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Old 09-03-2011, 07:00 PM   #1
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Attic Ventilation - new or add to existing?


I have been having a tough time deciding what to do for ventilation. I've attached a 2D sketchup model of my current situation. I have 1050 sq ft of attic floor space, there are 5 "roof louvers" at the locations shown (squares on the roof surface) that are 50 sq inches of NFVA each, there are 2 12"x18" gable vents and one 12"x12" gable vent, figured at 50% NFVA. And there are two of the rafter bays (I have open cornice overhangs with 2x6 blocking over the rafter bays...see photos) at 22.5"x5.5" with quarter inch hardware cloth (metal screen) over them. Figuring at 1 sq foot NFVA per 150 sq ft of attic I would need 7 sq ft (1008 sq inches) of NFVA. I currently have 760 sq in.

I am considering two options: 1) covering existing vents and adding vents under the eaves and ridge vent OR 2) adding to existing. I am looking for suggestions and opinions on the best way to ventilate. The home is in Portland, OR, and the decking is shiplap. There are 3 photos below, one shows the overhang at the northeast corner of the house (equivalent to the corner farthest right in the sketchup drawing), another shows one of the two existing undereave vents) and one shows the underside of the roof deck from the attic.









Thanks for all the help so far! I've almost got planning completed...


Last edited by diyroofer; 09-03-2011 at 07:02 PM.
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Old 09-03-2011, 07:06 PM   #2
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Attic Ventilation - new or add to existing?


Forgot to mention that the attached, unvented structure at bottom of the sketchup model is my garage. It has an open ceiling so I wasn't planning on venting it.

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Old 09-08-2011, 12:29 AM   #3
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Attic Ventilation - new or add to existing?


Update: I am going to install ridge vents and under eave intake venting in the blocking between the rafters...as long as the call I make to the city code authority goes as planned tomorrow (I am checking on potential structural code violations if I cut into the blocking).

The issue I am running into is that the ridge vent of choice provides excessive NFVA, such that balancing it with intake vents will be a challenge. If I install the coravent V300 I will have: 13.5 inches NVFA per linear foot x 52 ft of ridge = 702 sq inches of NFVA. As I described above, the attic need 1008 sq inches total if I figure based on 1 sq ft NFVA per 150 sq ft of attic floor. Balancing this amount of exhaust with an excess of intake ventilation will be tricky.

I was planning on drilling 3 x 3.5 inch holes in each piece of blocking (this will take awhile, I know). This leaves 1 inch of blocking above and below each hole and 3 inches on either side of each hole, I will back the blocking from inside the attic with 1/4" hardware mesh. This will provide a total of 882 sq inches of intake (figuring the holes in the blocking at 90% NVFA due to the 1/4" screen). For a total of 1584 sq inches of NVFA.

If anyone wants to chime in, I would like to hear your thoughts on whether this is too much ventilation (can you have too much?). And if the intake-exhaust ratio is sufficient.
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Old 09-08-2011, 07:38 AM   #4
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Attic Ventilation - new or add to existing?


Air seal the top plates and penetrations and your ventilation requirements will be reduced. You are going to have to make sure you have proper baffling at the eaves so seal up all of those exterior top plates.

I think the venting approach you are taking is the most comprehensive.
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Old 09-08-2011, 07:43 AM   #5
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Attic Ventilation - new or add to existing?


Sounds like you have it covered.
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Old 09-08-2011, 11:42 AM   #6
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Attic Ventilation - new or add to existing?


WoW,

I think I understand what you are describing, but hope you can provide a little clarification for a beginner.

By seal up the top plates and penetrations, I assume you mean and penetrations from the interior of the house to the attic, and sealing the wall top plates from inside the attic?

I don't know what you mean by proper baffling at the eaves and sealing up the exterior top plates. I am going to put baffles between rafters in the attic to direct flow from the undereave blocking vents toward the ridge. Is this the baffling your referring to? I shouldn't have any exposed top plates on the exterior. Were you referring to the top plates of exterior walls, not necessarily top plates accessible from the exterior?

Thanks for the info!
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Old 09-09-2011, 08:17 AM   #7
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Attic Ventilation - new or add to existing?


See below.

Quote:
Originally Posted by diyroofer View Post
WoW,

I think I understand what you are describing, but hope you can provide a little clarification for a beginner.

By seal up the top plates and penetrations, I assume you mean and penetrations from the interior of the house to the attic, and sealing the wall top plates from inside the attic?

Yes and Yes.

I don't know what you mean by proper baffling at the eaves and sealing up the exterior top plates. I am going to put baffles between rafters in the attic to direct flow from the undereave blocking vents toward the ridge. Is this the baffling your referring to? I shouldn't have any exposed top plates on the exterior. Were you referring to the top plates of exterior walls, not necessarily top plates accessible from the exterior?

Yes and Yes

Seal up the exterior wall top plates (i.e. foam or caulk) as you would the interior wall top plates and prior on installing the added insulation, put vent chute or baffles in to allow for air flow from the soffit.


Thanks for the info!

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