Division 7 Ebook Page 538 FOAMULARŪ XPS Insulation Resisting Water Absorption (Comparing Types of Rigid Foam Plastic Insulation)

Technical Bulletin FOAMULAR Extruded Polystrene Insulation Resisting Water Absorption, the Key for High Performance Foam Plastic Rigid Insulation time 5 year aging. The polyiso industry uses CAN/ULC/S770 7 , to accelerate aged R-value. The S770 method has been shown to overstate R-value or LTTR. 8 The Misconception of Foam Plastic Insulation as a "Double Vapor Barrier" If FOAMULAR XPS is used as sheathing on a wall, doesn't that automatically create a "double vapor barrier", potentially trapping moisture in the wall? The answer is, no. Walls must be assessed for vapor performance. The perception is, 1) liquid water runs off of foam plastic insulation, therefore, 2) it must be water impermeable, and, the misconception, 3) therefore it must be a vapor barrier. Vapor condenses in walls for three reasons: 1) too much water vapor gets into the wall; 2) it cannot escape out the other side fast enough, and; 3) the accumulating vapor reaches a surface cooler than "dew point" temperature, the temperature at which water vapor condenses to liquid. The solutions are, 1) limit vapor in with a vapor (barrier) retarder; 2) let vapor out faster than it gets in; and; 3) make surfaces inside the wall warmer using insulation strategies. This is where FOAMULAR XPS helps. (Keep in mind this discussion is about vapor PERMEATION only. More moisture gets into walls via AIR LEAKAGE, so, that is even more important to control than vapor.) To control vapor permeation, buildings in heating dominated climates are typically provided a vapor retarder on the "warm in winter" side of the wall. Vapor retarders are designed to limit the permeation of interior moisture vapor into the wall, where it may encounter cooler surfaces and condense to liquid as it moves to the lower vapor pressure outside. Prolonged condensation inside a wall assembly can lead to mold growth. "Perm" Ratings A vapor retarder limits the amount of water vapor permeating into the interior side of a wall, so that it is slower going in than it is going out on the exterior side. Comparing the perm rating of the retarder material to the perm rating of FOAMULAR XPS is the key to understanding why FOAMULAR XPS is not necessarily a "double vapor retarder". Although liquid water runs off of, and is virtually unable to penetrate, FOAMULAR XPS's closed cell structure, molecules of water vapor are another matter. Water vapor molecules can pass between the molecules of any solid material, such as FOAMULAR XPS. That process is called "permeation". The "perm" rating of a solid material is the measure of how rapidly it allows water vapor molecules to pass. The higher the "perm" rating, the faster water vapor passes through. Perm rating varies by thickness, decreasing as thickness increases, with the exception that the permeability of facers added to Figure 7 R-Value Retained in Roof Insulation after Sustained Exposure to Water Vapor 5 0 10 20 30 40 50 60 70 80 90 100 Polyiso Foam 250 1000 900 800 700 600 500 400 300 200 150 100 50 EPS % R-Value Retained Exposure Time in Days

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