Issue link: http://cp.revolio.com/i/281113
of interior ("cavity") insulation, and is spelled out in the US and Canadian codes, however, there are no building science principles that prescribe both a vapor barrier interior and exterior, so understanding the vapor transmission properties of the materials is still important. No material can withstand vapor transmission entirely over decades of use, rather they retard vapor to some degree. In the US model codes, several "classes" of vapor retarders are identified. Materials allowing less than 0.1 perm transmission are defined as "Class 1", between 0.1 and 1.0 are "Class 2", and greater than 1.0 are "Class 3". In Canada, products greater than 60 ng/Pasm2 (equivalent to 1.0 perm) are currently defined as not being a vapor retarder, although this is being further studied. Generally, Class 1 is regarded as a "vapor barrier", where Class 3 is the beginning of "vapor open" per the codes. Housewraps boasting perm 20 or higher would argue that "vapor open" begins at 10 perms, but in the case of walls and the codes, this is not the current practice. Now the really cool thing. Expanded Polystyrene (EPS) is a Class 3 vapor retarder in most common thicknesses used as wall insulation, and because of this, the designer can place the Class 1 or 2 vapor retarder elsewhere in the wall assembly without creating a "double vapor barrier". The designer can select if they want the wall to dry inward, our outward, and the EPS will not interfere. Further, manufacturers can add thin durable facers to EPS rendering it a Class 1 vapor retarder, allowing the designer another option. And as long as the interior does not also have a Class 1 vapor retarder, such Class 1 exterior EPS products are perfectly suited for cold climates per the model codes. Extruded Polystyrene (XPS) is a Class 2 vapor retarder at the thickness required for most cold climates, while foil faced polyisocyanurate foam is always Class 1. Only EPS offers lightweight rigid foam board as a Class 3 product solution for exterior walls. To reduce the risk of moisture in buildings, proper design, product choice and good construction practices are essential considerations. With careful attention to detail, inside and out, EPS foam does exactly what it's supposed to – insulate – in a safe, effective and predictable way. 13 continued from pg 12 Expanded Polystyrene (EPS) is a Class 3 vapor retarder in most common thicknesses used as wall insulation, and because of this, the designer can place the Class 1 or 2 vapor retarder elsewhere in the wall assembly without creating a "double vapor barrier."