What type of material should never be used as building paper for sheathing protection?

Waterproof plastic is not suitable as building paper for sheathing protection due to its lack of breathability. Building paper is designed to allow moisture vapor to escape from the wall assembly while preventing liquid water from penetrating the sheathing. Traditional materials like asphalt felt, fiberglass mesh, and non-woven synthetic options strike a balance by providing water resistance while remaining permeable enough to let moisture escape.

Using waterproof plastic can trap moisture within the wall cavity, leading to potential issues like mold growth, wood rot, and deterioration of the sheathing over time. Therefore, for effective moisture management and to maintain the integrity of the building envelope, materials that allow for vapor transmission are preferred.