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FatAugie 08-03-2007 06:05 PM

Attic Ventilation - importance of balance
I have a roof, 1100 sq ft. that I currently have 4 plastic boxes and 32 linear feet of vented soffit panels. I'm going to add about 40 ft of ridge vent which will boost the exhaust to 720. Call it 700 sq in for round numbers.

Now, I should have a total of at least 700 sq inches coming in at the soffits, correct? I'm assuming 350 sq inches from the front, and the same from the back, right?

If I need 700, I have a total of 240 or so coming in now. I really like the Smart Vent product for a number of reasons. So if I run the full length of the house on both sides, I'll have about 700 incoming from SV, what about the soffit? that makes a total of 940 incoming. That's now 25% more inflow than exhaust. I'm out of balance.

Ed, you say you'd rather have more coming in than exiting, but what about balancing the system? I thought that balance was what we should strive for to get maximum chimney effect.

Also, would having soffit vent below the slot for the Smart Vent allow for short circuiting similar to having gable vents with a ridge vent?

The soffit now is clear, wide open to the attic with no obstructions, especially in the front of the house. If it was stronger, I could crawl from the attic to the soffit the opening is that big. The back is more normal, but there are baffles in place and it looks like a clear shot for air movement.


(Ed, I'm not trying to be dense, just trying to clarify some things. I know you've been very patient answering my questions and I appreciate it.)

Ed the Roofer 08-03-2007 08:53 PM

The short circuiting aspect could not occur on the lower end of the roof if there were sufficient ventilation inlets and exhausts.

Many studies I have read point out that the intake ventilation is even more significant than the exhaust aspect. The intake vents alone, if substantially provided for, can actually flush out the attic environment. This goes contrary to the "Stack Effect", but real world scenarios also do not look like ventilation diagrams we see all too often.

I do believe in the proper amount of exhaust ventilation though, but most accounts state that the Intake should be at least 60 % of the ventilation equation.

Additionally, the steeper the roof and attic assembly are, the more ventilation is needed on a cubic foot method of calculations. This would require in most cases, at least an additional 20 % more total attic ventilation than what can be feasably achieved.

Where else is the additional ventilation going to come from?

Power Attic Ventilator? NO. It would compromise the ridge or static exhaust vents. Also, it pulls out the interior conditioned climate into the attic, which increases the energy load on the home and costs significantly more funds.

Alternative additional exhaust ventilation products? NO. Then the products would short circuit themselves and be self defeating.

Add to the Intake ventilation? YES. What would be the harm? Unless there is opposing pressurization in the attic, such as being caused from a powered attic ventilator, their is no NORMAL conditions which I have come across which would allow the intake aspect to malfunction. I can not account for extremely high wind, such as mountainous or coastal like conditions, as I have no first hand experience with those conditions.


FatAugie 08-04-2007 05:08 AM

OK, that I'll buy. What you're saying is, by having more intake, you create a sort of positive pressurization in the attic and help promote the airflow that way, correct? I know with HVAC in commercial buildings, they try to make sure the building has a positive pressurization to prevent the building sucking in outside air from around doors and windows.

Thanks, the explanation sounds reasonable.

Ed the Roofer 08-04-2007 08:31 AM

I'll have to defer to you regarding which type of pressurization I was referring to. I can't recall if it is positive or negative???

If there is not enough exhaust ventilation provided for and the under soffit fresh air intake vents are place too close to the exterior, I have heard and seen examples of the intake vents actually sucking in the exterior elements, such as a fresh snow fall.

Wierd, huh?


FatAugie 08-04-2007 09:28 AM

An extreme example is a inflatable domed stadium like the one at Syracuse University. When you open the door, you get a 20 mph breeze in the face, you have to lean into it to walk in (ok, I'm exaggerating a bit). That would be positive pressurization. Without it, the roof would sag. Imagine how much air they have to blow into the place to keep the kinds of snowfall Syracuse gets from collapsing the roof.

Ed the Roofer 08-04-2007 10:38 AM

I have been to the Minneapolis Metrodome and the Pontiac Silverdome for Bears games, so I know what you are talking about.


AaronB 08-04-2007 10:41 AM

Negative is when the building's airflow systems are sucking air in.

Ed the Roofer 08-04-2007 10:57 AM

That is a very simple and understandable explanation Aaron. Thanks, even I get it now.


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