Rigid Foam Insulation for attic


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Old 07-26-12, 05:21 PM
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Rigid Foam Insulation for attic

I would like some professional opinions.

I would like to use some rigid foam insulation in the attic on the tops of the exterior wall's studs. I'm thinking 2 or 3 inches thick and 2 feet long starting from the exterior wall's top edge. I would leave space so air can travel from the soffit vents into the attic itself.

I hear that you need to install fiberglass bats with paper side down, due to moisture reasons, will my foam cause a moisture problem too? I'm just placing a 12 x 12 x 3 inch block directly on top of the ceiling drywall.
 
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Old 07-26-12, 07:23 PM
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Have you checked into the cost of XPS lately?? It is really expensive in comparison to conventional insulation methods. Fiberglas batts vapor barrier down toward living space. Blown in insulation offers qualities for much lesser price as well. You mention you will only be placing this on the top of the studs. Isn't there a top plate on the studs?? Why would you install XPS on top of them?
 
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Old 07-26-12, 07:39 PM
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Sorry, I"m a real amateur. Ya, there is a piece of wood over the studs(top plate) as you said. I would be putting the foam over the top plate since fiberglass bats will block the air from enter the soffit intot he attic. My contractor said if you can't see light from inside the attic, then there is not much air entering the soffits. I just want to avoid using baffles. So I would be using the foam near the soffits and fiberglass bats on the rest of the attic. I do not like blown-in insulation since I go up a lot still to work on the network wiring and adjusting the antenna. I am installing an amplifier next when I get a chance to buy one. I dont' want to accidently step through the blown-in insulation and through the ceiling, otherwise I have thought about the blown-in stuff.
 
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Old 07-26-12, 09:55 PM
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I would be putting the foam over the top plate since fiberglass bats will block the air from enter the soffit intot he attic.
What would installing the XPS do that stapling some hardware cloth or chicken wire to the underside of the rafters, beginning where they land on the top plate, would not do more effectively and less expensively?
 
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Old 07-27-12, 02:40 AM
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Why do you want to avoid using a product made to aid in accomplishing the task at hand?? Baffles are simple to install and provide air from the soffits to the ridge. I'm not sure what you want to accomplish above and beyond that.
 
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Old 07-30-12, 08:19 PM
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You are on the right track, congrats! Canned foam them to the ceiling joists, tight to the drywall, stopping air/diffusion from below: http://www.buildingscience.com/docum...n-roof-venting

Best to use foil-faced poliso board, no moisture problems and acceptable for attic use without an ignition/thermal barrier. Many foams don't require them in attic applications, check locally with AHJ. UMass Amherst: Building and Construction Technology » Reroofing and Residing To Save Energy 3"= R-18 vs. R-11

Hard to get substantial R-value with only a few inches of room. Air seal, canned foam any wiring/plumbing holes first; I'm lazy, LOL: #3---- http://www.doityourself.com/forum/ba...ml#post1990328

Gary
 
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Old 08-02-12, 12:08 PM
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Do you have vents in both soffits and ridge?
Do you know that loft ventilation can mean that the passing wind pulls your expensive warm air out of your home?
Have you filled/sealed all the cracks and gaps in your home?
Especially those in the floors and ceilings.
 
 

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