Insulate Attic Knee Wall Spray Foam Cellulose Ventilation

Knee walls are used to close off space in the attic where the height is too short to use as room floor space due to slanting roofs. Knee wall spaces may be used for storage, ductwork, or furnaces, and can be vented or unvented.

The attic floor of a knee wall consists of the horizontal (flat) portions. The sloped portions are insulated similarly to cathedral ceilings (enclosed framing assemblies, vented or unvented).

The knee wall includes the sloped ceiling, vented or sealed roof space. The attic usually stays vented using cross gable vents and a power fan to draw cool air from the soffits across the roof deck to cool sunlight-heated shingles in summer and move out warm air in winter to prevent condensation/mold in the rafters. Vented rafters prevent insulation from blocking airflow, allowing fresh outside air to move through the assembly in all seasons.

Option #1 Insulate Knee Wall Slopes

Cathedralize the knee wall space if the storage area is conditioned. In a cathedralized assembly, air seal the knee walls along the roofline and insulate the rafters with cellulose.

Cathedralize the knee wall space if ductwork is located on the attic floor to bring the ducts within the conditioned envelope.

Cathedralize the knee wall space if there is no passive ventilation available. Continue the insulation from the attic down to the floor through the rafter. Add rigid foam board and seal seams for additional insulation (R-value) and a vapor barrier, or install drywall for a lower R-value with a vapor and fire barrier (intumescent barrier).

Option #2 Insulated Knee Wall

Insulate the knee wall space when the storage area is not used. In this approach, air seal the knee wall framing and insulate the vertical knee wall to separate the conditioned living space from the unconditioned attic area.

Hot in the summer / Cold in the winter.

Knee walls are often found in Cape Cod–style homes or bungalow-style homes in Chicago. They are vertical surfaces that typically use drywall or paneling to create a finished look inside a room. These rooms are often used as bedrooms around Chicago, and sometimes as a home office, playroom, or workout space.

The knee wall is sometimes load-bearing and helps support the horizontal sloped rafters. A purlin is a horizontal beam that runs along the length of a roof, resting on the main rafter and supporting the common rafter boards above the knee wall.

This assembly makes it difficult to cool the space in summer and increases heat loss in winter, which can lead to mold and condensation in the rafters. It can also create a colder, drafty first floor during winter due to heat escaping faster than in traditional attics.

In either method of attic insulation, it is essential to air seal the seams of the interior/exterior framing (see photo) to prevent drafts and air movement between the interior conditioned air and exterior weatherized air.  

Knee Wall Ventilation

If there are no soffits, the knee wall area may breathe, and attic ventilation is provided through cross gable vents (passive or active with fan motor). Ventilation can help prevent stagnant air buildup; however, a healthy, clean storage space can be damaged if excess humidity and moisture are introduced to organic building materials such as wood.

Sometimes the knee wall area of a second floor is not vented; however, ventilation baffles (air chutes) may be installed under the roof deck insulation to feed fresh air intake to roof and attic power vents or passive vents.

Related article: Knee Wall Insulation Tips-All You Need To Know