A knee wall is a short vertical wall roughly two or three feet high that blocks in that useless triangular space.
How to build a attic knee wall.
Push the knee wall firmly into place.
Use your saw to cut along this line.
Install an air barrier on the exterior of attic knee wall insulation and to block open floor joist cavities under attic knee walls.
Ideally confirm that the marks are level with a laser level if you have one and revise if.
Install insulation without misalignments compressions gaps or voids in all knee wall cavities.
But you do so at the expense of floor space.
Nail down the sole plate the bottom of the wall in three or four places into the attic floor.
Face the knee wall with the appropriate size of drywall.
Measure and mark the rafters at each end of the attic with the desired height of the knee wall.
Step 1 measure the attic to build your walls to the correct length measure the angle of the rafter by laying a piece of 2x4 inch lumber against the side of the rafter in a vertical position.
Mark the line that the angle forms along your 2x4.