In a recent advertorial on JLC’s website, the author exclaims the virtues of solid PVC siding and trim, including “not having to add a rainscreen.” Yet the lack of a rainscreen (or enough continuous exterior insulation) with any polymer-based siding (including vinyl, “solid” PVC, and similar materials that do not vapor permeable) in residences can potentially cause serious and widespread moisture damage to wall sheathing and framing.
It has long been known that water vapor produced indoors can migrate by air leakage or vapor diffusion into walls and condense on the wood sheathing or the exterior portions of framing in cold climates. That commonly and naturally happens during wintry weather. During winter, the moisture content in sheathing rises and peaks, eventually falling again with warmer spring and summer weather. The key is that wood or similar siding or fiber-cement siding is vapor permeable (what some call “breathable”) and allows moisture in the sheathing to slowly but surely dry to the outside air. If it dries enough, the sheathing moisture content drops below the fiber saturation level of about 26% to 30%. Below this level, decay cannot occur during warm spring and summer weather.
Read the full article at: www.jlconline.com