Issue link: http://cp.revolio.com/i/1121626
Polystyrene Foam Insulation in Long-Term Building Applications – Effective R-values Page 8 Moisture The R-value of insulation typically is determined under ideal, dry laboratory conditions. In many building applications, polystyrene foam insulation is protected from moisture, and the R-values determined under dry laboratory conditions are appropriate. Examples include insulation under roof membranes and wall insulation covered by a weather-resistive barrier. In these applications, no adjustment to the R-value is needed based on the insulation's exposure to moisture. Polystyrene foam in below grade applications may be exposed to moisture, and in such cases an adjustment to the laboratory R-value based on these conditions is appropriate. Under these conditions, the reductions in the R-values of both EPS and XPS materials are well documented in international standard ISO 10456. When the average moisture absorption is known, the adjustment of R-value due to this moisture can be calculated by Equation 1. Equation 1 R-value moisture adjustment factor = 1/e (a•Moisture % by volume) where a = 4.0 for EPS and 2.5 for XPS and e is Euler's number, 2.71828. Effective R-value Determination The adjustment of the R-value from ideal laboratory conditions to the conditions in building applications is straightforward. The effective R-value determination discussed herein is analogous to the thermal conductivity adjustment method recognized in international standard ISO 10456. The R-value determined in the laboratory (R LAB ) following the FTC R-value Rule is multiplied by three adjustment factors which determine the effective R-value (R EFFECTIVE ). There is an adjustment factor for aging (F AGE )where this value is a number equal to or less than one since the R-value decreases over time for some products due to the loss of captive blowing agents. There is an adjustment factor for temperature (F TEMP ) where this value may be less than or greater than one depending on the change in performance relative to the R-value determined at the mean temperature of 75°F (24°C). There is an adjustment factor for moisture (F H20 ) where this value is a number less than or equal to one since moisture reduces the R-value. Effective R-value The discussion on aging, temperature, and moisture demonstrated that R-value is affected by each of these considerations. A methodology that includes all three considerations can be used to determine the effective R-value under specific building conditions. Equation 2 R EFFECTIVE = R LAB x F AGE x F TEMP x F H20 where: R EFFECTIVE = effective R-value under the specific conditions considered Note: This equation is applicable to R-values (U.S. units) or RSI values (SI units). R LAB = R-value determined under standard laboratory conditions at 75°F (24°C) mean temperature per the FTC R-value Rule F AGE = adjustment factor for a product that is 50 years old F TEMP = adjustment factor for temperature F H20 = adjustment factor for moisture based on application