Vapor barrier frost


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Old 01-04-14, 07:25 PM
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Vapor barrier frost

Finishing the basement and I have one wall that is a walkout basement and is above grade. It has a half concrete wall and top half studded wall. The bottom concrete wall was framed an inch away from the concrete. I used faced r15 insulation. The temp got down to 5 degrees outside and I noticed there's frost behind the insulation on the concrete. It also happened on the exterior concrete wall in my garage. Is the faced insulation not adequate as a vapor barrier or should I install a VB over it? I'm planning on using green board in the basement.
 

Last edited by moreira85; 01-04-14 at 07:52 PM.
  #2  
Old 01-04-14, 07:58 PM
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I'll attach a link, fig 11 illustrates one approach. Basically, your insulation is keeping the heat away from the outside wall and moisture is wicking its way up from the soil below as well as leaking past the faced insulation. You need some rigid insulation against the concrete, thick enough to ensure the inside surface stays above the condensation temperature (dew point). For mass that is usually 1" or 2". Then your batt insulation can make up any additional code requirement. But an air barrier is the most important part, and a vapor barrier is optional above grade. Below grade omit the VB.

More links available as needed
Bud

BSD-103: Understanding Basements — Building Science Information
 
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Old 01-04-14, 09:15 PM
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So basically once the drywall is up it will act as an air barrier and keep the warm air from getting to the cold walls? Thanks for the link.
 
  #4  
Old 01-05-14, 04:06 AM
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But you still need the rigid insulation against the foundation to prevent moisture from wicking up, evaporating and then condensing further up where it is colder. Two problems needs two solutions.

Bud
 
 

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