Way More Intake To Supplement Minimal Exhaust Ventilation?

Way More Intake To Supplement Minimal Exhaust Ventilation? NOTE: If anything, can anyone assist me personally with the two requests that are in the Bolded Font? Regarding the Short Circuiting, I am not looking for manufacturers who just plain and simply state that it occurs. I want studies that prove that it happens and how the physics work, if possible. Thanks. Ed

This home owner is NOT a customer of mine, although I did submit a proposal on the project. So, there are no legal concerns about the advice you could share.

Lets disregard the typical spiel for the time being, attempting to obtain a properly balanced ventilation system. I have a home owner on one of the forums I provide advice on, who is now concerned about frost on the bottom side of his newly installed plywood deck sheathing. He had most of his decking replaced 6 months ago, due to mold issues collecting on the bottom of the decking.

It is a Hip roof with about 1200 square feet of attic floor space and about 9 feet of ridge length with an approximate 8/12 pitch roof.

Last fall, I provided advice to him to install 100% continuous soffit ventilation along with the Shingle Vent II Ridge Vent. In lieu of the 3" continuous strip soffit vent under the entire perimeter, I suggested a shingle over style intake vent, which at that time, was the Smart Vent.

I know the intake and exhaust would not be balanced, but the TOTAL NFVA for the attic would have met or exceeded his requirements, even considering the 1/150 ventilation formula. I advised him that due to not having a balanced ventilation system and also not vapor barrier, which has since been rectified, that he should not consider only meeting the 1/300 formula.

He changed from 12 of 4" x 16" to 12 of 8" x 16" soffit vents, which I precluded to still not offer enough intake ventilation. I also advised, that due to the 8/12 pitch, he actually further needed to upgrade the percentage of Total NFVA by at least 20% due to the additional attic volume of cubic feet of contained air.

My current advise, per comments by William Rose regarding soffit ventilation, is that he should still consider adding to the soffit ventilation to increase the total NFVA and that seems to be getting shot down.

Does ANYBODY have any reports and studies that I could obtain that studies the amount of air going IN AND OUT of soffit vents?

Per Rose, of Rose and Tenewold and from the BRC, the Building Research Council at the University of Illinois, he claims that soffit ventilation alone would be his preference, if he had to choose intake versus exhaust venting.

Here is the link to one of the articles purporting Rose’s opinion regarding the significance of Intake Ventilation.


Below, is the pertinent excerpt from that page linked to, which comes just below the paragraph labeled Soffit Vents and the building attic diagram.

Soffit Vents

Rose considers soffit vents to be very important.

“If a roof had only one type of vent device, I would choose soffit vents,” he says, “because they work well as inlets and outlets.”

There’s less problem with rain and snow getting in, because soffit vents point downward. To get maximum protection, locate soffit vents as far out from the wall as possible. If rain or snow blow into the soffit, it’s less likely to soak the insulation or drywall.

Soffit vents should always be installed whenever there are high vents. High vents, on ridges or gables, will pull air out of the attic. Without soffit vents, make-up air would be drawn through the ceiling, which increases heat loss and adds moisture to the attic.

Because of the importance of soffit vents, Rose believes that at least 50 percent of the vent area should be low on the roof.

Also, does anyone have any research or engineering studies that would substantiate the “Attic Ventilation Short Circuiting” claim, which some people, or at least 2 very studious debaters defy that logic, since there are no published actual scientific data to back that theory up.

I would LOVE to get my hands on those studies.



For clarity’s sake, someone on the Building Envelope Forum at JLC Forums requested a better wording of what I was seeking, so here goes, in case I lost anyone in the translation during that first post…

Okay…Attic Ventilation Short Circuiting

Myth Or Reality?

Their seems to be a common sense logic regarding this “Short Circuiting Theory” and this is one of the fundamental principles I consider when specifying ventilation solutions.

I have searched and searched and found NO actual studies regarding the concept of short circuiting the attic ventilation systems, although, I have found user documentation and also personally experienced the physics that have shown reverse air flow when opposing ventilation systems are installed, creating a suction portal for the exterior weather elements. But, that is just individual and possibly anecdotal evidence, not supported by the Scientific Method.

I even communicate regularly with Paul Scelci, from Air Vent Corporation, who is an engineer and the main administrator of the Attic Ventilation Seminars, and requested the research way over a year ago, with no supporting documentation returned back to me for analysis. He stated that they have some studies, but has never provided them to support that hypothesis.

So, where is the research that documents this?

Does anyone have a link to studies that prove this beyond a reasonable doubt, even to a devoted naysayer?

That was the first point.

Secondly…Total NFVA Requirements?

Most building codes I have run across specifically outline that attic ventilation must meet Minimum FHA Guidelines.

That brings you to the formula to calculate either the lesser 1/300 or the greater 1/150 formulas.

You need to upgrade the specifications to the higher 1/150 standard, if certain criteria is not met.

A. If there is no vapor barrier on the warm side of the attic insulation.

B. If the system, is not a balanced ventilation scenario.

Therefor, by mathematical calculations, all of the ventilation can be achieved for the Total NFVA Requirement utilizing just one or the other ventilation system, either all intake ventilation or all exhaust ventilation.

Now, time to Quote William Rose again:

Soffit Vents

Rose considers soffit vents to be very important.

"If a roof had only one type of vent device, I would choose soffit vents," he says,"because they work well as inlets and outlets."

With the aforementioned math calculations being done and determining that the Minimum Required NFVA Requirements can be achieved by Intake Ventilation Alone, specifically continuous soffit vents, so that my thought process can see and physically understand how wind pressures against the windward wall from any direction, could cause the opposing sides intake vents to act as exhaust vents, …

Are there any studies that show this to be true, rather than just an innocuous statement made by William Rose as an off the cuff comment in the previously quoted reference?

How can proof be met to show that the mathematically calculated proper 1/150 amount of JUST Intake Ventilation be shown to be a fact, both in physics and in actual studies that report on this phenomena.

I sure hope that was good enough English to get some replies and results, because I spent a lot of time being more specific and hopefully in proper layman’s terms.