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Community and Q&A

IRC 316.4 – Code interpretation on the use of rigid foam as air barrier on backside of attic knee wall

rocket190 | Posted in Building Code Questions on

My insulation plans for my attic kneewalls were to install two layers of 2″ eps with staggered and taped seams, then install damp sprayed cellulose in the 2×4 stud cavity, followed by 1/2″ drywall. I believe that a thermal barrier is required since I have usable attic space, but I am unclear whether this thermal barrier must be installed on both sides of the stud wall, or only the portion of the stud wall that faces the interior of the attic room. The gray area is whether or not the areas behind the stud wall are considered the “interior of the building”.

Per IRC , R316.4 Thermal barrier.
Unless otherwise allowed in Section R316.5 or Section R316.6, foam plastic shall be separated from the interior of a building by an approved thermal barrier of minimum 1/2 inch (12.7 mm) gypsum wallboard or a material that is tested in accordance with and meets the acceptance criteria of both the Temperature Transmission Fire Test and the Integrity Fire Test of NFPA 275.

The kneewall areas behind the attic will truly be inaccessible. I was not planning on access hatches as I won’t have plumbing in any of the knee walls. There will be outlets approx every 6′, but nothing else that would be a fire hazard. I would consider running all wiring in 1/2″ emt if there is a fire risk from the NM wire, but as far as I am aware, there are no issues with romex type wiring in an attic.


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  1. GBA Editor
    Martin Holladay | | #1

    If the rigid foam on the back side of a kneewall faces a triangular attic that can't be accessed, you don't have to put a thermal barrier or an ignition barrier on the attic side of the rigid foam.

    For more information on these issues, see Thermal Barriers and Ignition Barriers for Spray Foam.

  2. rocket190 | | #2

    Yes, that would be the scenario. The backside will be inaccessible. I don't see the need to ever access that area unless something catostrophic happens.

    Thanks for the assistance.

  3. rocket190 | | #3

    Martin, if I may pick your brain on this point....

    Is the intent of the ignition barrier/thermal barrier have anything to do with the risk of a fire starting within the wall cavity, i.e. electrical malfunction?


    Is it to protect from an "external" occupant caused fire? I.E. Candle tipped over, kid playing with matches, cooking fire, etc

  4. GBA Editor
    Martin Holladay | | #4

    The idea of the code provision is to reduce the risk of residential fires. Evidently the code writers have decided that the risk is substantially reduced if there is a layer of 1/2-inch gypsum wallboard between the occupants and the foam.

  5. Richard Beyer | | #5

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