To maximize the energy efficiency of a home’s structure, high-performance homes employ walls with high R-values (high-R walls). These wall systems use cost-optimized insulation and air-tightness to provide an extremely high-performance building envelope that contributes to the goal of reaching zero-energy-ready performance levels.
Increasing insulation quantity raises the wetting potential of walls, due to lower temperatures on the cold side of exterior sheathing or interior drywall. This setup also reduces the drying potential with less thermal flow through the assembly, resulting in an increased risk of moisture-related problems due to condensation, which can be significant without proper attention to detail. To help builders deliver zero-energy-ready homes with high-R walls, the U.S Department of Energy’s (DOE) Building America program provides proven research results for effectively managing high-R wall moisture risks in all climates.
Building America’s research teams have conducted modeling analysis as well as field studies of several different wall assemblies. They identified effective whole-wall R-values that take into account thermal bridging of framing members
—in other words, a conduit where heat leaks through the wall system. Researchers have also investigated critical moisture potential and durability issues addressing the reduced drying potential of high-R walls.
Evaluating Wall Types
With support from the Energy Department, the Consortium for Advanced Residential Buildings
(CARB) conducted several evaluations of different wall types between 2009 and 2012, including oriented strand board (OSB), closed-cell spray foam, and fiberglass batt
used in a code-minimum wood-sided house; extruded polystyrene (XPS) rigid foam, OSB, and fiberglass batt used in a brick wall; and a high-R wall with OSB and R-40 to R-60 blown cellulose.
In one study, CARB performed analysis on three typical cold-climate wall assemblies modeled at ASHRAE 160
interior conditions. THERM
, a free program provided by the Lawrence Berkeley National Laboratory to analyze two-dimensional heat transfers through building products
, was used in this analysis, as was WUFI
, a hygrothermal model developed by Oak Ridge National Laboratory and the Fraunhofer Institute that predicts moisture transport in building envelope systems over a period of time. Together, these tools helped teams to understand the thermal and moisture performance of various wall systems.
WUFI modeling allows the realistic simulation of the transient hygrothermal behavior of multilayer building wall and roof components exposed to actual weather conditions.
Hygrothermal essentially means the flow of heat and moisture through materials.
Building Science Corp., a DOE building science research team, investigated several variations of insulated double-stud walls as part of a study on insulation approaches for constructing high-R walls.
Wall Assembly Research Findings
The study found that drying to the interior is severely limited in high R-value walls constructed with the OSB on the exterior of all the insulation. It was recommended these walls employ a vented cladding to increase the wall’s capacity to dry to the exterior and reduce the risk for moisture-related damage. High R-value walls constructed with the OSB sandwiched between a permeable insulation on the interior and an impermeable insulation on the exterior should keep the OSB as warm as possible, reducing the potential for condensation and promoting drying to the interior.
A minimum of 50 percent of the total cavity wall R-value should be provided by the impermeable insulation in Climate Zones
4 through 6, with 60 percent recommended in Climate Zone 7. Nearly every wall in this study failed the ASHRAE 160 30-day criteria, with interior relative humidity levels reaching 90 percent in all cases despite air conditioning. Based on these findings, the researchers recommended further analysis of the ASHRAE 160 criteria for interior conditions in moist climates.
The Building Science Corporation
also evaluated 12 high R-value wall cases with 20 wall assemblies. According to its analysis, many of the wall assemblies perform significantly below their rated R-values, but the researchers identified seven wall assemblies that actually performed at whole-wall R-values of 30 or higher.