Attic ventilation question

Hello everybody. I work in the trades too, I do heating and cooling. However, I am definitely not a DIYer on roofing. I decided to hire a pro for the job to be done right! We are really happy with the company we are going with. I just have one problem. I need to buy a roof-mounted power ventilator. As a heating contractor I have assisted with the installation of these before, but never had to “size” one for appropriate ventilation.

Here’s the issue: My roofing contractor came out and did some calculations and said that my attic needs X-amount of sq. inches of intake ventilation in the soffit vents. We have a shallow-hipped roof and could not get the proper amount of ventilation from ridge vents, so he recommended that I purchase a power-vent. I want a balanced system. So I don’t want to get a vent with too little power to keep things cool and dry up there. But I also don’t want to get one that will be so powerful that it will exhaust more than can be intook from the soffits and start pulling from the structure itself.

First of all, I am wondering if I am just over-analyzing the situation? Should I just get a big 'ol power ventilator? Or do I need to do some calculation to determine the propler type/size of unit?

The problem is I know how many sq. inches of intake I will have, but I don’t know how to transfer that into how many CFM of exhaust I should have?

Air Vent Corp has a guide in their brochure regarding venting a hip roof to base the CFM on.

Also, if you are doing a tear-off, you can add 9 square inces per lineal foot for the entire perimeter edge by installing the Smart Vent made by DCI Products, Inc.

It is a tapered intake ventilation product that gets installed under the shingles.

Google that name and check out the web sitwe info.


Thanks for the reply. We are using Invisivent? Any thoughts on it. It seemed to greatly increase the intake air area and looks real nice, came highly recommended by the installing contractor…

I will check out that link.

Hey thanks. I found that link at airvent corp and it is very useful. It gave me a formula for determining the strength of the attic vent. Square Footage x 0.7 = Desired CFM Awesome! :mrgreen:

Invisi-Vent is a good intake product, since it is usually applied 100 % around the perimeter.

Your roofer sounds like he knows how to properly estimate the specifications.

Take his good advice. He is a keeper.


I did some calculations and called my contractor to confirm, it looks like I will have about 11 sq. ft. of intake area, which is really good. My attic square footage is approximately 1748, which according to my calculations means I need about 1250 CFM of exhaust air volume. The intake is going to be suitable for almost any fan that I purchase. Most I have found only need like 4-6 sq. ft. of intake to function well. I have read that depending on the way they are installed and static pressure you might only get about 75% of the actual CFM the fan is rated for. So I think I will go with a 1600 CFM power vent.

Does anyone have a recommendation on a brand that is available retail that will output about 1600 CFM and comes equipped with a thermostat and a humidistat? Looking for quality, but not too expensive…

Roofing supply houses such as Bradco, Allied Bldg Products, Shelter Distributing, and ABC Roofing Supply should carry the Air Vent products, which I think go up to 1,500 CFM.

It is important to get one that has a humidistat control as well as the typical thermostat control, to remove excess RH content when the temperatures are not high enough, even in the winter months.

I think the model is called Whisper Cool?

Do NOT over do the Powered Exhaust ventilation, because as you statede in your first post, you would be concerned about removing the interior conditioned air from the living quarters.