The fabric isn't the same as our harnesses, it's a little different. The fabric is breathable and often it's actually the heat created between the body and the packer that makes the product feel like it isn't so breathable. Because of this we recently made changes to all styles of the PETE UnderWear™ line. We removed two regular fabric panels and replaced them with two soft light weight mesh panels. This facilitates better body heat release, by allowing heat to flow around the packer and out, making it more breathable.
I hope this helps answer you question.
Also, Kindred is correct. We are very excited to be releasing a natural fiber version of the PETE UnderWear™ line later this year. Stay tuned!