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Old 11-04-2011, 11:17 AM   #1
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Attic Ventilation


We are replacing our roof (Cedar to Asphalt), so I believe the attic will generate a lot more heat, thus putting more importance on proper venting.

My preference would be to go with the ridge vent, but not sure we have enough intake, output & ridge. Here are my measurements...

Hip Roof Area (main house)
- Attic space: 1200 sf
- Ridge: 20'
- Soffit: 110'

A Gable (front entrance) & A Frame (garage)
- Attic space: 944
- Ridge: 30'
- Soffit: 50'

Other information that may be helpful...

Back Soffits will be replaced. Front Soffits will NOT be replaced, because of dental molding. I do not know the manufacture of soffits in back of house. Front soffits look to be in good shape & not covered with paint or insulation.

I have had many conflicting recommendations. The attic space is all connected. Some have recommended a ridge vent over the garage & A gable + a power vent over hip. However, my research tells me that it is either one or the other, since it is the same attic space.

Ridge vent or power vent? Whole house? Split up?

Thanks for your feedback.

Patrick
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Attic Ventilation-img_8325.jpg   Attic Ventilation-img_8326.jpg  

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Old 11-04-2011, 03:16 PM   #2
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Attic Ventilation


You have 2144 sqft in the attic, divided by 150 sqft equals 14.29 which represents the total sq feet of NFA(net fee area) ventilation you will need. To have an efficient system you need an equal amount of ventilation at both the highest point of your roof and the soffit.

At the highest point of your roof you need 7.15 sqft of ventilation.,
If you add the 50' of ridge vent (which I would do) it will get you 6.25 sqft.
I would put 2 roof vents with NFA of 50-60 flanking the power vent or add 3 and take out the power vent.



At the soffit level you need 7.15 sqft. or the equivelant of 22 -8x16 soffit vents with an NFA of 50.

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Old 11-05-2011, 11:22 PM   #3
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Attic Ventilation


Take the power vent out and go with 3 passive vents along with the ridge vents.

The more important question is why go back with asphalt and replace the beautiful shake? Have you considered any of the steel systems that look like shake?

There are some very nice systems out there.

Here is a job we did in Northern VA.

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Old 11-05-2011, 11:24 PM   #4
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Attic Ventilation


50-year roof and the look of shake.

If you figure on the replacement cost of asphalt in 18-24 years, the steel is much, much cheaper.
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Old 11-06-2011, 01:12 AM   #5
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Attic Ventilation


Quote:
Originally Posted by Windows on Wash
Take the power vent out and go with 3 passive vents along with the ridge vents.


Why would you do that? All having a ridge vent while adding the passive vents would do is short circuit the system.

The box vents will act as intakes for the ridge vent.
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Old 11-06-2011, 01:53 AM   #6
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Attic Ventilation


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Originally Posted by ParagonEx View Post
Why would you do that? All having a ridge vent while adding the passive vents would do is short circuit the system.

The box vents will act as intakes for the ridge vent.
Incorrect.

My statement was aimed at bringing the exhaust air up to relatively close match to the intake air and in this application I would suggest putting them on the hips and near the ridge line.

Box vents will disrupt ridge venting when the balance is out of phase with more exhaust venting than intake venting.

PAVs are never a good idea when passive is an option.

Venting should always err on the side of having more intake vs. exhaust if you have to be out of balance at all. Going the other way around just drive air exfiltration from the living space below as do PAVs.

These ventilation recommendations are based on the 1:150 ratio. If the homeowner air seals and insulates the attic properly, he can probably drop down to the 1:200-250 range safely and not have any issues because of the reductions in moisture accumulation in the attic. At those levels, the box vents will not be required at all.
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Old 11-07-2011, 08:53 AM   #7
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Attic Ventilation


Windows on Wash...I appreciate your feedback.

We have so many things to replace in this house (windows, kitchen, master bath, etc.) we have to consider value. We are going with Certainteed Landmark (lifetime warranty). We did look at Grand Manor, but could not justify cost since I would rather put that into windows, garage doors, etc.

We will be removing the fiberglass insulation, sealing openings & installing blown cellulose insulation (R49).

Do you still recommend 3 passive vents vs. ridge & soffit?

What type of passive vents do you recommend?

Thanks, Patrick
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Old 11-07-2011, 09:02 AM   #8
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Attic Ventilation


Airventinc.com will disagree with everything you just said Windowsonwash.

http://www.airvent.com/homeowner/whyVent/evaluate.shtml

the first bulletin point under "Mistakes to avoid"

Last edited by ParagonEx; 11-07-2011 at 09:09 AM.
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Old 11-07-2011, 09:10 AM   #9
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Attic Ventilation


You do realize that there is no such thing, despite the claims on paper, of a lifetime shingle. If you are going to be in the home for 15-20 years, you need to re-examine the numbers and what makes better sense. If the roof is much older than that when you put the home on the market, you are likely to being paying for it again whether or not you put a new one on the home. The roof is the first thing that most home inspectors will look at and try to depreciate the value of (somewhat for good reason).

Combine that with the fact that your asphalt roof will never look as good as the day it went down (streaking, mold/mildew) and steel looks even more favorable.

Have you even priced out that steel roof? What one person charges for asphalt varies from person to person and steel might be more affordable than you think.

I would never put a Grand Manor on my home. The idea of a 50 year asphalt shingle is laughable to say the least.

Don't remove the fiberglass unless you have had an infestation issue. There is not point in the expense and waste generated. The cellulose cap will make that blown in fiberglass function much better in this case as well.

Once you have sealed all the penetrations into the attic (top plates, lights, etc), you necessities for ventilation are not as dramatic.

You could get away with the ridge and soffit vent combo at that point without needing the additional exhaust ventilation.

Quote:
Originally Posted by ecoastpro View Post
Windows on Wash...I appreciate your feedback.

We have so many things to replace in this house (windows, kitchen, master bath, etc.) we have to consider value. We are going with Certainteed Landmark (lifetime warranty). We did look at Grand Manor, but could not justify cost since I would rather put that into windows, garage doors, etc.

We will be removing the fiberglass insulation, sealing openings & installing blown cellulose insulation (R49).

Do you still recommend 3 passive vents vs. ridge & soffit?

What type of passive vents do you recommend?

Thanks, Patrick
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Old 11-07-2011, 09:28 AM   #10
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Attic Ventilation


Quote:
Originally Posted by ParagonEx View Post
Airventinc.com will disagree with everything you just said Windowsonwash.

http://www.airvent.com/homeowner/whyVent/evaluate.shtml

the first bulletin point under "Mistakes to avoid"
Would Airventinc. recommend having a completely out of balance system with appreciably more intake than exhaust air? A system with a 2:1 intake to exhaust area is also not proper and creates its own set of issue.

My recommendation, see previous post, should have been more detailed but I do not think that the addition of a box vent on the hip side is going to short circuit the ventilation.

Archimedes' principle dictates that the warmer air is going to leave through the highest point. Nothing about the addition of a box vent is going to change that. As long as the attic has good intake square footage, it will vent properly and the warm air will rise and the bulk of the outside air will come from the soffits.
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Old 11-07-2011, 09:34 AM   #11
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Attic Ventilation


All I'm saying is that Air Vent sets the principals regarding ventilation. According to my GAF rep, they are who they base proper ventilation off of when doing warranty inspections.
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Old 11-07-2011, 09:37 AM   #12
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Attic Ventilation


Quote:
Originally Posted by ParagonEx View Post
All I'm saying is that Air Vent sets the principals regarding ventilation. According to my GAF rep, they are who they base proper ventilation off of when doing warranty inspections.
And I am sure that GAF would invalidate that shingle warranty if you didn't have a balanced ventilation system.

What would Airvent recommend in that situation if you have a 150:1 ratio and a 3:1 intake to exhaust ratio (low slop hip roof)?

Like I said before, if the customer air seals the attic floor and seals it up, go with 250:1 and the point is moot.
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Old 11-07-2011, 02:08 PM   #13
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Attic Ventilation


I am not familiar with a steel roof. I thought it was asphalt or cedar? What manufacture do you use?
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Old 11-07-2011, 02:47 PM   #14
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There are a couple in the steel arena but the one pictured is Decra.

http://www.decra.com/all-products/shake-xd

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