Issue link: http://cp.revolio.com/i/1121626
Polystyrene Foam Insulation in Long-Term Building Applications – Effective R-values Page 13 Moisture Adjustment Factor The R-value of insulation typically is determined under ideal, dry, laboratory conditions. In many building applications, insulations are protected from moisture. Examples include roof insulation under membranes and wall insulation behind weather-resistive barriers. In these applications, no R-value adjustment for moisture is needed. In below-grade and ground-contact applications, insulation may be exposed to moisture, and adjustments to the R-value based on these conditions are necessary. The adjustment factor for moisture can be determined by knowing the moisture absorption of polystyrene foams over long periods, coupled with an understanding of the decrease in the R-value associated with the absorption of moisture. Many building professionals often refer to ASTM C578 water absorption values published for polystyrene foam products. These values are the results of short-term quality control tests, and they should not be used as the values for the expected water absorption in building applications. As early as 1983, researchers from Dow Chemical concluded "that moisture gain in perimeter insulation cannot be predicted accurately by any single laboratory test" 21 . Numerous studies on the field performance of polystyrene foams have been conducted around the world. The findings of many of those studies are not directly applicable to products produced in North America, because the standards for the manufacture of polystyrene foam products in the U.S. and Canada are not aligned with international standards. Five independent studies conducted in North America 22-26 provide field testing information on the water absorption of products produced in North America. Three of the studies include results on EPS, and four studies include results on XPS. Figure 8 shows the resulting data on EPS and XPS with a 1.35 pcf or greater density which relates to EPS Types II, IX, and XPS Types X, IV. Figure 7. Below-Grade Water Absorp on of EPS and XPS with Density Above 1.35 pcf Over the Long Term 23-27 7 6 5 4 3 2 1 0 0 2 4 6 8 10 12 14 16 Years Moisture Absorption, % by Volume EPS XPS