Question: What are the requirements surrounding weather-resistive barriers over wood structural panel wall sheathing? Can this requirement be exempt?

Answer: Until recently, building code provisions and APA recommendations allowed omission of additional weather barriers over wood structural panel sheathing, which historically were recognized as water-repellent panel sheathing.

With changes to building practices and in an effort to reduce chances of moisture-related problems in wall cavities, APA revised its stance on the topic in 2001 to recommend the application of additional weather barriers in wall assemblies.

Leaks through exterior wall covering systems, if prolonged, can lead to mold and structural decay. This concern led to APA's revised recommendations and building code requirements for weather barriers behind all exterior sidings – an inexpensive insurance against potential damage.

Current model building code provisions call for the use of weather-resistive barriers between exterior wall cladding (e.g., all types of siding, brick veneer, stucco) and wood structural panel wall sheathing (e.g., plywood and OSB). Weather-resistive barriers are also required behind panel siding applied direct to studs.

While earlier editions of Evaluation Report NER-108 recognized APA Rated Sheathing (plywood and OSB) as water-repellent panel sheathing, as defined by the Standard Building Code and the Uniform Building Code, the July 1, 2003 edition (ICC Evaluation Service, Inc. Legacy Report NER-108) no longer included this recognition. NER-108 now states, "Sheathing shall be installed in accordance with the applicable Code provisions for wood structural panels of Sheathing Grade..."

In the past, some building code provisions allowed omission of additional weather barriers over wood structural panel sheathing recognized as water-repellent panel sheathing. The International Building Code (IBC) and International Residential Code (IRC) now require some form of protective barrier over wood structural panel (plywood and OSB) applied as exterior wall sheathing:

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 Type 1 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.

APA’s Build a Better Home web site provides recommendations to help ensure moisture protection. APA’s Build a Better Home: Walls, Form A530, addresses the proper use of weather-resistive barriers.

Technical Topic APA Building Paper Recommendations for Walls, Form TT-005, provides additional information.