Attic ventilation question


  #1  
Old 06-24-22, 11:37 AM
J
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Attic ventilation question

Hi folks,

Our house has a basement, a 1st floor (kitchen, family room, etc), 2nd floor (bedrooms), and an unfinished attic. The problem here is a very hot attic. I'm talking degrees easily into the 130s. We can feel the difference in temperature when we go to the 2nd floor bedroom level. The insulation in the attic was very bad, so we had all the gaps sealed and the insulation replaced (pink fiberglass stuff). It made a difference, but it's still too hot. After some research on the GAF website, I'm learning about a balance of intake air and exhaust air. From what I can tell, we have no soffit vents in the back of the house, but I'm not sure yet if they exist in the front (I have to get up there again). We have a ridgeline vent that runs the length of the house. There are also two gable vents, one on either side of the house. According to the GAF vent calculator, our square footage is 1,008 sq ft, and we need 242 Minimum Sq. In. of Net Free Area of exhaust needed at or near the ridge, and 242 Minimum Sq. In. of Net Free Area of intake needed at or near the soffit.

Question #1: How do I determine how much exhaust air I have at the ridge? Do I just measure the length and width of the ridge vent? So let's say the vent is 432" x 2", does that mean I have 864 sq inches of exhaust air? Something doesn't seem right with my math.

Question #2: A roofer wants to install an attic/roof fan. I'm all for it, but don't I have to figure out if I have intake at the soffit first? Otherwise if there is no intake in my attic, where in the world will it be pulling in the new air from?

MSPaint drawing included for fun.

Thanks!

 

Last edited by John Palatzo; 06-24-22 at 12:03 PM.
  #2  
Old 06-24-22, 12:19 PM
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So the ball park ratio is 1 sq ft per 150 sq ft to 300 sq ft of attic.

If you have 1008 square foot that equals between 3.5-7 square foot of vent, intake and exhaust.

From there you just need to determine if your free area is in the ballpark.

BTW attics are always hot and yes as you get closer you will feel.

and the insulation replaced
​​​​​​​So what is your current R-value?

 
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Old 06-24-22, 06:50 PM
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Thanks for the reply. I have to double-check on the insulations’s r-value as it’s been a while.

How do I figure out how much NFA my ridgeline vent has? It’s not really a product that’s on a website with specs. It’s just a gap in the rooftop, you know what I mean?
 
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Old 06-24-22, 07:28 PM
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I think this is more of an HVAC balancing issue - if you have done the air sealing and insulating of the ceiling, I don't think attic ventilation is going to make a difference inside the house. Attic temperatures mimic outdoor temperatures plus a little even when well ventilated. Venting is more about prolonging shingle life in the summer and preventing ice dams in the winter.
 
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Old 06-25-22, 12:46 AM
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How do I figure out how much NFA my ridgeline
​​​​​​​Its just a measurement of open area in the decking!
 
  #6  
Old 06-25-22, 12:48 AM
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Although I have installed Attic Gable Fans and they do work, the fans are quite energy consuming. Fans are great when it is dry heat but when it is humid, they do not do much. I rarely use them anymore except for airing out the attic.

In the recent years, I have found putting a Window AC Unit (leaving door open) serves two purposes while is more energy efficient. First, it cools me and my computers (OverClocking) directly. Second, it cools the entire second floor (box fan moves air), lowing the temperature in the attic.

This gable fan uses about 200 watts:
https://www.homedepot.com/p/Master-F...EGV6/205924917

This gable fan uses about 120 watts: Seems to be better but costs $15 more.
https://www.homedepot.com/p/QuietCoo...-2-0/314250784

Windows AC (and box fan) uses about 600 watts
https://homeairguides.com/how-much-e...hly-cost-guide
 
  #7  
Old 06-25-22, 12:41 PM
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Thanks for the additional replies. Upon some further research and inspection, it appears that I do NOT have a Ridgeline vent. it is simply a gap in the wood at the top of the roof and the shingles cover the gap. From what I’m reading and from talking to others, that is not a Ridgeline vent, especially when the shingles cover the gap. No air is being let out. Since I also do not have a soffit vent, appears that the only ventilation I have in the attic are the gable vents, which don’t here to do too much. I’m thinking instead of putting in an attic fan, maybe the best bet is to start with a ridgeline vent and a soffit vents first to at least let air in and out. Thoughts?
 
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Old 06-25-22, 01:48 PM
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I've always used an attic fan, either a gable vent fan or center roof fan, depending on your design. They do use some power BUT I have it set with a thermostat that they come with for 90 degrees and it is switched on all year. They do make a difference and I wouldn't be without one.
 
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Old 06-26-22, 05:01 AM
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especially when the shingles cover the gap.
Maybe, maybe not. Typ they use a ridge vent cap that provides an opening for air flow then it is shingled over. Need to look on the outside to confirm.


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