Garage/Attic Exhaust Fan

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Old 01-22-13, 01:03 PM
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Garage/Attic Exhaust Fan

Over the weekend I installed an exhaust fan in the wall of my garage. So far, so good, however I still have a few questions.

The previous owner of my home did a very nice job dry-walling the entire garage, including the ceiling. There is no room above the garage. The house stops where the garage starts. Above the ceiling in the garage, the previous owner installed a pull down ladder to allow access to the a-frame for storage. The ceiling between the garage are the attic is completely sealed, with the exception of the ladder and garage door opener.

I installed the exhaust fan inside the garage, not the attic. I found a gable vent on the outside of the house next to a side door in the garage. The previous owner had dry-walled over this vent. So essentially the garage is sealed. I used a few pieces of 2x2 to shim the fan away from the gable vent to allow the shutters on the back of the exhaust fan to open and close.

The fan works great when the garage door is cracked, or the door to the house is cracked.

So here is my question, intake vent placement. I know I should have my intake vent on the coolest side of the garage. Also, there are 3" Holes just below on the roof line venting the attic on both sides. Since the attic is "sealed" from the garage, would it be best to put a vent in the ceiling of the garage so the fan can draw the fresh air in from the 3" holes, through the attic, then into the garage and out the exhaust fan? Kind of killing two birds with one stone?! Or should the intake vent just be mounted on the opposite side of the garage?
 
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Old 01-22-13, 01:19 PM
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Air sealing is a tricky process, so when you say the garage is sealed from the attic and I presume from the house, it isn't a perfect seal. Running a powered exhaust fan in an attic has proven to be a problem in that it pulls expensive cooled air from the house into the attic. In your case, depressurizing the garage may also pull some cooled air from your house, so some extra sealing would be in order. Often many of the leaks, like holes for electrical can no longer be accessed so some loss will occur.

Pulling air from the garage attic is probably not the best as that would most likely be all hot air. The garage attic needs its own intake and exhaust, plus you should be sure those pull down stairs are air sealed. NE side or shaded area away from the heat of the sun would be best location for the intake.

Bud
 
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Old 01-22-13, 01:47 PM
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Thanks Bud!

I used the term "sealed" loosely. What I meant to say was, the ceiling of the garage was dry-walled to create an attic, and is not a perfect seal. (Neighbors homes are open) The attic in the garage is sealed from the house. (I've attached a photo of the home. you can see where the a-frame of the garage ends at the second level of the home. The garage, and the room behind the two front windows are the hottest spots in the house).

I thought about the fact that the air coming from the attic would be a lot warmer that the air in the garage. (Looking at the photo) I was thinking of installing a matching gable on the left side of the garage as the intake (fan is on the right). This side of the house is also shaded from the sun by the house next-door.

I call these tract homes the human filing cabinets, 2x4's and sheet rock, as fast as they can!
 
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Old 01-22-13, 04:53 PM
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I'll need more information on your current and planned venting, but typical pop-in round vents have about 19% NFA (net free area). So a 3" vent has about 7 in² of area, but functions like a 1-3/8" hole (1.5 in²) and that is not a lot. For every 1,000 ft² of attic floor you need 480 in² (3.3 ft²) both high and low. Ridge vents, roof vents, and soffit vents all have a NFA rating to give you a better number to calculate with. The above is based upon 1 ft² for every 150 ft² of attic floor.

To match that exhaust fan you will need something larger than the current fan opening. Can you go out through a wall? The construction co (roofers) may be able to cut one in when they are there, it isn't difficult. There are wall mount hoods to keep the weather out.

Bud
 
 

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