# condensation behind vapor barrier of walkout basement wall



## gregzoll (Dec 25, 2006)

It is because they are not breathing. The basement walls need to breathe, which means getting rid of the plastic sheathing, so that the system works properly. In other words, the insulation and building materials are not working as a whole to help keep the space warm.


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## fusion_ta66 (Jul 24, 2012)

Thanks for your reply. So if I remove the sheeting, what is to prevent condensation build up inside the wall when warm, moist air from inside meets the cold outside wall in the winter? I thought that was the point of a vapor barrier?


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## gregzoll (Dec 25, 2006)

Read the info over at buildingscience.com and it will explain further about how to properly insulate and maintain a conditioned barrier. A vapor barrier can be anything. It can be XPS, Kraft Faced Fiberglass, plastic sheathing, Tyvek, Drywall, etc..


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## trauma2 (Jul 25, 2012)

The poly on your basement walls is there to prevent moisture from the exterior to enter the living space. The poly should be to grade ( ground ) and run under the bottom plate of the framed wall next to the block wall. Concrete is very permeable to moisture and thus needs a moisture barrier if you will use your basement as living space. Also Not everything is a vapor barrier ie Tyvek ( this is a wind barrier but allows moisture to easily pass).


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## gregzoll (Dec 25, 2006)

trauma2, no tyvek is not a wind barrier. And Placing a plastic shield that does not breath is a recipe for disaster. That is why it is no longer a good reason to use, due to you want the walls to breath in a way, that moisture will not congregate and build up inside the walls.

Following the info at buildingscience.com is about as best as good info out there, due to it explains how the structure should work as a whole.


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## trauma2 (Jul 25, 2012)

Gregzoll
Please read the label on Tyvec. it is not a vapor barrier, it is an air infiltration barrier. If the interior poly has moisture behind it then there are two choices. 1 to control moisture on the outside by diverting water away from foundation. 2 - Take down the poly and dry the walls with fresh air or dehumidifier the use waterproof paint on the block. Then put the poly back and insulate.
Framed exterior walls Require a impervious vapor barrier on the warm side of the wall and a pervious material on the cold side( exterior)


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## gregzoll (Dec 25, 2006)

trauma2, suggest you go through the info at buildingscience.com and read the info from 3m on their Tyvek product. And no, using a imervious vapor barrier inside the home is a old way of thinking, that has been thrown out in the past ten years, due to it was found to not work, as some off the hip engineeer & architects thought that it would.

If you go through this board, you will find that everything you just stated in your two posts has been shot down as FUD, due to the info stated over and over by one of our regular's, who does this stuff for a living, will tell you that impervious plastic vapor barriers are crap and need to go away.


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## fusion_ta66 (Jul 24, 2012)

There is no cement or concrete behind the wall in question. It is 100% above ground.


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## gregzoll (Dec 25, 2006)

Then having the vapor barrier inside is a problem. That means that your home is not breathing as it should, to not allow moisture to build up inside the walls.


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## Gary in WA (Mar 11, 2009)

For your location, poly (plastic) is not recommended, above or below grade; pp. 18, Fig. 5: www.buildingscience.com/documents/reports/rr-0410-vapor-barriers-and-wall-design

You are seeing the outside air condensing on the poly which should be removed and replaced with Membrain* or asphalt paper-faced batts, above grade. No poly below grade. 

Tyvek is a water resistive barrier. If the laps are taped it is an air barrier, if not, the bulk of it is a wind barrier (except the laps) with a cladding over it. It has a permeance rating of 58, very open; to allow moisture to pass through in either direction. 

Gary


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