Oven Attic!


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Old 06-20-11, 09:25 AM
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Oven Attic!

I have a heat buildup problem in my attic.
My attic has under eave vents and ridge vent with two gable vents. The house was bought new and it was built this way. It is my understanding that the gable vents could be my problem. I donít know?
I have checked all under eave vents to make sure they are clear, which they are.
I am considering electric gable vent fan installation to move some of this heated air out.
Any input is appreciated.
 
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Old 06-20-11, 09:51 AM
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What kind of temps are you seeing during warm sunny conditions? 120-140 is not uncommon on a hot day even with good ventilation.

How many feet of each type vent do you have. I assume by under eave you mean soffit vents? Are the rafters/trusses exposed or covered. If covered, then you probably have whats called soffit venting either a continuous strip (vinyl or aluminum) or areas cut out with perforated covers.

If you don't have enough intake its not effective.

I can't find any of my references...but several very good sources said gable vents do not decrease the effectiveness of ridge and soffit vents...and in some cases can help.
 
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Old 06-20-11, 10:06 AM
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Originally Posted by Gunguy45 View Post
What kind of temps are you seeing during warm sunny conditions? 120-140 is not uncommon on a hot day even with good ventilation.

How many feet of each type vent do you have. I assume by under eave you mean soffit vents? Are the rafters/trusses exposed or covered. If covered, then you probably have whats called soffit venting either a continuous strip (vinyl or aluminum) or areas cut out with perforated covers.

If you don't have enough intake its not effective.

I can't find any of my references...but several very good sources said gable vents do not decrease the effectiveness of ridge and soffit vents...and in some cases can help.
I'm sure temps are in that range. Yes... I have ridge and soffit vents which are vinyl and run the length of the house (50ft on each side) with the two gable vents at each end. By late afternoon I can feel the heat radiating from the ceiling.That is the reason I am considering adding additional venting.
 
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Old 06-20-11, 10:11 AM
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How did you check the soffit vent? Did you go in the attic to verify they have unobstructed air flow from the soffit to the ridge?
 
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Old 06-20-11, 10:29 AM
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Yes...It's the setup that has a baffle to prevent blockage by the insulation. In fact that what has me stumped! It should be circulating much better than it is?
 
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Old 06-20-11, 10:42 AM
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I think what Gunguy is getting at is that 140F is pretty normal. Don't freak out over a normal condition.
 
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Old 06-20-11, 10:54 AM
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Originally Posted by XSleeper View Post
I think what Gunguy is getting at is that 140F is pretty normal. Don't freak out over a normal condition.
Who's freaking out? Yes I understand that 140 is high side normal but... this attic should be circulating much better than it is. If by late afternoon I can feel the heat radiating through the ceiling when the rafters are more than adequately covered with insulation then one can assume that the heat build up is not being vented properly as the heat is stacking up.
Are saying this is normal?
 
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Old 06-20-11, 10:57 AM
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Ridge Vent

Look from inside the attic to make sure the ridge vent is not obstructed.

I once saw a house where the roof sheathing was left with a gap at the ridge for the ridge vent but the roofers forgot to cut away the felt when the shingles were installed. The felt had the ridge vent completely blocked.
 
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Old 06-20-11, 11:41 AM
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Have you determined that you have sufficient inlet and exhaust vent area to ventilate your attic? The rule of thumb is to divide the footprint of the Conditioned Space (in sq. ft) by 150 which yields the total Net Free Area in square feet of the vents. Divide by 2 for the individual inlet and exhaust areas which should be roughly equal.. Louvered vent covers have only about 50 to 70% of the total size of the cover for NFA.

Let's talk about those gable vents. Are the tops at least 3' below the ridge? If not, then they aren't able to work effectively as Inlets - which is what they are supposed to be - and instead are circulating a very small amount of air from the bottom and exhausting it at the top with negligible useful effect for the attic. If they are too close to the ridge, remove them from calculations for ventilating NFA.

Is there a 2" to 3" wide gap between the roof decking sheets along the top of the ridge(s) to provide for exhaust through the ridge vents?

I'm not looking for answers to these questions. They are being presented as items for you to check as they are minimal provisions for passive attic ventilation. But, I am interested to learn if you discover anything interesting.

Good luck with your project.
 
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Old 06-20-11, 11:42 AM
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Even with normal/good airflow...it will always be hotter in the attic. Esp if your shingles are a darker color...though there's not as much difference between light and dark shingles as you might think. As the structural members and plywood sheathing heat up and the heat penetrates the insulation, it's going to radiate to the living spaces below.

On a typical 85 degree partly sunny day it might be 120 down low and 140 up near the peak. Whether its a calm day or windy can also make a difference.

Do you have an infrared non-contact thermometer you can check with? You haven't given specifics on things. How hot is it up there...how long is the sun exposure... how much is "adequate" insulation, and what type is it? It varies greatly depending on your location. Many places are recommending R50 in attics...that could be anywhere from 12-15 inches (or more) depending on the product.

I believe there are ways to check for correct air flow using smoke generators of some sort?
 
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Old 06-20-11, 04:18 PM
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Originally Posted by cbr549 View Post
If by late afternoon I can feel the heat radiating through the ceiling when the rafters are more than adequately covered with insulation then one can assume that the heat build up is not being vented properly as the heat is stacking up.
Are saying this is normal?
What I am saying is that an attic that is around 140F (or more, depending on where you live) is normal. Did you know that fiberglass and cellulose insulation does not stop radiant heat?
 
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Old 07-11-11, 04:53 PM
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Sorry, I have been busy with other projects and just now had time to respond. Just a thought but... if I installed two attic fans just below the ridge vent blowing up to add more air movement then air intake would be soffit vents + gable vents and air would exhaust out ridge vent.
Attic is 30'x50' with approx. 30 degree pitch.
 
 

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