Watch the video on Insulating Attic Knee Walls.
To increase the home's living space without increasing the size of the home, builders frequently use knee walls—short vertical walls with attic space directly behind them—to create low-cost living space in the attic. While knee walls are often insulated, they are, unfortunately, rarely sheathed on the back side, which can create comfort and performance problems. Knee walls that are properly insulated, sheathed, and sealed, will help to create a comfortable environment in the attic that can add valuable space in the home.
The comfort problems related to not using sheathing 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.
The best practice is to encapsulate the insulation on all sides by installing an airtight layer of rigid sheathing on the attic-side of the knee wall and then to seal the perimeter of the sheathing. Follow these steps:
1. Cut and install batt insulation in the attic-side of the knee wall.
2. The interior of the knee wall will be used as living space and should be sheathed with drywall. The exterior will be exposed to unconditioned (not heated or cooled) air in the attic space and so should be sheathed with foam or some other type of sheathing.
3. Seal all perimeter joints with caulking or spray foam.
And don't forget to insulate and seal the knee wall door. A home's attic access, like a knee wall door, often goes uninsulated. This gap in the attic insulation increases heat loss in the winter and heat gain in the summer. Knee wall doors also often aren't sealed properly. A 1/4-inch gap around the perimeter of an attic access can potentially leak the same amount of air supplied by a typical bedroom heating duct. Knee wall doors need to be weatherstripped. They also need a latch that pulls tightly against the weatherstripping and the frame to achieve a solid seal.
The Department of Energy (DOE) recommends using construction adhesive and screws to attach rigid foam insulation to the attic side of the door. Pay special attention to the clearance between the insulation and the door frame and air sealing details. For more details on knee wall insulation, sheathing and sealing, visit the DOE Website.
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