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