By Brad Oberg
Knee walls, which establish the minimum interior height of a bonus room, are the short walls that are formed by the intersection of a sloped roof and a vertical wall. These walls separate the conditioned interior space and the unconditioned attic space.
But despite the common use of knee walls in attics, as well as in bonus rooms and mechanical rooms, they are rarely insulated properly -- a situation that can result in cold spots and more serious thermal performance issues. Because their exterior surfaces ares exposed to the attic conditions and their interior surfaces to the conditioned room, knee walls are exposed to even more extremes of temperature and humidity than exterior walls (see Figure 2).Therefore, the methods of insulating a knee wall should mirror those used to insulate an exterior wall, althought they don't require the weathering surface.
A properly insulated knee wall must be airtight and well insulated. For highest performance, install rigid foam insulated sheathing on the backside of the knee wall (Figure 3). Alternative materials for the backside of the wall include housewrap or thin profile sheathing. Seal all edges, joints, and penetrations per manufacturer’s recommended installation instructions.
In addition to the sheathing, fill the wall cavities with a high-performance batt insulation designed for the depth of the cavity. Remember that compressing insulation to fit into a space will reduce its R-value. You may need to increase framing dimensions to accommodate the insulation.
A knee wall that is not airtight on the back side is an invitation for thermal problems. Air driven by wind or temperature differences overpowers the insulation’s resistance to air motion and moves through the insulation, which reduces its R-value. Additionally, the air movement through the walls may also cause condensation at the drywall surfaces.
In the winter, this air movement can cool the interior drywall surface to the point where condensation occurs, leading to deterioration of the drywall or promoting mold growth. In hot, humid climates, warm, humid air can come in contact with the backside of the drywall that is cooled by the air conditioning of the house, another formula for condensation and promoting mold growth.
Insulation is only part of the story, though. Electrical, plumbing and mechanical penetrations to adjacent conditioned spaces should be sealed to reduce the overall infiltration rate.
Special care should be taken when attic walls terminate over garages and unconditioned spaces to seal the top and bottom plates, as well as any other penetrations that could let poor quality air into the bonus room (Figure 4). Make sure that floor segments over unconditioned spaces are insulated as well.
Brad Oberg is co-founder and chief technology officer of IBACOS,a building-science company based in Pittsburgh. He has directed extensive research into the integration of ventilation strategies, residential ductwork design and performance, improved residential airtightness approaches, and durable construction approaches.
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