another vapor barrier question

[billconner]

My current project converting garage to habitable space. Because of 12" ICF stem walls, building an inner wall so cavity is about 12". Existing "outside" wall has craft paper faced fiberglass. I plan 8" batts in inner wall with a 6 mil poly vapor/air barrier on inside. Can I leave the craft paper? Doesn't seem with poly it matters. I could slash it but feels a little silly. I should add the craft paper is not stapled - either face or inside studs to please the drywallers - but simply friction fit. Lots of yellow visible.

With a much more complete and sealed vapor barrier on the interior side, is it a double vapor barrier problem?

 

[The Cobbler]

We no longer use Kraftpaper backed insulation here , but my understanding is you slash the paper anytime you're adding insulation

 

[billconner]

Thank. I would buy it but it's here. Want to suggest a slashing quantity? Maybe a diagonal nearly edge to edge every foot or two?

 

[kbs2244]

I would slash it to avoid trapping moisture in the wall

 

[billconner]

yup. I'll slash it.

 

[Kpaige]

Never have two vapor barriers slash it.

 

[billconner]

the craft paper is already not a good vapor barrier - 9' studs so it's pieces and not stapled at all. Many gaps already in paper.

 

[billconner]

Good reading on vapor barriers - and proper labels.

BSD-106: Understanding Vapor Barriers | buildingscience.com

buildingscience.com

BSI-092: Doubling Down—How Come Double Vapor Barriers Work? | buildingscience.com

buildingscience.com

The double vapir barriers that are fine doesn't apply to me, and craft paper is "smart". Trying to figure out if that applies to me.