I call this tin foil on steroids because that s what it is.
Attic knee wall insulation.
Knee walls that share space with a conditioned room in your home should have a combination of both traditional insulation and foil radiant barrier insulation.
First you will want to add traditional insulation between the studs this can be batt insulation spray foam or even rigid foam insulation.
There are two ways to insulate triangular attics behind kneewalls.
How to insulate knee walls.
Rigid air barrier material could include rigid foam insulation drywall plywood or osb among others.
Any penetration between the conditioned interior air and the knee wall space needs to be sealed tight.
The fourth point is between the roof rafters over the knee wall top plate.
The third point is the knee wall and crawlspace floor drywall.
This includes built ins recessed lights vent fans duct work and electrical outlets.
A better approach is to insulate the roof slope above the attic.
This method can be made to work but the necessary air sealing details are demanding and fussy.
Then you can add atticfoil on top of the traditional insulation by stapling it to the wall studs.
Install a continuous air barrier on the exterior side of the attic knee wall framing with a rigid air barrier or other supporting material to prevent the knee wall cavity insulation from sagging and to create a continuous thermal barrier.
Knee walls have r 13 fiberglass rolls also called batts in them covered with stapled on reflective insulation.