A knee wall is short wall, perhaps a metre high or so, supporting roof rafters typically in a room with vaulted / sloped ceilings. The cavity created by this area between the knee wall and the roof rafters forms an attic space, and insulating this area is often not done properly. Knee wall attics are common in older storey and a half homes, but can be found in new homes too.
Sometimes batt insulation is installed between the roof rafters in these attic spaces, suggesting it is to be a “warm space” (see image above). As such, heating and air circulation should be provided in that space just like any other area within the home. If insulation is installed between the rafters it can also reduce the life span of the shingles and other building related materials and cause concerns, such as, moisture, mould, mildew, and fungus as there is less ventilation (and often insulation) than say with “traditional” attic spaces.
Another method of insulating this area would be to remove the insulation between the rafters and put insulation in the knee walls themselves and in the floor areas (along with appropriate vapour barriers). In this case, the knee wall area becomes a “cold space” like traditional attic areas. Proper air blocks / air barriers, for example, covering the insulation on the attic side and between the attic floor joists below the knee wall should be present. As well, the access door should be insulated and sealed appropriately. It is crucial to determine if the knee wall attic space will be a warm or cold space, and to insulate it appropriately. Unfortunately, far too often we see these areas improperly insulated which can create issues with condensation, wood rot and mould.