Venting an attic before putting in insulation

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Old 11-03-02, 09:15 PM
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Venting an attic before putting in insulation

I have an older house, built in 1949. I want to finish off the attic into a master bedroom and bathroom in the future, but I need to insulate it before winter hits. Currently the floor joists of the attic are rough cut 2x6's and have r-13 on top of some old fiberglass/newspaper loose insulation. The rafters are also only 2x6's approximately 14-16 inches on center. I know they should be more evenly spaced, but they are not it varies. I want to put up drywall soon, but I need to insulate first.

There is a recently installed ridge vent. I can see outside where the rafters meet the outer wall, about a 2 inch gap. I know that I need to place some polystyrene venting in the cavity but does it need to be airtight where the roof meets the outer wall? If it isn't and I use batting in the wall am I in for trouble down the road?

thanks for any advice you can offer
 
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Old 11-05-02, 04:18 PM
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Free venting is when you have low vents and high vents. If you were to block off that 2 inch gap at the bottom of the rafters, you would not have free venting. The best way to illustrate this in your situation is a straw you dip into a fluid. Then put your finger on one end of the straw and take it out of the fluid. The fluid remains in the straw. This is the equivalent to blocking the 2 inch gap. Lift your finger from the end of the straw, the fluid leaves the straw, this illustrates free venting.

The polystyrene represents the straw, the ridge vent and the 2 inch gap represents the ends of the straw. You cannot let air out of this venting system without letting in the same volume of air.
 
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Old 11-05-02, 08:08 PM
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Makes sense

Thanks that makes sense, do I need to have an airtight seal to the 2" gap or just running the venting to gap is good enough? My concern is that what happens if there is a gap between the venting and the fiberglass insulation? Is this a problem, do I need to make sure that the outside air can only go up the vent to the ridge vent or is it okay so long as it has a path?

Check out the attached .gif for clairification
 
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Old 11-05-02, 08:11 PM
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Angry opps didn't attach go here to see what I am talking about

http://www.4-id.net/Vent_graphic.gif
 
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Old 11-05-02, 08:40 PM
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Make sure the polystyrene goes all the way to the gap so you can install insulation all the way to the floor without blocking the gap. A gap in the insulation at that point may cause condensation. You want a clear unblocked path for the venting and at the same time, you don't want gaps in the insulation.
 
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Old 11-06-02, 10:57 AM
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I am in a similar situation. I ran the baffles an inch or so over the corner of the ceiling (between the floor joists) down into the soffit cavity.

Since I have mostly loose fill insulation, I raked that back and used expanding foam to fill in the gap (only a couple of inches) between the corner of the ceiling and the baffles. Once dry, I pushed the loose fill back up against this barrier.
See:
http://www.geocities.com/toddmn68/diagram.gif

Now I could have bought those paper "keeper" baffels that you fold up and place in this gap holding back the insulation from falling in the soffits, but I think that the foam creates a more efficient barrier. Like you, at this corner I only have about a two inch gap between the cold air baffle and the ceiling of the room below. I want to do everything I can to keep any warm (moist) air from downstairs from coming up and meeting that cold air flowing thru the baffle (and cause a rainstorm). The spray foam in the can is more expensive than the cardboard, but it does block these vapors, insulate, and keep the loose fill from falling in the soffits all at the same time. I know this great seal doesn't extend more than a few inches towards the center of the house, but the insulation quickly ramps to 10-12" thick as you move towards the center of the house. I feel that's enough buffer to prevent significant condensation.

I posted here to suggest this to you and for the selfish reason of validating my logic. Any thoughts anyone?

Thanks...
 
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Old 11-06-02, 12:09 PM
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I like both the diagrams that are presented here, it makes it a lot easier to visualize. The foam is also a very good idea and you would be surprized on how many structures I've inspected where condensation was a problem in that same exact area. That area is a very tight area to get to and most people don't realize the problems that they create with moisture if they don't do a good job with insulating and providing proper venting for that area.
 
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