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Old 01-04-2012, 10:14 AM   #1
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new wall insulation question


is it necessary to remove existing silver foil fiberglass insulation anchored to concrete when building new basement wall. can i keep there, build the new wall against the old insulation, then put a new insulation between all studs? i figure this will have combine R value of both insulation.

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Old 01-04-2012, 10:49 AM   #2
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new wall insulation question


The insulation never should have been glued to the foundation in the first place and it's backwards if the vaper barrier is on that side.

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Old 01-04-2012, 11:17 AM   #3
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The insulation never should have been glued to the foundation in the first place and it's backwards if the vaper barrier is on that side.
it was anchor to the foundation using concrete screws. the silver foil(not the vapor barrier, since it can pass through air) is facing toward inside the house. this was done in a Unfinish basement. however i'm planning to build a finished basement.

so order from fundation to wall
1. concrete fundation
2. old/ existing fiber glass with reflective cover point toward the room.
3. I will add new plastic vapor barrier between 2&4.
4. wall & stud
5. new insulation with barrier porint toward the room.

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Old 01-04-2012, 11:21 AM   #4
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new wall insulation question


With 2 layers of insulation, I'm not sure how the vapor barrier should be. Commonly, it would go on the other side of the studs than your drawing. This can depend on your location though. Where are you?
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Old 01-04-2012, 04:52 PM   #5
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new wall insulation question


Remove that existing insulation. Place at least 1" of XPS (I recommend 2" for better vapor retarding capacity) against the foundation wall and then put up your stud wall. Only use un-faced batts within the stud bays and DO NOT put a vapor barrier on the inside face of the studs.
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Old 01-05-2012, 10:30 AM   #6
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Remove that existing insulation. Place at least 1" of XPS (I recommend 2" for better vapor retarding capacity) against the foundation wall and then put up your stud wall. Only use un-faced batts within the stud bays and DO NOT put a vapor barrier on the inside face of the studs.
i can understand the vapor barrier, but is it necessary to remove all existing insulation then put up new foam one against the concrete? would it be ok to romove or keep the existing insulation, then build wall 1" from foundation?

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Old 01-05-2012, 10:46 AM   #7
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new wall insulation question


I cannot recommend keeping fiberglass against concrete...
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Old 01-05-2012, 12:54 PM   #8
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I cannot recommend keeping fiberglass against concrete...
is it because fiberglass doesn't keep moister from concrete to stud? but if i put a vapor barrier both side of the wall, wouldn't it solve the issues?

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Old 01-05-2012, 02:16 PM   #9
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new wall insulation question


It is impossible to 100% seal the fiberglass insulation with poly on both sides. Nails for trim, screws for drywall, the un-noticed little gap or tear. We used to use fiberglass in basements just like in the main floor of a house. It was wronge.

A good article http://www.buildingscience.com/docum...lation-systems

Look at this other recent thread on insulation and insulating basements. lots of good info.
EPS or XPS

Last edited by mae-ling; 01-05-2012 at 02:30 PM.
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Old 01-05-2012, 02:57 PM   #10
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Originally Posted by AGWhitehouse View Post
Remove that existing insulation. Place at least 1" of XPS (I recommend 2" for better vapor retarding capacity) against the foundation wall and then put up your stud wall. Only use un-faced batts within the stud bays and DO NOT put a vapor barrier on the inside face of the studs.
I am in Minesota and doing exactly what AG recommends. I am using 1" board and will monitor the humidity level, making sure it is low enough in the summer. I saw on one of the Building Sciences links from this board that you should keep the humidy level below 50% if you use less than a 2" board.

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