Impermeable means that stuff can't go through it.
Wax is impermeable to water. Paper towels are not.
Glass is impermeable to air. Mesh cloth is not.
Some things are impermeable to one thing but not another.
Clear plastic is impermeable to water and air, but not light.
Lead metal is impermeable to x-rays, but not heat.
Which layer is impermeable depends on what it is impermeable to.