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Old 02-15-2009, 09:42 AM   #1
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Attic Ventilation


Converting attic space in my Cape Cod home to master bedroom. I currently have 3" round soffit vents on both sides and a roof ridge vent. I also have gable vents on each end of house. I was thinking of putting a gable fan at one end with a thermostat.
Is this overkill? I live in Western Mass.

If gable fan is recommended to exhaust hot air out, what brand name do you recommend and does it matter what end of the house I put it at?

After putting the air channels in, insulation, and sheet rock, the sq footage of open space for air to circulate at the top is approx 250 -300sq ft . What size fan should I use?

Any help would be greatly appreciated.

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Old 02-16-2009, 07:13 AM   #2
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Attic Ventilation


If the area of the soffit and ridge vents are equal, you shouldn't need any additional venting. Especially not a powered fan.

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Old 02-17-2009, 10:45 PM   #3
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Attic Ventilation


The ONLY thing that matters at this early point of the discussion, is the actual calculations of the Intake and the Exhaust Vents that you have in place currently.

Here are the NFVA ratings for those minature 3" circle vents. Unless they are installed one right next to each other all the way across the soffit, they would Never be able to produce enough Intake Ventilation.

You also need an equal amount of Exhaust Ventilation.

When you have a Balanced Intake to Exhaust ventilation system, then you can loosely follow the 1/300 ventilation formula.

If you do NOT have a properly balanced ventilation formula, the Total amount of all venting NFVA must be doubled, by figuring out the 1/150 formula.

Unless you can inform me of the outer measurements of the perimeter of your home, I can not tell you what is needed, but I can tell you for pretty darn good certainty, that you do not have enough, Yet.

Ed



General Specifications
Overall Size (in)
Net Free Area (in2)
Hole Size (in)

Screen & Louvers
3
1.63
2-1/2

Screen only
3
2.34
2-1/2



The other thing needed to be known, is if their are insulation baffle chutes installed where the insulation is tightly packed inside of the vaulted cieling or steep slope roof sections.

Without air flow, the ventilation hardly permeates the insulation at all, so you might as well consider the Intake Null and Void.

Ed
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Old 02-18-2009, 01:27 PM   #4
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Attic Ventilation


Quote:
Originally Posted by Ed the Roofer View Post
The ONLY thing that matters at this early point of the discussion, is the actual calculations of the Intake and the Exhaust Vents that you have in place currently.

Here are the NFVA ratings for those minature 3" circle vents. Unless they are installed one right next to each other all the way across the soffit, they would Never be able to produce enough Intake Ventilation.

You also need an equal amount of Exhaust Ventilation.

When you have a Balanced Intake to Exhaust ventilation system, then you can loosely follow the 1/300 ventilation formula.

If you do NOT have a properly balanced ventilation formula, the Total amount of all venting NFVA must be doubled, by figuring out the 1/150 formula.

Unless you can inform me of the outer measurements of the perimeter of your home, I can not tell you what is needed, but I can tell you for pretty darn good certainty, that you do not have enough, Yet.

Ed



General Specifications
Overall Size (in)
Net Free Area (in2)
Hole Size (in)

Screen & Louvers
3
1.63
2-1/2

Screen only
3

2-1/2



The other thing needed to be known, is if their are insulation baffle chutes installed where the insulation is tightly packed inside of the vaulted cieling or steep slope roof sections.

Without air flow, the ventilation hardly permeates the insulation at all, so you might as well consider the Intake Null and Void.

Ed
Ed, the length of the area to be finished off is 38'. The width is 28'. It is a cape cod home with a sloping roof. I just re-measured my soffit holes that I drilled in each bay. They were drilled at 3 1/2" in each 16" oc bay along both 38'sides.
(Note: I have perforated vinyl that covers the 3 1/2" holes. ) I have a full ridge vent across the length of the house. I am in the process of converting this attic space into a master bedroom. I plan on putting baffle chutes in each bay. I did not want to fur out the rafters due to limited head space, so I am using R21 (5 1/2") batts and 1" (R7)foam board before putting up sheet rock.

THanks in advance for your response and suggestions.

Last edited by Vermont1; 02-18-2009 at 01:44 PM.
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Old 02-18-2009, 07:10 PM   #5
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Attic Ventilation


Based on the size of your mini-louver vents under the soffits on both, the front and the rear of your home, using a table I found at the Lomanco Ventilation site, http://www.lomanco.com/ProductPAGES/CirkVents.html

Each one of those miniature vents provide approximately 2.57 square inches of NFVA per opening, not deducting for any blockage from dust, insulation or the impeded air flow from the vinyl covering.

I figured about 28 per side, so you have about 71.96 square inches per side, so 72 x 2 sides = 144 square inches.

For your ridge vent, if it is fully functional and not the Roll Type, then it should provide 18 square inches of NFVA per lineal foot, so, 38 feet x 18 square inches = 684 square inches of Exhaust NFVA.

You need an additional 540 square inches of Intake NFVA to add to your current 144 square inches of Intake, so to round things up for simplicity, you need 5 times the amount of Intake Mini-Louver Vents to be added.

Yes, that seems like a lot, doesn't it? It all comes down to the calculations. That is why between 90% and 95% of all shingle roofs are inadequately ventilated per the required manufacturers specifications and
the Local Building Code Requirements.

Technically, you do meet the "Minimum" buiding code formula using the lesser requirement of 1/300, which means you need One Square Foot of Total Ventilation NFVA for every 300 Square Feet of Attic Floor Space or in your case, the horizontal footprint, which is 38' x 28' = 1,064 square feet.

But, since it is not Near Equally Balanced Ventilation, you need to double the amount required by using the 1/150 formula, or increase the amount of Intake Ventilation to Balance the amount of Exhaust Ventilation.

You would be better advised to install a Continuous Intake Strip Soffit Vent, which would provide 9 square inches per lineal foot on each side of the home, thereby balancing out the 18 square inches per lineal foot from the ridge vent.

Ed
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Old 02-19-2009, 08:21 AM   #6
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Attic Ventilation


Quote:
Originally Posted by Ed the Roofer View Post
Based on the size of your mini-louver vents under the soffits on both, the front and the rear of your home, using a table I found at the Lomanco Ventilation site, http://www.lomanco.com/ProductPAGES/CirkVents.html

Each one of those miniature vents provide approximately 2.57 square inches of NFVA per opening, not deducting for any blockage from dust, insulation or the impeded air flow from the vinyl covering.

I figured about 28 per side, so you have about 71.96 square inches per side, so 72 x 2 sides = 144 square inches.

For your ridge vent, if it is fully functional and not the Roll Type, then it should provide 18 square inches of NFVA per lineal foot, so, 38 feet x 18 square inches = 684 square inches of Exhaust NFVA.

You need an additional 540 square inches of Intake NFVA to add to your current 144 square inches of Intake, so to round things up for simplicity, you need 5 times the amount of Intake Mini-Louver Vents to be added.

Yes, that seems like a lot, doesn't it? It all comes down to the calculations. That is why between 90% and 95% of all shingle roofs are inadequately ventilated per the required manufacturers specifications and
the Local Building Code Requirements.

Technically, you do meet the "Minimum" buiding code formula using the lesser requirement of 1/300, which means you need One Square Foot of Total Ventilation NFVA for every 300 Square Feet of Attic Floor Space or in your case, the horizontal footprint, which is 38' x 28' = 1,064 square feet.

But, since it is not Near Equally Balanced Ventilation, you need to double the amount required by using the 1/150 formula, or increase the amount of Intake Ventilation to Balance the amount of Exhaust Ventilation.

You would be better advised to install a Continuous Intake Strip Soffit Vent, which would provide 9 square inches per lineal foot on each side of the home, thereby balancing out the 18 square inches per lineal foot from the ridge vent.

Ed
Thanks very much Ed for the research and advise. I will increase my soffit vent intake by cutting a strip along both sides of house. You mentioned "roll type" vent. This is what I used for the top. Does this change things? Also, I have gable vents and was thinking of putting a fan on one side with a thermostat? Does that make sense? Also, I did not use those mini louver vents for my soffit vent. I just cut a 3 1/2" hole in each bay and replace perforated vinyl along the length. It probably still doesn't matter. Thanks again for your response.

Last edited by Vermont1; 02-19-2009 at 08:26 AM.
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Old 02-19-2009, 12:03 PM   #7
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Quote:
Originally Posted by Vermont1 View Post
Thanks very much Ed for the research and advise. I will increase my soffit vent intake by cutting a strip along both sides of house. You mentioned "roll type" vent. This is what I used for the top. Does this change things?

Also, I have gable vents and was thinking of putting a fan on one side with a thermostat? Does that make sense? Also, I did not use those mini louver vents for my soffit vent.

I just cut a 3 1/2" hole in each bay and replace perforated vinyl along the length. It probably still doesn't matter.
Thanks again for your response.
The ridge vents, like Shingle Vent II or Cobra Snow Country, which are rigid plastic 4 foot sections and also contain an External Baffle and do not get squished when getting nailed in place, are far more functional than most Rolled Ridge Vent brands. The Cobra Roll Vent has the most complaints for getting nailed down too firmly and decreasing the Exhaust flowage.

I don't think you will need any type of Powered Exhaust, either a roof top Powered Attic Ventilator or an Powered Gable Vent if you do the intake the right way and change the brand of ridge vent to what I suggested.

Your probably right, but no tests have been done on that scenario that I know of.

Ed
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Old 03-29-2011, 09:59 AM   #8
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Attic Ventilation


This is the best Powered Attic Ventilators, A 14 in. diameter steel blade specially designed for maximum air movement., Operates automatically. Adjustable thermostat built-in.-->>Buy the BROAN - NUTONE ( NAUTILUS ) - Broan 358 Roof Mounted 1200 CFM Attic Ventilator at TheHardwareCity.com

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