Attic Ventilation


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Old 09-18-12, 11:46 AM
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Attic Ventilation

I am looking to increase the efficiency of my home, not to "be green" but to "save green".

Details:
Single story
1674 sq/ft
2.5 Ton AC (2008 Manufacturer)
Built: 1986

In my attic, the insulation is really low in some spots (~R-10 average). Really bad...and I am going to be blowing up to R-38 (or more). One thing I was looking in to was solar attic fans. Being that I have gable vents on each side of the house, I figured these would be the easiest for a "not confident enough to cut a hole in my roof" DIYer.

I am looking at 14" Honeywell solar fans. They claim 1250 sq/ft ventilation, so I plan on adding two. One on each gable vent.

My gable vents are larger than 14". I am fine creating a mounting frame for the fan, but my question is around whether or not I need to restrict the gable vent to *just* the fan, or can I put the fan there and leave the rest of the gable vent as is? The best way I can explain it is if you were to imagine a 14" box going in to a 20" box. There is a void between the two. Do I need to limit these fans to just a 14" blow out, or am I ok leaving them as is and putting the fans in front of them?

 
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Old 09-18-12, 11:52 AM
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Welcome to the forums

Where do you see the make-up air coming from? I gather from your question you see both fans blowing out from the attic to the outside.
 
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Old 09-18-12, 12:45 PM
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Correct. I envision both fans blowing air out, with intake occurring at the soffit vents.

Thanks for the welcome! I look forward to learning a lot here!
 
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Old 09-18-12, 12:48 PM
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Yes, in this case, I would block the vent opening other than where the fan is.
 
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Old 09-18-12, 12:54 PM
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Perfect!

I'm thinking I'll frame it out, throw a sheet of plywood over the frame, and cut out what I need...a little caulking to seal it up and I should be good to go.

Thanks for all the help!

 
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Old 09-18-12, 02:38 PM
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Hi pb, a question.
I know you are planning on solar powered vents, but when the sun isn't shining you will be relying of static venting. Two things would be desirable, a vapor barrier and a well sealed attic floor (ceiling). Air sealing is a win/win all by itself, especially any large bypasses like drops above kitchen cabinets, plumbing paths, can lights, and chimney chase ways. There are more, but those are some of the big ones. The vapor barrier is a bit more difficult as an conditioning climate would want that on the warm side to keep the humid air away from the cold surfaces below. R-38 may be sufficient for that as long as air passages have been addressed.

Install baffles of course to keep the insulation away from the soffits, but also add wind blocks to prevent the inflow of air from washing through the insulation. The wind has been known to blow two or three feet of insulation away from the outside walls.

Depressurizing an attic with a fan runs the risk of pulling your conditioned air out of the house and into the attic. Thus another benefit for air sealing.

Any recessed lights will need to be IC/AT rated or otherwise protected.

Guidance for your static vent area is between 6 and 12 ft�, half high and half low. 6 if you have a good vapor barrier and 12 if not. Using just the insulation you fall somewhere in between.

There is another option, install the insulation first and see if the fans are still necessary. Once you have abundant insulation, with air sealing, often that is more than enough. Worth discussing.

Bud
 
 

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