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Moisture Absorption

      
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Insulated siding is designed to be an exterior cladding, not a water-resistive barrier. The siding is designed to allow the material underneath it to breathe; therefore, it is not a watertight covering. Because of its design and application, it provides a supplemental rain screen that enhances the water-resistive barrier system by reducing the amount of water that reaches the underlying water-resistive barrier.

A water-resistive barrier system is a system that includes water shedding materials and water diversion materials. Water-resistive barrier systems commonly consist of a combination of exterior cladding; flashed wall openings and penetrations; water-resistive barrier material; and sheathing. Effective water-resistive barrier systems will shed the water initially, control moisture flow by capillary and diffusion action and minimize absorption into the wall structure. The level of water resistance required is determined by the applicable building code and structure.

Besides improving a structure’s ability to keep heat in, insulated siding does not inhibit efforts to keep moisture out. Insulated siding provides a supplemental rain screen that reduces the amount of water that reaches the underlying water-resistive barrier. With a properly applied water-resistive barrier, insulated siding minimizes moisture penetration from the exterior into the wall assembly and provides a way for moisture to readily drain and dry. The presence of a layer of thermal insulation filling the space between the insulated siding and the wall sheathing also aids in the moisture management system.

Studies of foam sheathing and insulated siding make the case for insulated siding’s moisture management:³⁵

• A technical bulletin published by the EPS Molders Association highlighted third-party testing conducted by SGS US Testing Company Inc. to evaluate EPS’s resistance to fungi and mold growth. Using ASTM C1338-08 Standard Test Method for Determining Fungi Resistance of Insulation Materials and Facings, the research indicates that under a 28-day incubation period, the EPS had no trace of mold growth. EPS insulation is a closed-cell foam, and exposure to moisture has little, if any, effect on its thermal performance and dimensional stability.
  
  
• The National Association of Home Builders (NAHB) Research Center, a subsidiary of the National Association of Home Builders, conducted a 22-month field investigation of nine different north- and south-oriented wood-framed wall assemblies to determine the moisture performance of various wall construction types, most of which incorporated absorptive cladding. The study was conducted 20 miles east of Washington, DC, in a mixed-humid climate. Moisture content of the sheathing and wall cavity temperatures were measured at various points in each wall section. The primary performance measure was moisture content of the wood-based structural sheathing. Walls with non-absorptive cladding (i.e., vinyl siding and insulated siding) had among the lowest sheathing moisture contents recorded in the study; this was the case for walls that faced either north or south.
  
  
• A field study conducted on 20 multi-family structures in Minneapolis, Minnesota, examined nearly 90 percent of the buildings’ exterior structural sheathing after seven years of use with insulated siding. No instances of moisture problems relating to insulated siding were found in any areas inspected, including the siding, the water-resistive barrier and the sheathing.
  
  
• Third-party, insitu testing to evaluate EPS foam insulation in below-grade applications found that, following a 15-year period, the EPS performed as required by ASTM C578 with regard to R-value and moisture absorption.