Knee doors are an ideal way to make better use of the space in your home. The space behind the knee wall, mostly remains unused. With the installation of a knee door, the space behind the knee wall can become an ideal storage space for objects. A knee door has a fire resistance rating of EI 30. EI is the ability of a knee door as a partition wall to meet certain requirements during a fire. Fire resistance is expressed in minutes. Ud - flap centre factor.