In terms of tile underlayment materials and construction, you’ll hear a lot about “paper” and “felt,” but most often, these are used interchangeably and referred to as “organic felt paper.”
But felt isn’t the only material you’ll run into: fiberglass mesh may be substituted for this more traditional material. That stated, underlayment materials are regularly categorized by weight.
For example, “90-pound paper” describes a cheap, low-quality gravel-coated material to be used only if there is nothing else around.
A better choice would be a product that consists of 2 layers of 30-pound, asphalt-impregnated felt. This material is likely to last from 20 to 30 years. Upgrade to a double layer of heavy-duty, SBS-modified asphalt underlayment and you’re going to be set for the long haul.