Do I need a ridge vent if I have an attic fan?


Do I need a ridge vent if I have an attic fan?

Asked by Chrissy

I just recently had my roof done and now people are telling me the roofers should have put in a ridge vent. I had called and the company has told me a ridge vent is not needed when i have an attic fan. Who do i believe??

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Polly Osborne, FAIA, LEED AP's picture

My first thought is to suggest you invite an architect or general contractor over to help you with this. If you have a friend in the trades they could give a quick look and you'd know.

You haven't supplied enough information to know the answer from afar.

  • Do you have eave vents?
  • Is there a gable vent? Those are the ones on the walls at the ends of the ridge.
  • Does the attic fan send the air from the house to the attic or from the attic to the outside, or both?
  • Is the fan solar, electric or draft driven? (There seems to be fan for every cause! (whoops on the pun.))
  • What shape is your roof?
  • And most importantly, what is the square footage of attic and what is the square inch size of venting?

The purpose of attic vents and fans is to circulate the air so that moisture forming from condensate dries out. The moist air is caused by the differential of heat between the interior and the exterior air. If this isn't done sufficiently it can quickly cause the roof to rot. It also helps prevent ice dams and releases excess heat from the attic.

So to sum it up, attic vents help with physical comfort, preserve building materials, and reduce energy consumption. The venting should be distributed uniformly so you don't have dead spots where the air cannot flow in and out of the attic.

  • This is why ridge vents are recommended.
  • But a fan can also move the air that would otherwise be sitting still up on the ridge.
  • If the fan relies on electricity and that could go out, you still need the vents.
  • If the fan relies on draft, then you'll would, again, want to calculate the airflow based on the size of your attic.

I never use an attic fan instead of venting, but only to augment the airflow. You must have adequate venting. Venting is actually calculated as a percentage of your attic square footage. The standard amount is 1/150 or 1/300, depending on your specific conditions. These are minimum numbers and the conditions that apply to each are specified in the building code for your area.

So check you local building code or invite your neighbor the contractor or architect over for dinner. Before he/she comes, warn them what you need and do this homework:

  1. Figure out the attic's square footage. Multiply it width by it length or multiples there of and add them up.
  2. Divide the area by 150.
  3. Determine high and low intake/outtake by dividing that number in half and figure half the venting should be low and half high. If you have eave vents, this means half the venting would be in the eaves and half in a ridge vent. If you just have big vents on the walls of the attic at the ends of your gables then this area would be divided evenly among them.
  4. Multiply your answers by 144 to get the square inch area necessary for venting. Remember, vents have metal structure so the size of the vent might not be the size of the actual air flow area.
  5. Cook your neighbor a nice dinner.