What is the primary purpose of building paper (asphalt felt)?

The primary purpose of building paper, often referred to as asphalt felt, is to serve as a protective layer for the sheathing of a building. When installed behind siding or roofing materials, building paper acts as a secondary barrier against wind and rain, preventing moisture from infiltrating the structure. This is crucial because moisture accumulation can lead to problems such as mold growth, wood rot, and other structural damages.

Building paper is specifically designed to allow water vapor to escape while minimizing the entry of bulk water, which helps maintain the integrity of the sheathing material. By protecting the sheathing, building paper contributes to the overall durability of the building envelope and enhances the structure's resistance to moisture-related issues. While it does not provide insulation or directly enhance structural integrity, its role in moisture management is vital in promoting a safe and long-lasting building environment.