Threadcount is only part of the story when considering a fine downproof fabric for your down comforters. Even more important is the type and size of yarn, and the density with which it is woven (different than threadcount). (plumeriabay.com)

This 30 denier 1.1 oz/sq yd downproof nylon ripstop is breathable and extremely lightweight. It is a high quality choice for shells, ultralite quilts and sleeping bags. (thru-hiker.com)

Placing reliance on thread count as an indicator of the downproof quality of fabric is a mistake. What is important is the type and size of yarn, the density with which it is woven (which is a different issue than thread count), and the specialized finishing techniques needed to insure the fabric's downproof qualities. (DeWoolfson Down)