A knee wall attic is the unconditioned space located behind a short, vertical wall (a "knee wall") built under a sloped roof, often adjacent to a finished or conditioned living area.
Understanding the Knee Wall Attic
Imagine a room built into an attic space, often on an upper floor with a sloped ceiling. A knee wall is the short, vertical wall that forms the outer boundary of this finished room where the sloped ceiling meets the floor. The space behind this knee wall, between the wall and the exterior roofline, is the knee wall attic.
According to common building terminology, an attic knee wall is a vertical separation between attic space on one side and conditioned space on the other side. This definition highlights that the knee wall itself acts as the barrier between the heated/cooled interior room and the unconditioned attic area. The knee wall attic refers to the unconditioned space defined by this wall.
This space is typically unheated and uncooled, similar to a main attic, and is subject to outdoor temperature variations.
Why Do Knee Wall Attics Exist?
Knee wall attics are created when sloped rooflines intersect with vertical walls during construction, particularly in homes with:
- Finished attic rooms or bonus rooms
- Dormers
- Houses with upper stories built directly under the roof structure
They allow builders to maximize usable living space while navigating the geometry of the roof structure.
Common Issues with Knee Wall Attics
While creating usable space, knee wall attics can pose significant challenges for energy efficiency and comfort if not properly addressed. Common problems include:
- Poor Insulation: The knee wall itself and the roof deck or floor joists bordering the space may be inadequately insulated.
- Air Leaks: Gaps around the knee wall, at the floor, or ceiling allow unconditioned air to infiltrate the conditioned space, leading to drafts and higher energy bills.
- Temperature Extremes: The knee wall attic space can become extremely hot in summer and cold in winter, making the adjacent room uncomfortable.
- Moisture Problems: Poor ventilation and air sealing can lead to condensation and potential mold growth within the knee wall attic space.
Improving Performance: Insulation and Air Sealing
To ensure comfort and energy efficiency in adjacent rooms, it's crucial to properly air seal and insulate the knee wall attic assembly. Key strategies include:
- Air Sealing the Knee Wall: Seal all gaps and cracks in the knee wall structure, especially at the top and bottom plates, around electrical outlets, and where different materials meet.
- Insulating the Knee Wall: Add appropriate insulation (such as batts, rigid foam, or spray foam) to the knee wall cavity. Ensure a complete thermal barrier.
- Insulating the Sloped Ceiling/Roof Deck: The section of the sloped roof above the knee wall attic space often needs insulation and ventilation strategies similar to a cathedral ceiling.
- Insulating the Floor: If there is a floor between the knee wall attic and a space below (like a garage), it should be insulated.
- Creating a Barrier: Install a rigid air barrier (like drywall or rigid foam) on the attic side of the knee wall over the insulation to prevent air movement through the insulation.
- Ventilation: Ensure proper ventilation in the knee wall attic space itself to prevent moisture buildup, especially if the roof deck above it is not insulated directly.
Addressing these areas is vital for maintaining consistent temperatures, reducing energy consumption, and preventing moisture-related issues in rooms adjacent to the knee wall attic.
