Confusion between Vapor barrier and Air barrier[edit]
The function of a vapor barrier is to retard the migration of water vapor, which are not typically intended to retard the migration of air. This is the function of air barriers.[8] Air is mixed with water vapor. When air moves from location to location due to an air pressure difference, the vapor moves with it. This is a type of migration of water vapor. In the strictest sense air barriers are also vapor barriers when they control the transport of moisture-laden air.[9] It must be mentioned that the designated perm ratings do not reflect the diminished permeability of a given vapor retarder medium when affected by temperature differences on opposing sides of the medium.[10] A discussion about the differences between vapor barriers and air barriers can be found in Quirouette.[11]
Materials have a moisture vapor transmission rate that is established by standard test methods. One common set of units is g/m²·day or g/100in²·day. Permeability can be reported in perms, a measure of the rate of transfer of water vapor through a material (1.0 US perm = 1.0 grain/square-foot·hour·inch of mercury ≈ 57 SI perm = 57 ng/s·m2·Pa). American building codes have classified vapor retarders as having a water vapor permeance of 1 perm or less when tested in accordance with the ASTM E96 desiccant, or dry cup method.[1] Vapor retarding materials are generally categorized as:ASTM F1249 - 13
Standard Test Method for Water Vapor Transmission Rate Through Plastic Film and Sheeting Using a Modulated Infrared SensorSignificance and Use
5.1 The purpose of this test method is to obtain reliable values for the WVTR of plastic film and sheeting.
5.2 WVTR is an important property of packaging materials and can be directly related to shelf life and packaged product stability.
5.3 Data from this test method is suitable as a referee method of testing, provided that the purchaser and seller have agreed on sampling procedures, standardization procedures, test conditions, and acceptance criteria.
1. Scope
1.1 This test method covers a procedure for determining the rate of water vapor transmission through flexible barrier materials. The method is applicable to sheets and films up to 3 mm (0.1 in.) in thickness, consisting of single or multilayer synthetic or natural polymers and foils, including coated materials. It provides for the determination of (1) water vapor transmission rate (WVTR), (2) the permeance of the film to water vapor, and (3) for homogeneous materials, water vapor permeability coefficient.
Note 1—Values for water vapor permeance and water vapor permeability must be used with caution. The inverse relationship of WVTR to thickness and the direct relationship of WVTR to the partial pressure differential of water vapor may not always apply.
1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.