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Old 09-07-2011, 03:16 AM   #1
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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.

QUESTIONS

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.)
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Old 09-07-2011, 10:13 AM   #2
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Ventilation - Opposing gable, limited soffit + box vents = Screwed up?


QUESTIONS

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?

It is not ideal but not every home requires soffit venting to be adequate.

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

Totally depends on what you are wanting to spend and do. The fact that you don't appear to have mold or mildew indicates that your soffit venting is adequate but not ideal. Attic ventilation is for moisture control and not roof temperature modulation.

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.

No. Do not do this. Attic ventilation should be balanced, however, if you are going to error on one side or the other, you should have more intake air than exhaust air. By blocking off the soffits, you will further exacerbate the issue and create more stack effect in the attic and draw more conditioned air out of the space below. Do not block off your soffits.

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

I would leave well enough alone for right now. What you have has worked so far. If you were going to add anything, you might try more eave venting via a vented fascia board or similar.

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?

Avoid powered attic fans as they usually create more than solve problems.

Properly air sealing the attic floor and sealing up any penetrations will mitigate your necessity for attic ventilation and allow your current arrangement to be more adequate. It will also help with utilities, comfort, and overall efficiency. I would do this prior to adding cellulose.
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