Air flow - roof and attic ventilation

[dmcroof]

Hi all, I need to know if I'm on the right track with my thinking. I've drawn a diagram that hopefully is not too simplistic and shows what I am getting at. One of these links should work.

http://www.roofersreview.com/d/9390-2/Hip+roof+diagram+calc_+copy.jpg

http://www.roofersreview.com/v/Vent...n+Basics/Hip+roof+diagram+calc_+copy.jpg.html

The question is, are my notes/conclusions on the diagram right? Is there any reason to think that the upper vent (a) would draw air from the lower vent (b).

Are there any other threads / discussions on this subject? I did not see a search option for the forum.
Is there a good resource for learning more on this?

Thanks

 

[Averagesupernova]

The diagram that shows the air intake on the soffit and exhaust at the peak is the best system of the two in my opinion. As air in the attic is warmed it rises to the top and exhausts through the top vent. Cooler air is drawn in on the soffit vents. This is why vented aluminum soffit is readily available. One disadvantage I can think of with the soffit vent is that when you insulate the attic floor, there is a buildup of insulation against the roof out at the edges. This blocks off air movement. The usual remedy is to build a 'scoop' into the underside of the roof that forms a duct that allows air from the soffit to travel up freely into the attic. It is an extra step in construction. This is done in multiple places in a building. Typically the approach is: The more air movement in an attic, the better. Too little air movement causes condensation in the winter and overheating in the summer. The life of the shingles is reduced by excessive heat.

 

[dmcroof]

Thanks for the reply

But I am more interested in the physics and equations. And the question, will the upper vent draw air from the lower vent.

 

[FredGarvin]

I really do not understand your notations. Let's think of it in the most simple of terms; Air, like electricity, is going to follow the path of least resistance. Even though there is a very slight difference in elevation between the two peaks, I would not think that difference to really equate to a change in head required for the same flow. That being said, the two main items driving the flow would be the areas of the individual soffit vents and the areas of the ridge vents. Assuming they are equal, then the two will have very close to the same flow.

There are a bunch of things that can complicate this analysis. Most of which are the items that will add resistance to the flow. Items like rafter density, attic baffles,, insulation, gable vents, etc...

 

[dmcroof]

Thanks Fred,

I think you understand what I'm getting at, but I really don't know the best way to make the notations.

I had read in a ventilation manufacturers specifications that the upper vents should not be installed as in the diagram. The idea being that there would be stronger low pressure area at the upper vent created by a higher wind speed at the higher altitude. This stronger low pressure would draw air from the lower vent and not the attic or soffit vents.
I figure that the air resistance at the lower vent (the air being drawn out by the slightly less low pressure) would not allow that. Instead all the air would be drawn from the attic and soffits. As long as the soffit vents were equal or greater in area to the upper vents.
And we're not dealing with any other variables.

I'm just not sure how to explain that in the diagram.

 

[russ_watters]

And the question, will the upper vent draw air from the lower vent.

It might, but I doubt it.. I guess I'd say if anything, the lower vent may be superfluous, but I don't think it can hurt.

 

[Miguel Gonzal]

soffit intake vents

I needed an opinion -

I have 3 different options for intake vents for an attic -all 3 are 2" wide and 8' long the difference is thickness. All 3 vents have the same net free area of 9 nfa per sq. ft.

In your opinion do all 3 have the same airflow or is there resistence because of difference in thickness?

1. is 1/8" thick

2. is 1/4" thick

3. is 7/16 thick

 

[FredGarvin]

At the flows you should be seeing, which are pretty low, the difference in thickness should be pretty negligible. The only way I see the thickness as an issue would be if the thickness contributed, someway, in deflecting the flow. The flow is a function of the opening's cross sectional area.

 

[LURCH]

Yes, the differential in windspeeds will cause the upper vent to exhaust air from the attick, which will cause air to be drawn in through the lower vent. This is similar to the way prairy dogs dig their tunnels, always with a mound at one end and a hole in the flat ground at the other. This makes air flow through the tunnel whenever there is any wind, so they don't all suffocate in there.

The airflow will be very slow, very low-volume, on most days, but it should be enough to keep your prairy-dogs from suffocating if you store them in the attick.

 

[Miguel Gonzal]

Haha - thanks