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Old 08-03-2012, 06:20 PM   #1
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Vapor barrier question does extruded foamboard breathe?


I'm finishing a basement and kneewalls in my 2 year old home in central Minnesota. I'm getting different suggestions on if I should add a poly vapor barrier and if so where in the order of construction?

Here is the order of construction: Water proofing membrane, block wall, 1.5" foil faced Thermax board glued to block wall (foil side to inside of house), all seams and edges taped with foil tape, 2x2 furring strips with 1.5" extruded foamboard between furring strips, drywall, paint.

I don't think I want to add a poly vapor barrier because my Thermax foil is my vapor barrier, but will I have condensation problems with the Thermax board up against the Extruded foamboard? Is this a common practice? I think I would want more insulation than just the 9.8 R value of the Thermax so thats why I would like to add the 1.5" extruded foamboard. My other option is to just leave a air gap and no extruded foam between furring strips Not sure what that does with the R value in that case.

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Old 08-04-2012, 10:42 AM   #2
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Vapor barrier question does extruded foamboard breathe?


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Originally Posted by crankcase View Post
I'm finishing a basement and kneewalls in my 2 year old home in central Minnesota. I'm getting different suggestions on if I should add a poly vapor barrier and if so where in the order of construction?

Here is the order of construction: Water proofing membrane, block wall, 1.5" foil faced Thermax board glued to block wall (foil side to inside of house), all seams and edges taped with foil tape, 2x2 furring strips with 1.5" extruded foamboard between furring strips, drywall, paint.

I don't think I want to add a poly vapor barrier because my Thermax foil is my vapor barrier, but will I have condensation problems with the Thermax board up against the Extruded foamboard? Is this a common practice? I think I would want more insulation than just the 9.8 R value of the Thermax so thats why I would like to add the 1.5" extruded foamboard. My other option is to just leave a air gap and no extruded foam between furring strips Not sure what that does with the R value in that case.
You are correct. No VB needed.

Might want to go 2" on the Thermax and 1" on the XPS. If you have any moisture issues, XPS is a better against the wall.

Use the XPS in the airspace. Air is okay but XPS is a better insulator.

Make sure the drywall is air tight and sealed and you will not have any issues.

Be sure to seal the bands/sill prior to insulating.

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Old 08-05-2012, 05:37 PM   #3
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Vapor barrier question does extruded foamboard breathe?


I agree, Case 5, pp.53: http://www.buildingscience.com/docum...study-analysis

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