What is considered the worst underlayment for bathroom tile flooring?

The worst underlayment for bathroom tile flooring is 1/4" particle board, primarily due to its susceptibility to moisture. Bathrooms are prone to high humidity and occasional water spills, and particle board is not designed to withstand such conditions. When it absorbs moisture, it can swell, warp, or break down, which compromises the integrity of the tile installation above it.

In contrast, materials like cement board are specifically engineered for wet areas. Cement board is water-resistant and provides a solid, stable base that is less likely to deteriorate when exposed to moisture. Plywood and OSB also have certain structural advantages, although they are not as ideal as cement board for tile installations. Thus, the choice of underlayment directly impacts the durability and longevity of the tiled surface in a bathroom setting, making particle board the least suitable option.