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03-13-2013, 06:31 PM   #1
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Calculation Question

Is it ok to have more intake soffit vent volume than exhaust volume?

My calculations have me at 232.32 sq inches total using the 1 in 300 rule(insulated, vapour barrier attic). So assuming

116.16 sq in for intake with selected product will give me 196.8 sq in
116.16 sq in for exhaust with selected product will give me 120 sq in

This is using standard hood vents at the peak on a gable roof and aluminum basket weaved soffits on one side entire length only(can only have vented soffits on one side due to fire code restrictions).

03-13-2013, 06:39 PM   #2
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Yes, the more the merrier. Soffits get clogged up with atmospheric dirt, pollen, etc in short order.

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03-13-2013, 06:45 PM   #3
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Quote:
 Originally Posted by jagans Yes, the more the merrier. Soffits get clogged up with atmospheric dirt, pollen, etc in short order.
Awesome. Regarding orientation, since I'm restricted to having vented soffits on one side only, is there any difference if I have roof vents on the same side or the opposite?

 03-13-2013, 06:50 PM #4 Exterior Construction     Join Date: Aug 2011 Location: Washington DC Metro Area (VA, MD, DC) Posts: 16,310 Rewards Points: 5,554 Blog Entries: 5 Why are you restricted to having soffits on only one side? That is not ideal by any stretch and as jagans said, always better to have more out of balance in favor of intake volume as opposed to exhaust volume.
03-13-2013, 06:55 PM   #5
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Quote:
 Originally Posted by Windows on Wash Why are you restricted to having soffits on only one side? That is not ideal by any stretch and as jagans said, always better to have more out of balance in favor of intake volume as opposed to exhaust volume.
In 2006 there was a \$20 million fire in a new area under construction(largest ever in the city). Because of this the City had to take "action" and introduced fire codes such as:

- No vented soffits on a side that measures 4ft or less from the overhang to the property line

- Fire treated OSB or fire treated house wrap(or both, not sure) on habited dwellings

You should see the havoc this has created in construction due to "interpretation" of the code.

 03-13-2013, 08:07 PM #6 Exterior Construction     Join Date: Aug 2011 Location: Washington DC Metro Area (VA, MD, DC) Posts: 16,310 Rewards Points: 5,554 Blog Entries: 5 Wow....that is pretty flipping dumb. I would put in a vented fascia board then. Look at GAF literature. That or run hidden vented soffit and should shouldn't be able to tell.
 03-13-2013, 08:51 PM #7 Roofmaster   Join Date: Oct 2012 Location: Maryland Posts: 3,726 Rewards Points: 2,210 The Actual main reason for below deck ventilation is twofold: To prevent ice damming at the eaves, and to prevent condensation on the underside of the roof deck due to differential temperature and humidity. In other words, in the winter time, you want the temperature and humidity under the roof deck to be as close as possible to the outdoor temperature. As it gets colder out, and assuming you keep the interior temperature at around 70, and the humidity at around 50, vapor pressure increases and warm humid air flows through your insulation package more readily. With this in mind, you have to make sure you have enough insulation to restrict heat loss, and make sure you don't have openings dumping hot, relatively moist air into the attic space. Considering the fact that you are not allowed to have ventilation at the eaves on one side of your home, I would think that you would have to increase the insulation throughout, and possibly install a powered fan or two to equalize the temperature throughout the attic. Im not sure why a "during construction" fire would prompt changes in the codes where you live, because how something would burn before it is completed might be radically different from how it would burn when completed. Allowing buildings to be built very close to each other in order to collect more taxes, then telling you to block your soffits to prevent fire from jumping from building to building is kind of ridiculous, but then it is a bureaucracy, so what do you expect. This would not be the first time that what is required by code is not what is required by mother nature. It sounds like what they are requiring by code may not have taken all factors into consideration, and I hope you do not end up paying the price for someone else's shortsightedness. __________________ " Most people would rather die than think, and most do " Bertrand Russell
 03-13-2013, 09:08 PM #8 Member   Join Date: May 2008 Location: Edmonton, Alberta Canada Posts: 787 Rewards Points: 518 I appreciate your informative reply and I cannot agree more. Could I add an additional roof vent or two near the eaves to compensate?
 03-13-2013, 09:21 PM #9 Roofmaster   Join Date: Oct 2012 Location: Maryland Posts: 3,726 Rewards Points: 2,210 You can, but I don't know whether it will do any good. You want correct and even distribution of ventilation. What you are taking about is breathing through one nostril. I hate that, don't you? __________________ " Most people would rather die than think, and most do " Bertrand Russell
 03-13-2013, 11:01 PM #10 Registered User   Join Date: Mar 2009 Posts: 11,730 Rewards Points: 526 __________________ If any ads are present below my answer or words underlined/colored, I do not condone/support/use the product or services listed/linked to, they are there without my consent. 17,000 dryer fires a year, when did you last clean the inside of the dryer near motor or the exhaust ducting?
03-14-2013, 08:38 AM   #11
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Quote:
 Originally Posted by jagans You can, but I don't know whether it will do any good. You want correct and even distribution of ventilation. What you are taking about is breathing through one nostril. I hate that, don't you?
Exactly. I had sinus surgery and can't believe I didn't have it earlier. Who would have thought that breathing was so underrated.

It has to be sufficient and even to be effective. This is best accomplished by a balanced soffit-ridge venting set up.

Take a look at the hidden vent soffit and put that up. You can't tell that it is vented unless it is off the home. Trust me.

 03-14-2013, 11:57 AM #12 Member   Join Date: May 2008 Location: Edmonton, Alberta Canada Posts: 787 Rewards Points: 518 Is there any point having vented soffits on the gable soffits since they don't tie into the attic space? (like the picture above)
 03-14-2013, 12:16 PM #13 Exterior Construction     Join Date: Aug 2011 Location: Washington DC Metro Area (VA, MD, DC) Posts: 16,310 Rewards Points: 5,554 Blog Entries: 5 Not really. I was just posting the picture so that you could see that it does not appear to be vented but still is.
03-14-2013, 12:18 PM   #14
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Quote:
 Originally Posted by Windows on Wash Not really. I was just posting the picture so that you could see that it does not appear to be vented but still is.
Thanks for the ideas, only risk is IF there is a fire and it doesn't burn to the ground I'll be liable if they find out.

03-14-2013, 12:31 PM   #15
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I have another option. I can rotate my trusses 90 degrees and have the eaves point to my backyard and the back alley. This would meet their 4ft code crap. Hmmm.

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