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Fact Check – Insulating a knee wall in a vented attic- Zone 4a

BarnyardBuilder | Posted in General Questions on

For an attic to bedroom conversion in vented attic,  Im planning on insulating my truss/knee wall 2×4 16 oc  bays with r16 rock wool batts and rigid polyiso foam.
Prior to install of the rock wool, I will install 2″ rigid foam with foil backing against the exterior side (facing attic space) with foil facing cold side and then foam air sealed.  This will gain a overall r value of 26. The warm side of the wall will be rocked with 1/2 inch drywall.

Question:  Will this work? From what Ive read on this forum, I think it will but I still get a bit confused with so much good info.

Thanks in advance.

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  1. GBA Editor
    Martin Holladay | | #1

    That will work. As with any insulation job, pay attention to airtightness.

    For more information, see this article: “Two Ways to Insulate Attic Kneewalls.”

  2. BarnyardBuilder | | #2

    Thanks Martin.

  3. BarnyardBuilder | | #3

    One step forward, 1/2 step back. Need a solution for replacing strongbacks running on the knee wall described above. An insulation contractor said they can come off. The truss manfacturer says they need to stay on. So, Im thinking do what I mentioned above but before applying the ISO on the exterior side, also install 7/16 OSB wall sheathing to replace the 2x4 strong backs. I think from all I've read, this will work.


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