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Old 06-14-2008, 01:21 PM   #1
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Hip Roof Ventilation Question


I currently have a 1500 sq foot home with a 4/12 pitch hip roof. Soffit has proper ventilation around the entire home. When i purchased my home, it had 3 standard square roof vents( the type that the majority of roofs have) on the rear side of the roof. A few years ago i removed the center vent and replaced it with a 1600 cfm power vent. I am getting estimates for a new shingle roof and this is one area that i am addressing. I have about 24 feet of ridge that i could install a ridge vent. After reading some info on the internet, i found it is not a good thing to install a power vent close the other vents as it will suck air from the closest vents vs the overhangs. Any recommendation on what types of vents and how many i should have to properly vent my attic?

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Old 06-14-2008, 02:55 PM   #2
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Hip Roof Ventilation Question


A 1,500 sq foot attic floor space with the "Minimum" code requirement if all conditions are met, would require 5 square feet of Total ventilation, best optimized by equal or slightly more intake ventilation as compared to exhaust ventilation. If the ventilation is not balanced between the intake and ridge exhaust, or if you do not have a vapor barrier under the warm side of your attic insulation, then the requirements for "Minimum" code and warranty approval must be Doubled, so you would then need 10 square feet of total ventilation NFVA.

24 feet of Ridge using the Shingle Vent II ridge vent or the Cobra Snow Country ridge vent, will provide 432 sq in of NFVA, which is exactly 3 square feet.

Now, ensure that you have at least 3 square feet of soffit intake ventilation or have the Smart Vent under shingle ventilation product installed along the perimeter eave edges of your home, which will provide 9 sq in per lineal foot installed.

OC makes a "Ridge Vent" product, which I will NOT use as a ridge vent, but they warrant it for applications on Hips of roofs, if you wanted to consider venting the Hips in addition to your ridge. This goes against most advice given about ventilation, and I would NOT recommend it, unless you have a vaulted/cathedral ceiling instead of an actual attic area, which I do not thik you do, based on your usage of a PAV previously.

Ed

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Old 06-16-2008, 07:22 PM   #3
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Hip Roof Ventilation Question


house is exactly 1440 square feet...roof is a 4/12 pitch hip roof.. I currently have 2 standard roof vents (OC states same size vents give 50 sq inches of vent area) and one roof power vent. The power vent is between the 2 standard vents and about 8-9 feet away from each vent(middle standard vent removed for power vent install) I think it is 1600cfm but could be wrong(it is at least 1200cfm. Based on Ed's formula i come up with 1008 cfm. I am getting a new roof soon and would like to make sure the exhaust end of the ventilation is correct. It sounds like the power vent alone is enough for exhaust. Is this a correct assumption? Would i be better off removing the 2 standard roof vents and only have the power vent? After reading through this forum, i wasnt sure if the proximity of the other vents to the power vent was defeating some of the airflow( power vent sucking air from the closer roof vents vs overhang vents). Another option i am looking at is to remove the 2 vents and power vent and install 23 feet of ridge vent...

Last edited by tecboy; 06-16-2008 at 07:29 PM.
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Old 06-16-2008, 07:58 PM   #4
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Hip Roof Ventilation Question


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Originally Posted by tecboy View Post
house is exactly 1440 square feet...roof is a 4/12 pitch hip roof..

1,440 % 300 = 4.8 sq ft x 144 sq in = 692 sq in with the minimum standard formula, presuming you have 50% venting from the soffits or having a vapor barrier under the warm side of the attic insulation, otherwise, double the amount needed.

I currently have 2 standard roof vents (OC states same size vents give 50 sq inches of vent area) and one roof power vent. The power vent is between the 2 standard vents and about 8-9 feet away from each vent

Poor air flow dynamics with that set-up.

(middle standard vent removed for power vent install) I think it is 1600cfm but could be wrong(it is at least 1200cfm.

Based on Ed's formula i come up with 1008 cfm.

Correct.

I am getting a new roof soon and would like to make sure the exhaust end of the ventilation is correct. It sounds like the power vent alone is enough for exhaust. Is this a correct assumption?

Would i be better off removing the 2 standard roof vents and only have the power vent?

Yes, if you choose to keep on using the energy guzzler PAV.

After reading through this forum, i wasnt sure if the proximity of the other vents to the power vent was defeating some of the airflow( power vent sucking air from the closer roof vents vs overhang vents).

That is precisely what is occurring, so you are not pulling air flow in through the farther soffit intake portals as readily as you should be.

Another option i am looking at is to remove the 2 vents and power vent and install 23 feet of ridge vent...

23 feet of Shingle Vent II or Cobra Snow Country Ridge Vent products would provide 594 square inches of exhaust ventilation NFVA.

I hope that answers your current batch of questions adequately enough.

Ed
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Old 06-16-2008, 08:18 PM   #5
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Hip Roof Ventilation Question


Thanks for the info Ed...with all the info at hand...i have been wanting to do away with power vent...besides being noisy it is an energy guzzler as you said. 23 feet of ridge vent is only a little of 3 sq feet of ventilation. The 3 original roof vents only gave 50 sq in each. I know it is not recommended to mix exhaust vent types...what would you recommend Ed? I am leaning towards closing up the 3 roof vents and installing ridge vent only. Would the difference between the 3 sq ft of ridge ventilation vs the 5 sq ft your required exhaust calculation comes up with be acceptable? I think it would take 12 or so standard vents @ 50 sq in to get 5 sq feet of exhaust. Really do not want to see 12 vents in the roof..
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Old 06-16-2008, 08:56 PM   #6
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Hip Roof Ventilation Question


The 4.8 square feet was the Total ventilation NFVA.

That can include the Soffit Intake Ventilation that you stated that you have already, or as you said, " Soffit has proper ventilation around the entire home".

Now, what do YOU mean by proper ventilation all the way around.

Each ventilation product has a manufacturers NFVA rating for these too.

A continuous strip vent typically provides 9 square inches per lineal foot.

A 4" x 12" rectangular louvre vent is about 26 square inches each.

An 8" x 16" rectangular louvre vent is about 56 square inches each.

Add up all of the total intake vents you have and arrive at your total intake ventilation numbers.

Ed
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Old 06-17-2008, 05:19 AM   #7
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Hip Roof Ventilation Question


That was where i was getting confused...4.8 sq ft was the total for both intake and exhaust vent area. My overhangs have vinyl soffit . Each piece is divided into 3's where the center section is perforated with small round holes for the intake. I could not find the exact sq in intake specs for this type of soffit. Any guestimates Ed?
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Old 06-17-2008, 09:37 AM   #8
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Hip Roof Ventilation Question


First, you have to make sure that the guys who installed the soffit panels actually cut out the wooden soffit material to the exact dize as the perforated panels. Sounds logical, doesn't it? But, in most cases, it is either not cut out at all, or they just drilled a 3"-4" hole saw in or close to the area.

Soffit panels vary from 6 square inches to 9 square inches per square foot of actual vented soffit panels.

So, if your overhangs are 12" extended past the walls of the home and you have the triple 4 panel, the center vented panel should be 4" wide x 12" of overhang equaling 1/3 square foot or 2 square inches to 3 square inches of NFVA per vented section in the panel.

You should be able to look up the brand of vinyl soffit and check out the manufacturers website to see if they list an actual rating of NFVA for it.

One other thing, even if the wood soffit is cut out, you have to make sure that no attic insulation is covering the vent holes and also clean away any dust or cobweb particles and debris from the vent holes to allow them to function.

Ed

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