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