Builders frequently use knee walls to create low cost living space under the roof rafters. Unfortunately, these knee walls are rarely sheathed on the back side, which can create comfort and performance problems. The comfort problems stem from uncontrolled air leakage. Air that blows in through the soffit vents can cause drafts around outlets, access doors, and at the floor-to-wall intersection. Performance problems occur because the air that moves through the wall degrades the performance of the insulation, which can lead to condensation on the drywall surface. In the winter, this air movement can cool the drywall to the point where condensation can occur, which could lead to mold growth. In hot-humid climates, this air movement can cause warm, humid air to come in contact with the backside of the drywall which is cooled by the air conditioning in the house, also potentially leading to condensation and eventually mold growth. Builders generally prefer to use soffit vents to meet code-required attic ventilation, so the knee wall needs to be constructed to eliminate the possibility of air moving through it.
One strategy is to encapsulate the insulation on all sides by installing an airtight layer of rigid sheathing on the backside of the knee wall. The other is to move the insulation layer to the roof plane, thereby putting the knee wall completely within conditioned space. If insulating the knee wall, the floor system must be insulated and air sealed, so wind does not blow through the floor system as well. The figures below detail these two ways to improve control of air movement in knee wall construction. Planning for frequent inspections during construction to verify the proper sequence is being followed is also important to ensure proper implementation. Download a PDF of the following two best practice details.
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This unsheathed wall is exposed to temperature conditions and air movement from the attic.
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