Attic Ventiliation

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  #1  
Old 08-03-10, 03:18 AM
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Attic Ventiliation

I am in the process of getting estimates to install proper vent to properly ventilate the attic in our cape. I am hearing different ideas and getting confused.

What is the correct way to install a proper vent to get the air from the knee wall/soffit area to the main attic?

Does the proper vent need to be installed in each bay starting at the soffit and going up the slope to the attic?

Some contractors say just create a space where the air from the soffit area can get past the insulation on the slope into the attic while others say to start the proper vent right at the soffit and continue into the attic.

Please help clarify.

Thanks in advance!

Paul
 
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  #2  
Old 08-03-10, 07:46 AM
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Hi Paul, One of the challenges up here is to get enough insulation in each bay AND leave a gap for air to pass. Each bay needs venting and usually around 1" of space. Those baffles they slide in are a bare minimum. If you have 6" rafters, minus the one inch, you're left with 4.5" for insulation, about half of what you need in Maine.

Is everything open, or are you trying to work with the sheetrock in place?

Just to clarify, the objective is not simply to transport the air from the soffits up to the attic and out, it is to cool the bottom of the roof to prevent snow melt and subsequent ice dams. One of the difficulties in doing this the natural way, convection, is that it requires heat for anything to move. So by definition, you can never get all of the heat out unless you eliminate the source.

Bud
 
  #3  
Old 08-04-10, 05:50 AM
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Thank you Bud

I have been told by a couple of contractors that I only need to open air space (baffles) where the insulation is blocking air flow from the knee wall area-this is approx 6 ft long.

Other contractors have told me that I need to run the baffles from the main attic all the way down to the soffits. The distance from the soffit to the main attic is approx 10 ft.
Obviously there is more expense/labor in running the baffles all the way down to where the soffits are cut into the eaves.

I need to know who is telling me the correct way to have these installed.

Thanks again
Paul
 
  #4  
Old 08-04-10, 06:48 AM
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The only reason to run the baffles the full length would be if the rafters are to be filled with insulation. Insulating the full length of the rafters is only necessary if the side attic space behind the knee wall is to become conditioned space. If the knee wall and the floor below is well insulated and air sealed and the access door is insulated and sealed, then the side attic can remain an attic.

Bud
 
  #5  
Old 08-05-10, 06:31 PM
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I agree with Bud. Keep in mind if using fiberglass batts on the knee wall to protect them from wind-washing by the incoming cold air from the soffits. Run the house wrap or foam board right up to the baffles and down between the floor joists. Remember to leave 1" between baffle ends, per manufacturer's instuctions. Hint, the plastic ones are 30% more air volume. Durovent - Economical Foam Attic Ventilation Channels
Berger Building Products
http://www.simplesavings.coop/simple...ee%20walls.pdf

Be safe, Gary
 
  #6  
Old 08-07-10, 12:01 PM
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Thank you Gary-pictures speak a thousand words
 
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