Attic ventilation with mixed soffit heights.

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

Replies to This Discussion

Permalink Reply by Guy DuBois on March 18, 2013 at 8:55am

I'm going to take a stab at this from the perspective being climate zone 4; mixed humid. There are not many days in the summer here where you can leave the windows open and hope to stay somewhat cool. Point is, we rely on conditioned air most of the year. The best way I have seen to "fix" these Capes with and without shed or duspan details is by totally encapsulating the roof deck and all ambient air facing surfaces with spray foam. Forget about exterior ventilation. This has to be done by a competent installer and all other existing above ceiling insulation must be removed. For proper installation, new access openings may have to be made in ceilings, kneewall and dormer sides. Usually the slopes can be foamed from the ceiling flats and via the knewall interiors.

After installation, anything below the roof is conditioned space. Humidity levels should be monitored to insure that the HVAC system is properly configured for moisture control. This is the same prescription for FROGs; Finished Room Over Garage. (You can use that but you probably heard it here first!)

After this I have never heard a homeowner comment anything other than the comfort and utility saving improvements being "Dramatic".

Permalink Reply by Bud Poll on March 18, 2013 at 10:00am

Hi Guy,
Although your approach would certainly solve the problem of sizing the vents, they all become zero, it doesn't answer the question of balancing high, medium, and low vent areas when needed, and unfortunately, we still need to improve a few million many homes out there and stay within the home owners limited budget.

I view the hot roof approach as the default, when all else fails. What I was hoping for was some advice from the research oriented people who might have tested various combinations and concluded some workable balance, but nothing so far.

Thanks for commenting
Bud

Permalink Reply by Guy DuBois on March 18, 2013 at 10:46am

I got nothing either Bud. Very tough problem with no clear answer.

Permalink Reply by tedkidd on March 19, 2013 at 12:54pm

Seems to me ventilating such a component, in the attempt to keep it cool with all the thermal bridging, etc, is a bit of a two steps forward three steps backward approach. It's putting a lot of effort into keeping a surface intimately connected to your living space cool during the heating season, and sacrificing air sealing and insulation opportunity to do it.

And you'll have a crappy component that will have to be addressed later if you do decide to build up the roof and fix it right.

You might polish a turd, but keep it away from water! All this effort to ventilate heat away, what's the upside? Is this an attempt to break all the rules and claim victory?

Anybody comfortable making this type of recommendation to a client? Sometimes getting to the ideal takes time (money not currently available), but shouldn't we try to avoid recommendations that move away from the ideal if possible?

Maybe it would be less energy intensive to simply dense pack, air seal, leave the gables open for moisture mitigation, and use heat strips. This means no backpedaling later. I guess it depends upon the climate, but, how much would you even need to run the strips? Just when it snows, right?

Permalink Reply by Bud Poll on March 19, 2013 at 4:08pm

Sorry ted, I have no idea why you think as you do, but you are drifting and I do not want to follow. Please avoid the poo and discuss how to assign vent areas to low, medium and high locations as the thread is asking. If you want a thread on the merits of eliminating all ventilation, start your own.

Bud

Permalink Reply by Bud Poll on April 16, 2013 at 12:24am

I've made some progress on how to assign the vent areas to achieve the air flow direction we want. There will eventually be some associated math, but for now the basic logic will at least provide some guidance, and note, that guidance is opposite other very notable advice.

In a very simple arrangement, a low soffit vent area, a high ridge vent, and one mid height soffit which we want, for now, to act as an intake vent. This pretty much matches the diagrams at the start. If you consider just the high and low vent areas, not mid height opening for the moment, there will be a Neutral Pressure Plane (NPP) somewhere inbetween those levels, just like a house. If we make the high vent area larger than the low, that NPP will move towards the top and hopefully above the mid height soffit. Once it is above the mid height soffit, we can assign whatever vent area we need to the mid level and the NPP will remain above the mid soffit allowing it to act as an intake.

Where I'm headed will be an equation or process where you input all vent locations and you obtain corresponding vent areas that will result in all areas acting as they were intended, some are intake and some are exhaust. The wind will of course shift this around at times, but as a starting point, air will at least flow in the right direction.

An interesting observation from selecting some common vent areas and locations is, many homes with this mid soffit situation are probably experiencing poor ventilation or a reverse air flow through that mid level. The notable advice to always make the low vent area the larger does not perform well and in many cases will result in poor venting. Pray for wind.

Bud

Permalink Reply by Bob Blanchette on April 16, 2013 at 3:58am

I've found even a mild wind of 5MPH blows all the NPP theories straight out the window. Structure your vents to work with the prevailing wind rather than NPP if you live a windy climate.

Permalink Reply by Bud Poll on April 16, 2013 at 5:46am

Hi Bob, I agree that there are other factors that must be considered when deciding upon how much and where our venting should be, but natural air flow is still part of the equation and the NPP is important, especially when the standard advice is wrong for mid height vents. That's unless you want those soffits to be exhaust vents. I'm in cold country with potential ice problems and I need those mid soffits to be intake vents. And when I look at the inventory of homes in my region, I'm far from alone.

Bud

Permalink Reply by Bob Blanchette on April 16, 2013 at 7:23pm

In our area attic ventilation is primarily about not overheating the attic, ice dams aren't an issue for us. There is almost always a breeze of at least 5MPH, south in summer, north in winter. If the wind shifts in the day the temperature can drop 40 degrees in 1 hour.

Permalink Reply by Bud Poll on April 17, 2013 at 2:31am

Bob, what do you use for guidelines for ventilation, 1/300 with 40% high and 60% low? Where do you want the air to enter and exit? I would assume some of your buildings, new and old, have soffits located all over the place, so given your preference for a wind driven design, what guidelines would you suggest?

Bud

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