ThermalStar® LCi-SS™ Water Vapor Transmission Rates Date: April 15, 2015 ThermalStar LCi-SS sheathing products are a composite of OSB, expanded polystyrene, and polymer film. The model codes in the United States define “vapor permeable” as having vapor transmission rate, for the thickness of the product defined in the assembly, greater than 5 perm. Products with lower transmission rates are deemed “vapor retarders,” and fall into several classes as defined in the model codes. From IRC 2012: “R702.7 Vapor Retarders. Class I or II vapor retarders are required on the interior side of frame walls in Climate Zones 5, 6, 7, 8 and Marine 4. Exceptions: 1. Basement walls. 2. Below grade portion of any wall. 3. Construction where moisture or its freezing will not damage the materials. R702.7.1 Class III vapor retarders. Class III vapor retarders shall be permitted where any one of the conditions in Table R702.7.1 is met. CLIMATE ZONE Marine 4 5 6 7 and 8 CLASS III VAPOR RETARDERS PERMITTED FOR Vented cladding over wood structural panels Vented cladding over fiberboard Vented cladding over gypsum Insulated sheathing with R-value ε 2.5 over 2 x 4 wall (use LCi-SS R3) Insulated sheathing with R-value ε 3.75 over 2 x 6 wall (use LCi-SS R5) Vented cladding over wood structural panels Vented cladding over fiberboard Vented cladding over gypsum Insulated sheathing with R-value ≥ 5 over 2x4 wall (use LCi-SS R5) Insulated sheathing with R-value ε 7.5 over 2 x 6 wall (use LCi-SS R7.5) Vented cladding over fiberboard Vented cladding over gypsum Insulated sheathing with R-value ε 7.5 over 2 x 4 wall (use LCi-SS R7.5) Insulated sheathing with R-value ε 11.25 over 2 x 6 wall (use 1” LCi over LCi-SS R7.5) Insulated sheathing with R-value ε 10 over 2 x 4 wall (use 3/4” LCi over LCi-SS R7.5) Insulated sheathing with R-value ε 15 over 2 x 6 wall (use 2” LCi over LCiSS R7.5) 8240 Byron Center Ave SW, Byron Center, MI 49315 www.atlaseps.com (800) 917-9138 R702.7.2 Material Vapor Retarder Class. The vapor retarder class shall be based on the manufacturer’s certified testing or a tested assembly. The following shall be deemed to meet the class specified: Class I: Sheet polyethylene, unperforated aluminum foil. Class II: Kraft-faced fiberglass batts. Class III: Latex or enamel paint. VAPOR RETARDER CLASS. A measure of the ability of a material or assembly to limit the amount of moisture that passes through that material or assembly. Vapor retarder class shall be defined using the desiccant method with Procedure A of ASTM E 96 as follows: Class I: 0.1 perm or less Class II: 0.1 < perm >1.0 perm Class III: 1.0 < perm >10 perm” OSB at 7/16” thick is generally considered to have a vapor transmission rate of 2 perm at 50% relative humidity. The EPS and polymer film portion of LCi-SS have a vapor transmission rate exceeding 2 perm for perforated film configuration (standard), and between 0.6 and 0.9 perm when not perforated (special order only). These values are based on actual ASTM E96 testing at both desiccant (Procedure A) and “wet cup” (Procedure B) conditions of the polymer faced EPS. In practical terms, LCi-SS R3, R5, & R7.5 in standard product configuration are considered “Class III” vapor retarders. The special order configuration is “Class II”. Both configurations provide a path for water vapor transmission to the exterior. The products may be installed in conjunction with either a Class I or Class II interior vapor retarder without concern of creating a “double vapor barrier”. Where a Class III interior vapor retarder is used, observe the requirements in Table R702.7.1 when selecting R value of the exterior assembly. Another factor to consider regarding water vapor transport in a wall system is interior air leakage into the wall cavity. Drywall with normal seam sealing and paint is an effective air barrier, but some leakage can be expected. By using the LCi-SS system on a 2x4 wall, condensation potential in a given climate is 50% less than could be expected for a similar R-value 2x6 wall, since the OSB sheathing is protected from exterior temperature extremes by the rigid insulation. The facts about the vapor transmission rates of LCi-SS are provided so that a design engineer may judge the suitability of the product in non-typical or extreme conditions. The prescriptive advice found in the model codes may not apply for special use areas such as where high indoor relative humidity is maintained, or where local jurisdiction may dictate some design details. This bulletin is current as of the date above. Please visit our website at www.atlaseps.com for the most recent technical information. 8240 Byron Center Ave SW, Byron Center, MI 49315 www.atlaseps.com (800) 917-9138
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