Attic Knee Walls

Knee walls, the walls that separate conditioned from unconditioned space in an attic, need to be constructed to insulate and prevent air flow from the unconditioned area to the conditioned. They can be a source of significant air leakage if a continuous air barrier is not provided to prevent unconditioned air from flowing under the knee wall and under the floorboards of the attic room. There are two ways to block off this air flow: 1) a continuous air barrier can be installed on the exterior of the kneewall framing from the top of the knee wall down to the attic floor, including the spaces between the attic floor joists from the bottom of the knee wall to the ceiling deck below, or 2) a continuous air barrier can be installed along the underside of the attic roofline from the top of the knee wall to the top plate of the home’s exterior wall. With either method, the air barrier should be installed before installing attic floor insulation in the unconditioned portion of the attic.