Attic Knee Walls

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This project is much easier during construction, but can also be completed as a retrofit. If you have more than one of these spaces, I’d consider hiring a Pro for this job, trust me I’ve done it before. As with all seal and insulate projects, it is critical not to skip any steps. Heat travels the path of least resistance, you must seal all of the paths to make the impact. A one inch hole will transmit about the same heat as a 4×8 sheet of drywall.

A typical attic knee wall installed without an exterior air barrier is about 25% effective. If you enclose that R-13 insulation with 1-inch foam board, you’ll go from R-3 to R-18, and that’s a difference you’ll feel.
R-13 x 25% = R-3
R-13 + R-5 = R-18
R-values in this calculation are not taking into account framing, installation grade, etc…
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In this image you can see all of the HOT yellow spots in this bedroom built over a garage.
Due to the improper installation of the insulation and the complete lack of an air barrier, there were many areas of the wall over 90 degrees. Only the pillows on the bed were at the thermostat setting of 72 degrees.
The lower wall sections that are yellow, were from the insulation not touching the drywall.
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Also, don’t forget about those small doors, you’ll need to seal and insulate them to the same level as the wall.
The yellow around the small door at the bottom is attic air leaking into the home.
The yellow spot on the wall is where insulation fell, a typical problem when insulation is not enclosed with an air barrier.

From Building America Solution Center

Knee walls, the walls that separate conditioned from unconditioned space in an attic, 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 floor boards 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. We prefer the first method for a few reasons; it is the only choice when installed over a garage or other unconditioned space and you rarely can get enough insulation installed properly along the roof line to meet your insulation needs.

An air barrier is defined as any durable, solid material that blocks air flow between conditioned space and unconditioned space, including necessary sealing to block excessive air flow at edges and seams and adequate support to resist positive and negative pressures without displacement or damage. Air barrier material can include thin sheet goods such as rigid insulation, dry wall (though we don’t recommend this, due to the potential of moisture in the attic) , OSB, plywood, or rolled batt insulation that is covered with spray foam. These materials may be installed by insulators, framers, or drywallers. This task should be included in the contract for the appropriate trade depending on the workflow at the specific job site.

Air barrier effectiveness is measured at the whole-house level. High-performance branding programs and the 2009 IECC require that builders meet specified infiltration rates at the whole-house level.

How to Air Seal Knee Walls along the Roofline

  1. Insulate and air seal the ceiling of the attic room.
  2. Continue the insulation along the roofline to the roof edge (Figure 1).
  3. Cover the insulation with a sheet material (drywall or rigid foam insulation) that is caulked where it meets the plywood floor sheathing, which is extended to the outside wall.

one way to air seal and insulate kneewalls – add insulation and a rigid air barrier along roof line of unconditioned attic space outside kneewall

Figure 1 – One way to air seal and insulate kneewalls – add insulation and a rigid air barrier along roof line of unconditioned attic space outside kneewall.

How to Insulate and Air Seal Floor Joist Cavities under Knee Walls

Step 1a: Insert solid wood blocking or rigid foam board in the floor joist cavity openings under the kneewall (Figure 2). Seal the edges with a continuous bead of caulk or foam sealant.

Air seal floor joist cavities under kneewall with rigid foam, plywood or OSB caulked in place

Figure 2 – Air seal floor joist cavities under kneewalls with rigid foam, plywood, or OSB caulked in place.

-OR-

Step 1b: Stuff Floor joist cavities with rolls of fiberglass batt and cover them with spray foam to the edges (Figure 3).

Stuff cavities under kneewalls with rolls of fiberglass batt and spray foam in place

Figure 3 – Stuff cavities under kneewalls with rolls of fiberglass batt and spray foam in place.

Step 2: Apply caulk to the exterior face of the framing of the top plate, bottom plate, and framing at each side of the kneewall. Install rigid foam or another solid air barrier over the knee wall framing (Figure 4). Seal any seams in the rigid barrier with tape or caulk.

Step 3: Fill the attic floor joist bays with insulation (batt, blown, or spray foam) to meet or exceed the code minimum R-value (Figure 4).

Cover insulated kneewall with rigid foam, caulked at edges. Add attic floor insulation

Figure 4 – Cover insulated kneewall with rigid foam or other solid air barrier, caulked at edges. Add attic floor insulation.

Summary

Install an air barrier on the exterior of attic knee wall insulation and block open floor joist cavities under attic knee walls.

  • Install a top and bottom plate or blocking at the top and bottom of all knee wall cavities.
  • Install insulation without misalignments, compressions, gaps, or voids in all knee wall cavities.
  • Install a continuous air barrier on the exterior side of the attic knee wall framing with a rigid air barrier or other supporting material to prevent the knee wall cavity insulation from sagging and to create a continuous thermal barrier. Rigid air barrier material could include rigid foam insulation, drywall, plywood, or OSB, among others.
  • Seal all seams, gaps, and holes in the air barrier with caulk or foam.
  • If spray foam insulation is used for the wall cavity insulation, the spray foam can serve as the air barrier if it is at least 5.5 inches thick if open-cell or at least 1.5 inches thick if closed-cell spray foam insulation.
  • Install blocking in the joist bays below the knee walls to prevent air flow under the knee walls.