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Ventilation - Opposing gable, limited soffit + box vents = Screwed up?

Ventilation - Opposing gable, limited soffit + box vents = Screwed up?

Old 09-07-11, 02:17 AM
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Ventilation - Opposing gable, limited soffit + box vents = Screwed up?

I'm about to add more insulation to my attic, and I'm now thinking my attic's ventilation is really screwed up. I know the previous owner replaced the roof a number of years back, and I'm thinking they made a mistake on it.

Description of my attic -- Pictures are below!

My attic has one peak, which slopes toward the front and back of the house. The sides of the house go straight up to the peak, and each side has a gable vent.

The front of the house has a front overhang covering the walkway to the front door, which extends about 2 feet out from the house and has long soffit vents on it.

The back of the house just has gutters -- there's no siffit vents. There appears to be no veltilation provided here, except for that I can see small cracks of daylight from within the attic showing a minute amount of air is coming in.

Most of the way toward the top of the roof, on the side sloping toward the back, has 7 passive box/grill vents that are a diameter of around 8-10".

The house appears to be built using balloon construction methods. That is, the exterior walls are made from 2x4's that go from the foundation sill all the way into the attic, with no top plate.

Reason why I'm thinking ventilation is screwed up

I can't add additional soffit vents because the roof doesn't overhang the house at all.

So, my attic appears to have opposing gable vents. But, it also appears to have a soffit/box vent ventilation system that's potentially weak due to limited soffit vents -- but maybe not since they're such long soffit vents.

I ran across a bunch of websites and forum posts today that seemed to be from knowledgable people stating that opposing gable vents aren't great, but seem to get by... But that if you have that system, you don't want to have box vents near the top of the roof because it sort of short-circuts the opposing gable vent system, and you wind up having air traveling between a gable vent and its nearest box vents.


1. Is my current attic ventilation screwed up, by having two ventilation methods which are so-so by themselves, but together are counteracting each other?

2. Before I add the new cellulose insulation, what changes should I make to the ventilation?

NOTE: I need to lean toward quicker fixes. I'm doing this myself, but am recovering from back problems from a car accident, so can't get into something too involved.

The things I've considered are:

a) Leaving things be, and blowing in the additional cellulose

b) Trying to improve the opposing gable vent system, by blocking off the soffit vents and box vents. Although this would make the gables more effective, I have no idea if this would do more harm than good overall.

c) Trying to improve the soffit/box vent system, by blocking off the gable vent nearest the soffit vent.

d) Something else? The gable vents are pretty small - perhaps using an exhaust fan on one, or that plus an intake fan on the other? Perhaps adding box vents closer to the bottom of the roof to act as soffits?

Front of house - The area with an overhang, with soffit vents

Front of the house - The area with an overhang, viewed from the inside. There's reflective insulation blocking most of the area, but there is open room at the top to allow airflow. I'll measure, but I think this reflective insulation will act as its own baffle.

Front of house - The area without an overhang, no soffit vents

Back of house - No overhang, no soffit vents

Back of house - Showing the 7 "box vents"

Side of the house - Each has a gable vent

In attic - Looking toward back of the house.
Might be hard to see, but the drywall goes to a piece of wood laying flat, and then there's a drop-off area of a few inches

In attic - Looking down side exterior wall (a gable vent is directly above us.)
Old 09-07-11, 08:01 AM
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I think the roof vents were installed to help air flow. Basically you have none, since you have no soffits. The minimal soffiting at the front of the house does little for the remainder of the house.
The only thing I can see that would help would be to enlarge the gable vents. They appear to be woefully inadequate to allow for cross ventilation as was their purpose when the house was built. You may even want to consider installing a power fan on one of the gable vents to assist in evacuation of the air. Technically the roof vents are passive and should evacuate the hot air by themselves, but they have nothing to draw from. I would not install power vents on the roof. They are a waste of money and do little in your situation.

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