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. Here are the ways to use the knee wall. Storage, ductwork, furnace, vented, or unvented.
The attic floor of a knee wall is the horizontal ( flat ) portions. The horizontal (slopped ) portions are insulated similar to cathedral ceilings ( enclosed framing assembly 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, and fresh outside air in all seasons will also move through the rafters.
Option #1 Insulate Knee Wall Slopes
Cathedralize the knee wall space if the storage area is conditioned. The cathedral option: air seal knee walls along the roofline and insulate the rafters with cellulose.
Cathedralize the knee wall space if ductwork is on the attic floor.
Option to cathedralize the knee wall space if there is no passive ventilation for the space. 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 vapor and fire barrier ( intumescent barrier).
Option #2 Insulated Knee Wall
Insulate the knee wall space if a storage area is not used. The cathedral option: air seal knee walls along the framing and insulate the vertical knee wall.
Hot in the summer / Cold in the winter.
Knee walls are often found in Cape Cod-style homes or Bungalow ( Chicago ) style homes. 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 home office playroom or workout space.
The knee wall is sometimes load-bearing to support the horizontal sloped rafters. The purlin is a horizontal beam along the length or a roof, resting on main rafter and supporting the common rafter boards such as a knee wall.
This assembly makes it hard to cool in summer and hard to keep from wasting heat and creating mold and condensation in the rafters from heat escape in winter. This also creates a drafty colder first floor in winter due to heat escape at a faster rate than 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 prevent a space from being dry. A healthy clean storage space can be destroyed by introducing humidity and moisture to the organic building materials such as wood with ventilation.
Sometimes the knee wall area of a second floor will not be a vented, however, ventilation through baffles (air chutes) may be placed under the roof deck insulation to feed fresh air intake to the top of the roof / attic area power vents and passive vents.
Related article: Knee Wall Insulation Tips-All You Need To Know
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