Fiberglass and cellulose insulation do not stop cold air.
Floor cavities run under the entire floor.
Exterior walls can stop cold air. But what happens when you have an interior wall where one side is conditioned and the other side is unconditioned? See the picture below:
What we see here is a wall cavity and a floor cavity. On the other side of the wall cavity is a wall. The cold or hot air that hits the insulation pass through but is stopped by the drywall on the other side. The wall cavity should be covered with either Tyvek or foamboard to stop the damage to the insulation from being hit by air.
The floor cavity has nothing on the other side but more floor cavity. Cold or hot air pass through the fiberglass insulation stuffed in the cavity and passes through the entire floor (that can be under your bedroom, office, closet, etc). This is what makes knee walls especially cold in the winter. The solution is to bay block. Take foamboard and seal off the floor cavity to stop air from passing under the entire floor, as you can see below.
Remember, bay blocking is a form of air sealing. Stopping air from entering or leaving the home can be dangerous if you air seal too much. At Sellair, we make sure to air seal the proper amount to keep you safe, energy efficient, and comfortable.