 pa, 10 high and 10 low, which isn't going to happen, about 360 F° for a ▲t.

If a ridge vent is contributing nothing in a static environment, then how best to vent our attics in a hot southern climate?  I'm going to assume strong objections to whirlybirds as they just look like they will leak and one that is built extremely well would cost a lot more than the el-cheapos we see most often.

Thinking outside the box, we see cooling towers, is a ventilation tower out of the question?

Bud

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