A ply includes a fibrous nonwoven web fabric forming a foundational structure, wherein the foundational structure includes fibers forming first pores and is partially filled with particles, wherein the particles at least partially fill the first pores so as to form regions filled with particles. A ply of the type can be designed and improved such that it combine low thickness with high porosity and high thermal stability following inexpensive fabrication. The particles in the filled regions form second pores, the average diameter of the particles being greater than the average pore size of themajority of the second pores.