Vapor barriers have many classifications and standards that most plastic sheeting doesn’t meet. There is often confusion between the types of materials that pass as a vapor barrier or vapor retarder. Contractors install vapor barriers throughout the building cavity to control indoor air quality, energy efficiency, and building integrity.
The three common barriers include air barriers, which control airflow, vapor barriers to control moisture, and radiant barriers to control temperature. In this article, you’ll discover what makes plastic sheeting a class a vapor barrier or vapor retarder.
Related Article: What are Vapor Barriers?
How is a Vapor Barrier Classified?
IRC Class I, II, and III Vapor Barriers
Vapor barriers are defined as either Class I, II, or III by the International Residential Code (IRC) based on how permeable they are to water vapor. The IRC is the comprehensive code for single and multi-family residences and covers all building, mechanical, fuel gas, and electrical requirements.
- Class I – 0.1 perm or less, very low permeability
- Class II – 0.1 perm to 1.0 perm, low permeability
- Class III – 1.0 perm to 10 perm, medium permeability
ASTM E1745 Class A, B, and C Vapor Barriers
Class A, B, and C vapor barriers are all Class I barriers, and Class I barriers are defined by ASTM E1745 classifications. ASTM creates the recommended standards for testing materials. E1745 specifically tests the flexibility of sheet membranes used in vapor retarders.
These barriers all have the same extremely low permeability. Tensile strength and puncture resistance changes with each class, with Class A barriers being the strongest.
- Class A — minimum 45 lb per inch tensile strength, 2200 grams puncture resistance.
- Class B — minimum 30 lb per inch tensile strength, 1700 grams puncture resistance.
- Class C — minimum 13.6 lb per inch tensile strength, 475 grams puncture resistance.
ASTM E84 is identified with classes A, B, and C but project managers should note that ASTM E84 has nothing to do with ASTM E1745. E84 measures fire resistance.
Other ASTM Test Methods Related to Vapor Barriers that are Good to Know
Class A vapor barriers follow various testing standards:
- ASTM E154: simulates conditions the vapor retarder will face
- ASTM D882: tests tensile strength
- ASTM D1709: tests puncture resistance
- ASTM E96/E96M: tests permanence using the dry cup method
- ASTM F1249: tests permeance using an infrared sensor.
- ASTM G21: tests fungal resistance of plastics and polymeric materials.
Related Article: Right Mil Thickness For Your Crawlspace Vapor Barrier
What is an ASTM E1745 Class A Vapor Barrier Used For?
Class A vapor barriers are the strongest class, meeting A, B, and C requirements for plastic water vapor retarders used in contact with soil or granular fill. Common applications are underslab beneath concrete, and crawl space encapsulation. In geographical areas where radon or methane gas is a concern, there are class A vapor barriers that also block gasses, like 20 mil VAPORBLOCK® PLUS. Prevent mold growth in these areas with the ASTM E1745 and G21 certified Pro Crawl Anti-Mold embedded with MPT™.
Browse our full selection of Class A Vapor Barriers and Vapor Retarders today.
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