In a recent advertorial that appeared 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-base siding (including vinyl, “solid” PVC, and similar materials that are 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, and eventually falls 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.
It is important to note that decay occurs only when wood is both wet and warm for extended periods of time. The optimal temperature range for the growth of decay fungi is about 65°F to 95°F (18°C to 35°C). There is little or no growth below 50°F (10°C).
It also has been known since the 1950s or so that in cold climates, the vapor permeability of exterior cladding must be much greater than the permeability of the interior surfaces of the wall to allow indoor water vapor that enters a wall cavity to dry to the outdoors. That means that the outside cladding system must be vapor permeable. However, siding materials like cellular PVC, poly-ash, metal (steel or aluminum), and vinyl are not vapor permeable. They may act like vapor barriers on the wrong side of the wall. In such cases, the water vapor that enters the wall cavity from indoors and condenses in the sheathing and framing leads to much higher sheathing moisture contents (higher, that is, than in walls with vapor-permeable siding materials) and cannot dry out as easily during warm weather when decay can occur. Thus, it is possible that the sheathing moisture content will remain above the fiber saturation level well into warm weather and lead to decay of sheathing and framing. Mold growth inside the wall cavity is also possible, along with deterioration of WRBs.
Whether or not the sheathing and framing get wet enough to cause moisture damage depends on several factors. Interior vapor barriers can reduce the amount of water vapor entering wall cavities, but in practice, it is hard to keep all interior moisture from entering walls given the myriad ways air and water vapor can enter them.
One key is to keep the relative humidity of the interior air low. This will minimize the moisture load that is driven into walls. Most damage in walls has been found to be in walls adjacent to bedrooms where relative humidities are typically higher than in other indoor locations. Oftentimes, indoor ventilation, especially in bedrooms, does not keep indoor relative humidities low enough to prevent damage in walls.
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