Interfacing material for a PV material

[McMurdo]

For PV I would say the interfacing is not needed, as they can be washed in a machine. None of my PV kilts has it.

[ChattanCat]

For PV I would say the interfacing is not needed, as they can be washed in a machine. None of my PV kilts has it.

Interesting. So why do wool kilts need the interfacing? I assumed with the lighter weight PV, you would need more.