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