Too much roof/attic ventilation?

[Joshua L. Frederick]

Ridge vent and roof vents combined with soffit vents. Thoughts & comments appreciated.

[Joe Funderburk, CMI]

I don't think "too much" is a concern.

[mnahrgang]

I agree, I don't think too much is an issue. What may be an issue is that the different types of vents will be working against one another...

Soffit vents are to let air in, ridge vents let it out. If the roof vents let the soffit air out, then all the air from the roof vents to the ridge could stagnate, heat up, and cause the shingles to fail. The ridge vent will only work if there is some other air to replace what is leaving. I'm not saying this is happening, only that it's a possibility. If there are gable vents too, then the fresh air could come from them for the peak.

It's all about the "design" and the building science behind it. If you install two things that are supposed to work together, and then add a third that isn't. That third is just going to get in the way...

[James H. Bushart]

I have read one study (provided at a seminar two years ago) that shows that the additional roof vents can reduce ventilation of the total attic space.

Air moving over the ridge vent draws air from the attic. The added vents a few feet away from the ridge can be providing the replacement air instead of or more than the soffit vent, as originally intended. Or, the air that the added vents are actually drawing is coming in through the ridge. In any event, under these conditions there is insufficient ventilation in the total attic space.

Look, for signs of moisture/damage in the bottom third area.

[David A. Andersen]

Did you do a smoke test and determine air flow?

As Mark said, it may not work as intended.
If You want to talk about it, you need to gather the required data.

What does the SOP state? (if I told you, you'd never read it).

[Joe Funderburk, CMI]

I personally wouldn't reference building science with regard to attic air flow in this case unless I was truly knowledgeable about it. You may open the worm can.

[Brian A. MacNeish]

Originally Posted by jfunderburk

I personally wouldn't reference building science with regard to attic air flow in this case unless I was truly knowledgeable about it. You may open the worm can.

If you were to contact Dr. Bill Rose (Phd building scientist at U. of Illinois), he would tell you you can draw all the arrows you want on diagrams about "theoretical" or "designed" natural air flow, but ensuring that the air flows that way is not guaranteed. He's been studying roofing, venting, insulation, condensation for years.

To quote him from a 1992 Journal of Light Construction article, The Science of Venting: "In real attics, however, the airflow rarely follows the arrows."

[Chuck Evans]

Manufacturer (i.e., AirVent) recommends not mixing static and ridge vents on the same roof as it can shortcut the designed flow from soffit to ridge.

Personally, I think you tend to get flow from windward to leeward, but I have never done my own independent testing, so I go with what the manufacturer states.

[Marcel Gratton]

Below copied from: http://www.ventilation-maximum.com/English/faq.html

Question: The building I want to ventilate is 2000 sq. ft. The ventilator, model #301 can ventilate up to 1200 sq. ft., and the model #303 can ventilate up to 800 sq. ft. Can I install one model #301 and one model #303 on the same roof?

Answer: No, you must always use the same model of ventilator, otherwise the stronger ventilator will draw from the weakest, therefore short circuiting the system. This is why we recommend that you block off or remove all other vents on the roof.

Food for thoughts!

[Brian A. MacNeish]

Originally Posted by mgratton

Below copied from: http://www.ventilation-maximum.com/English/faq.html

Question: The building I want to ventilate is 2000 sq. ft. The ventilator, model #301 can ventilate up to 1200 sq. ft., and the model #303 can ventilate up to 800 sq. ft. Can I install one model #301 and one model #303 on the same roof?

Answer: No, you must always use the same model of ventilator, otherwise the stronger ventilator will draw from the weakest, therefore short circuiting the system. This is why we recommend that you block off or remove all other vents on the roof.

Food for thoughts!

From the same site:

Question: Would your ventilator not draw the heat out of my house having such a powerful drawing power?
Answer: No, if the house in question is well insulated per standards, has a proper vapour barrier and the soffits are unobstructed and balanced or exceeds the opening of the exhaust vents, this shall never be an issue.

Notice they never mention that the attic should be firstly airsealed to prevent house air containing moisture and heat (that you paid for) from exiting the house to the attic. One of the biggest items the energy audit programs are promoting is airsealing for obvious reasons.......(1) stop warm air from leaving at the ceiling level and (2) prevent house moisture from getting into the attic where it may condense.

They wouldn't want to stop this moisture from getting into the attic.......then they'd have less of a reason to sell their vents......there'd be little or no attic condensation. From the Journal of Light Construction article The Science of Venting by Dr. Bill Rose: "The best protection against condensation and mildew problems in the attic is an airtight ceiling plane- one that allows no air leaks from the attic below."

[Marcel Gratton]

Originally Posted by Brian A. MacNeish

"The best protection against condensation and mildew problems in the attic is an airtight ceiling plane- one that allows no air leaks from the attic below."

I agree with above statement but that is practicaly impossible to achieve. Newer homes are build better but they still leaks.

[David P. Valley]

http://www.masscertifiedhomeinspections.com/?D=116

[Brian A. MacNeish]

Originally Posted by mgratton

I agree with above statement but that is practicaly impossible to achieve. Newer homes are build better but they still leaks.

Yes, it's virtually impossible to achieve a perfect airtight house. The tightest home our company ever tested had .25 ach @ 50 pascals pressure differential......but that is extremely tight. The R2000 requirement in Canada is 1.5 ach or less @ 50 pascals.

When homes are that tight they have air exchangers or HRV's to control the interior RH of the home so any air leaking out is relatively dry and the amount is so little so as not to cause a problem. It was the "wet" houses (wet crawlspaces/basements-in effect a 24/7/365 humidifier; poor or non-existent interior house venting, lots of plants/aquariums; indoor clothes drying/unvented dryers) with lots of air leaks to the attic that brought on the need for so much venting.

Now we know the underlying causes/moisture paths/building science and should be able to provide other options to begin to solve the problem other than only "vent the h ell out of the roof".