Airflow dynamics over house and effect on roof vents

[dmcroof]

I have been trying to find some information or study concerning a widely held idea concerning attic ventilation.
Conventional wisdom states that when air passes over the ridge vent on the peak of a house, the resulting low pressure will draw air from the attic. The air that is removed from the attic will be replaced by air drawn into the attic through vents at the roof eaves or lower edge.
If there is another roof vent or gable end vent near the ridge vent, air will be pulled into the attic from this upper vent instead of the lower eave vent.
I have not been able to find any evidence to support this and my common sense leads me to believe otherwise.
So I made a simple model to illustrate.

Could you please take a look at the following video and let me know what you think.
Why is it that the flow of air is exiting from the end vent instead of being drawn into the attic space? There is a mild breeze supplied by a forced air furnace simulating a makeshift wind tunnel.

https://www.youtube.com/watch?v=<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/kpHah-x39GI&hl=en&fs=1"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/kpHah-x39GI&hl=en&fs=1" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>

Thank you

 

[LURCH]

There's something I can't quite tell by looking. Deos the roof model have ends?

Also, can you repeat the experiment using dry ice to replace the (cigeret?) smoke. The smoke may have just been exting every opening near the top of the roof because the smoke was much warmer than the sarounding air, and rising wherever it could. Not saying this is the case, just suggesting a bug that could scew your results, and can be easily tested for.

Of course, since the air inside an attic is often hotter than the air outside, your experiment might be a more realistic depiction of what actually happens in a real attic. BTW, why didn't you just use a real attic on a windy day? Just find one with gable vents and put some paper fringes by both sets of vents (the ridge and gable vents), and see which direction they point. Nobody could say that's not "real-world."

 

[dmcroof]

Hi Lurch, thanks for the reply.

The roof model is closed on both ends with small cut outs to simulate gable vents. A sealed attic floor is below. There are openings at either eave/overhang to simulate soffit vents.

The smoke is incense. I used that to simulate the hot air that is in the attic space which we are trying to exhaust.

Timing and other factors make it hard to do real world. And hard to control conditions.

I'm just trying to figure out why the smoke/hot air is exiting the end vents when it is supposed to be drawn into the attic by the ridge vents.

 

[LURCH]

Could you re-film using the incense outside the structure, so we can see the entire path? I'd like to see if it is going in at the soffits.

(Oh, and a little different angle on the lighting, please.)

 

[dmcroof]

Already set up for a different model, but it shows the path of the smoke.

There is air going in at the soffits. Otherwise no air would be coming out the top.

The lighting is probably the hardest part of the whole darn experiment!

https://www.youtube.com/watch?v=<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/hjHl4T-R7Os&hl=en&fs=1"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/hjHl4T-R7Os&hl=en&fs=1" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>

 

[LURCH]

Yep, that's definitely going in the soffits, and coming out at the ridge vents and the end-vents. Almost seems like soemone should be saying "myth busted," or something.

Possible explanation; perhaps it is because the soffit intakes are so much larger than the ridge vents and the gable vents, that they provide a path of least resistance for air to come in, allowing enough airflow to sustain the exhaust from all other vents. The way to test this, I should think, would be to progressively block off more and more of the soffit vents, and see if you still get exhaust from the gables, or do they become intakes. (just a suggestion)

This might give you the "why," but it seems like your previous expiriment has already told you "if".

 

[FredGarvin]

I have finally been able to view the videos (I can't see them at work).

While I like what you have done there are a few questions I have about your set up and execution:

- What external speed is the air moving? Did you take into account Reynolds effects into your calculation of the required speed?

- Did you vary the wind speed at all to see if different results were obtained?

- What is the inside of the model like? Is it fabricated like a real house or is it an empty shell?

- Did you scale the roof penetrations/vents to the size of the house?

In the end, you may be right under some circumstances and wrong in others. I am sure that there are many myths that get passed along without being questioned. It happens in engineering as well.

This could be tested on a real house easily enough. Just use some light tissue paper an tape it along the top edge over a gable vent to see if there is indeed a low pressure inside drawing air in or higher pressure pushing out.

 

[dmcroof]

Thanks for the replys.

Lurch, I agree with your suggestion about closing the soffit vents. Sooner of later either the gable or ridge vent will turn into intake instead of exhaust.

Unfortunately, I don't have the resources, (measuring instruments, facilities, etc) or the smarts(Reynolds and coefficients give me headaches) to do much in the way of thorough scientific testing.
And, there are just too many variables in the real world, from building methods to climate and terrain, to try and be precise.

I guess my main goal is to prove that the wind induced low pressure affecting the ridge vent is also affecting the gable vents(as well as the downwind soffit vents). As long as there is sufficient intake at the soffits, both ridge and gable vents will act as exhaust vents.

The problem is that many roofing contractors and others have been "marketed" into believing that the low pressure exists only at the ridge vents.

Do either of you know of publicly available resources, cfd or aerodynamic studies, that could help me with more information.

Thanks

 

[FredGarvin]

The one thing I am thinking that should help with the standard flow idea is that there is no natural convection inside your model. A real house will have a convective flow due to the heat rising inside the home. Whether that is a deal breaker here or not, I don't know. It is however a variance from reality in your experiment.

It would be a good thing to see if you can vary the wind speed to see if higher velocity winds produce different results.