Ridge vent question

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  #1  
Old 06-04-16, 03:47 PM
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Ridge vent question

With respect to externally baffled ridge vents. If you install a 4 foot vent on the roof peak, will you get the full 4 feet of available ventilation?

This is to say that...I have approx 5 feet of roof peak by design at the top of my roof, (at one area it's sort of looks like a pyramid), the roofers installed a 4 foot vent at the top. The opening, (cut out for ventilation),at the top is probably about 4 feet as well...since they no doubt placed cap shingles beneath the vent, if they capped 1 foot in on either side of the vent, will attic air still be able to manuever up and over the capped portion of the roof in order to take advantage of the entire roof (ridge) vent area on either side?

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Old 06-04-16, 04:18 PM
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Every brand of ridge vent will list the net free area of ventilation for their product. I'm sure you can find that product info on their website if you know the brand and model of vent. If you have a 5 foot long ridge, but only 4 feet is cut out, your ventilation will be limited by the size of the hole they left in the roof. Let's say that your brand of ridge vent says it features 17 sq in of net free area per foot. In their instructions, they might recommend cutting 1" out on each side of the ridge... which would be 24 sq in of ventilation per foot. So why the decrease in volume? Air slows as it passes through baffles, filters and twists and turns, offering resistance to airflow. So in short, it depends on how much space was cut out of your roof, and what the advertised net free area is of your vent. If that answers your question.

Since a square foot is 144 sq inches, I don't think it either being 4 or 5 feet is going to make a huge difference in ventilation. Its still minimal ventilation, and you probably need more.
 
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Old 06-04-16, 06:22 PM
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As X said, that single section of ridge vent is probably less than you should have. On a hip roof they will sometimes install hip ridge vents to increase the high venting.

Easy to calculate the recommended net free area for high and low venting, did they do that for you?

Bud
 
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Old 06-04-16, 07:22 PM
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Hi guys,

Thanks for the replies...

My roof is sort of a crazy design. (I will try and post a pic). It is approx 1500 square feet and they have installed ridge vents over the garage (24 feet of vent) and over the peak (4 feet).

We calculated that I would need 24 feet of ridge vent to obtain my 1/300 ratio and have essentially exceeded this, (mathematically anyway) by 4 feet.

The problem with this is in my mind is that, although the garage roof is open to the main roof...they seem somewhat separate in my thinking as the air ventilation requirements in the garage would have negligible effect on the main roof area....would this be correct?

I really don't like the idea of hip vents or other types of vent units that make the roof line unappealing. This roof was 16 years old before it was replaced and was not in terrible condition. All the sheathing was golden brown with no mold, mildew or black marks inside. And we have done nothing but improve on the attic ventilation....except for the fact that the old roof had a 5 foot "rolled" vent at the peak whereas now I have a 4 foot externally baffled vent.

This company I hired also ensured that the soffit and internal air baffle arrangement was working properly, and had to actually install additional baffles as the original roof only had them in every second truss network...they also tacked them in place inside the sheathing so they would not fall down or move, then confirmed air was flowing before moving on to the next truss.

With respect to the opening at the peak, it is a good 5 feet, and the roofer advised me that is was cut back on either side of the truss by at least 2 inches. Therefore I believe he capped over the opening on either side and then installed the 4 foot ridge vent.

I was just wondering if getting the roofer to come back and increase the vent from 4 feet to 5 feet will give me any added benefit ?
 
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Old 06-04-16, 07:51 PM
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Like I said... I don't think an additional 17 sq inches of net free area (guessing) for that one additional foot of ridge is worth the time it took to type this... let alone getting on the roof to redo the ridge.

You would be better off closing up the 4' of ridge and adding a few can style vents (Lomanco 750's) on the rear and sides of the house... you will probably double or triple the net free area if you do. Your 4 feet of ridge probably equals the effectiveness of two 750's.
 
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Old 06-04-16, 09:06 PM
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Since you have more venting than just the top 4' ridge and since it has worked fine for 16 years, you can sleep well tonight. You also switched to a baffled ridge vent style which claims better efficiency.

With the additional baffles everything in this project went in the right direction.

Bud
 
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Old 06-04-16, 10:21 PM
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Thanks Bud and Xsleeper for your replies.

I am just doing the math on rolled vents versus externally baffled vents.

The way I calculate it, with the original 5 foot rolled vent, and assuming 4 feet of available opening at the peak, my net free would equal approx 60 sq. inches of ventilation. This is based on 12 inches of net free area per linear foot, (which seems to be the standard NFA for rolled ridge vents...please correct me if I am wrong).

With the externally baffled ridge vent, they can deliver 18 sq. inches of net free area per linear foot...so if I assume the 4 foot vent at the top of my peak is allowing approx. 3.25 feet of vent space, then my ventilation is 54 sq inches, or approx the same amount as the original rolled vent.

I think I am still going to get the roofer to put a larger vent on the peak, bringing me up to 72 sq. inches of ventilation. If nothing else for piece of mind. I probably still don't have the required exhaust numbers in this area, but it will have to suffice.

@Bud9051, the roof in question probably, although not in horrible shape, was beginning to deteriorate quite quickly. Lots of loose granules on the ground after a high wind storm.

I attribute the short roof life to a number of items, namely the weather we get here in Canada, and the fact that the original roof did not have any type of underlay. They installed the original roof with no felt, or nova (blue) seal, or ice and water shield.

Another reason why I wanted to replace the roof.
 
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Old 06-04-16, 11:03 PM
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There are many reasons to not be concerned, among them is that the starting point of 1/300 is and has long been just a wild guess. That number was adopted in 1942 and wasn't based upon testing then and has never been questioned since:
http://www.structuretech1.com/wp-con...entilation.pdf

Another is, there are no testing standards for NFA. Mfgs are free to claim whatever they can get away with. If the baffled vent has any type of bug screening inside then it is less than the 18 inē per linear foot they claim. But does it matter? NO!

If you end up with a lot more low venting than high venting, that is good because it reduces the air leakage from house to attic. As much as 75% low with 25% high works well.

If you want to get picky (I do often) I was avoiding mentioning the connection between the garage attic and house attic. It shouldn't be just a hole, it should be a full length opening as tall as possible. The garage ridge is already lower than the house so its vent area does not add directly to the ridge. It gets technical, but as you increase the top ridge vent you decrease the contribution of the low ridge vent.

It becomes an exercise in futility as getting it perfect gets blown out the proverbial window as soon as the wind blows. Passive venting with zero wind is not all that common. Most of your air exchange comes from the wind.

Sorry, but the lack of good information on this topic drives me wild.

If replacing the vent as you suggested helps you sleep, it is your choice. But it will not change anything measurable and the roofer will not be pleased.

BTW, very nice looking home.
Bud
 
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Old 06-05-16, 03:41 AM
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72 sq in of net free area doesnt even equal the net free area of 2 Lomanco 750's. (Not even 1 1/2) By closing the ridge and putting in just 2 of them you would increase the venting by 50%. I don't know why you would put the effort into it just to go from 60 to 72 sq in. But whatever.
 
 

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