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Old 08-04-2016, 01:19 PM   #1
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Need Help with roof ventilation calculation


As mentioned in an earlier thread, my home inspection revealed inadequate roof ventilation due to blocked soffit vents. In addition to opening up the soffits, I want to be certain that roof exhaust venting is adequate. There are four 8-inch diameter static vents and one powered vent (I'm estimating 12" dia.) on the roof. The powered vent is located midway between two of the static vents, which I understand will cause short-circuiting if there's not enough inlet air from soffits. According to a calculator, my house requires a minimum of 212 sq inches of soffit venting. Consequently, I assume that an equal amount of roof venting is required.

The four 8" vents provide 200.96 square inches of exhaust ventilation, based on my calculations. I'm not sure how to calculate the addition from the fan and I don't have any literature on it, or even know what brand it is. It may even be taking away from ventilation due to short-circuiting. How much ventilation does the fan provide during summer when it's running, and during winter when it wouldn't be powered.

After opening up soffits, should I continue to run the powered vent during summer or let it act as a static vent. Are any other roof vents needed. Thanks for your input.


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Old 08-04-2016, 03:40 PM   #2
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Re: Need Help with roof ventilation calculation


In summer it would indeed pull the attic very negative and may well pull air in through those other roof vents. But in doing so it will also be increasing the airflow through the soffit vents so the explanation of short circuiting is incomplete as the powered vent is still improving the ventilation up there.

Adding 2 decimal points to a target number that begins with a plus or minus 50% accuracy is unnecessary. The truth is, they don't really know whether you need 100 inē or 300 inē, citation available. And yes, it is one of my hot buttons. The more or less question is determined by other factors.

What % net free area did you estimate for your vents?
What ratio did you use, 1/150 or 1/300?

If the attic floor/ceiling below was not well air sealed then depressurizing the attic with a powered exhaust will be pulling the air conditioned air from the house into the attic. Worse yet is that the lost air must then be replaced by hot outside air. Powered fans are generally to be avoided.


If the powered fan is left in place but powered off it should act like a normal roof vent and some % of its open area can be added to your high vent area.

I'll wait for answers or more questions.

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Old 08-04-2016, 08:17 PM   #3
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Re: Need Help with roof ventilation calculation


Thanks for the reply. I will keep the fan running for the summer.
Decimal places were just to "show my work". The first calculator that I used suggested twice the ventilation than the one I referenced, but it didn't take my non-gable roof into account. So I deferred to the second.

I'm not familiar with free area or ratio. I guess my research was lacking. This is my first attempt at it.

"Powered fans are to be avoided"? Doesn't that contradict your first paragraph, or did I misunderstand?

My plan is to cut a continuous channel out of the plywood soffit and re-cover, but with screened vinyl instead of the mostly solid vinyl that's on there now. Keep the roof vents as they are. Replace attic insulation and add rafter channel vents. Sound reasonable?
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Old 08-04-2016, 09:31 PM   #4
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Re: Need Help with roof ventilation calculation


Hi Larry,

Before re-roofing this time, I had an architect calculate all of this for me and make recommendations. It was worth the price because I learned a lot. I recommend someone with a lot of experience. I had a roof fan on the house already and I noticed that the upstairs was cooler in the summer after I had it installed many years ago. The architect told me that technically I don't need it for proper ventilation. I was advised instead to add more soffit vents and install a better ridge vent when I reroof. The size of the ridge vent and the amount of soffit vents depends on the calculations. I can't tell you what to do for your house; I'm just recommending what I found out. I am still going to keep a roof fan because it helps with the problem of my upstairs getting hot in the summer in my non-air-conditioned house. He said that was ok and I could get a switch to turn it on and off if I wanted to and there's a thermostat and humidistat in those. I hope this helps.
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