One of the best methods is rigid foam board with sealed joints over insulation in the knee wall.
Insulate attic knee walls.
There are as many ways to do it as there are people who do it.
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.
If you plan to insulate the roof deck of your attic then you would use spray foam or fiberglass batts.
Knee walls are found in many older houses where the ceiling on the top floor is an attic or.
This method can be made to work but the necessary air sealing details are demanding and fussy.
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 re insulating an attic the knee walls are important.
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.
There are two ways to insulate triangular attics behind kneewalls.
The best materials for the attic and thus the knee wall are spray foam fiberglass and cellulose.
A better approach is to insulate the roof slope above the attic.