The best materials for the attic and thus the knee wall are spray foam fiberglass and cellulose.
Insulate attic knee walls.
This method can be made to work but the necessary air sealing details are demanding and fussy.
Knee walls are found in many older houses where the ceiling on the top floor is an attic or.
They are short walls usually no more than 3 feet high used to support the rafters in roof construction with attic space behind them.
If you plan to insulate the roof deck of your attic then you would use spray foam or fiberglass batts.
Insulating only the attic roof rafters and the walls at the end of the attic is one method some use to insulate the knee wall area underneath but carson dunlop author of principles of home.
One of the best methods is rigid foam board with sealed joints over insulation in the knee wall.
Insulating knee walls other spaces in the attic.
This is not very easy in most of the older houses we work on because the hatches and attics are too small to get rigid foam board through.
The traditional approach is to insulate the kneewall and the attic floor behind the kneewall.
When it comes to insulating knee walls you will want to insulate this space using the same material and method as the rest of your attic.
A better approach is to insulate the roof slope above the attic.