Question: Hasn’t the building code been revised to now require the use of building paper under all siding?

Answer: Due to changing building practices and to reduce chances of moisture-related problems in wall cavities, APA revised its stance on additional weather barriers over wall sheathing with the publication of the January 2001 edition of APA’s Engineered Wood Construction Guide, Form E30. Recommendations in the revised Guide called for the application of additional weather barriers in wall assemblies. Recommendations in earlier guide editions (e.g., 1996 version) allowed the omission of additional weather barriers in wall assemblies.

The concern of water leakage through exterior wall covering systems, which if prolonged can lead to mold and structural decay, led to APA's revised recommendations and subsequent building code requirements for weather barriers behind all exterior sidings. Additional weather barriers are an inexpensive insurance against potential damage.

Beginning with the 2004 Supplement, the International Building Code (IBC) and International Residential Code (IRC) now require the use of weather-resistant barriers between exterior wall cladding (e.g., all types of siding, brick veneer and stucco) and wood structural panel wall sheathing (plywood and OSB). Weather resistant barriers are also required behind panel siding applied direct to studs.

2004 IBC Supplement Section 1404.2: "Water-resistive barrier. A minimum of one layer of No. 15 asphalt felt, complying with ASTM D 226 for Type1 felt, shall be attached to the studs or sheathing, with flashing as described in Section 1405.3, in such a manner as to provide a continuous water-resistive barrier behind the exterior wall veneer."

2004 IRC Supplement Section R703.2: "Weather-resistant sheathing paper. One layer of No. 15 asphalt felt, free from holes and breaks, complying with ASTM D 226 for Type1 felt or other approved weather-resistive materials shall be applied over studs or sheathing of all exterior walls. …" The section goes on to describe acceptable application, referring to Table 703.4, and certain limited exceptions. Revisions to Table R703.4 are such that options for omitting paper provided in previous editions of Table R703.4 have been virtually eliminated.

For more information on recommendations to help ensure moisture protection, visit APA’s Build a Better Home web site, or download APA publication, Build a Better Home: Walls, Form A530, which addresses the proper use of weather-resistant barriers.