BA-1210: Moisture and Structural Analysis for High Performance Hybrid Wall Assemblies

Executive Summary

The project title is High Performance Hybrid Assemblies and has been given Subtask 2.1 under Task 2.0 – Evaluation of Advanced Efficiency Measures for New Construction. Based on past experience in the Building America program, BSC has found that combinations of materials and approaches—in other words, systems—usually provide optimum performance. No single manufacturer typically provides all of the components for an assembly, or has the specific understanding of all the individual components necessary for optimum performance. Integration is necessary and is the reason for the teaming approach that has been taken with this research project. The hybrid walls analyzed utilize a combination of exterior insulation, diagonal metal strapping, and spray polyurethane foam and leave room for cavity-fill insulation. These systems can provide effective thermal, air, moisture, and water barrier systems in one assembly and provide structure.

The optimal wall from the Building Energy Optimization (BEopt), thermal, hygrothermal, and structural analysis is Hybrid Wall 3 (described below). Hybrid Wall 3 has the lowest associated incremental cost, lowest associated air leakage condensation risk at less than 1% of the year in Minneapolis, the best structural performance—based on American Society for Testing and Materials (ASTM) E72— as well as the second best annual energy savings at 34% in Minneapolis and 29% in New Orleans. We believe that this is one of the most promising technologies for high performance residential wall assemblies in the mass built production house arena. This wall consists of the following:

• Exterior vertical wood strapping for cladding attachment
• 1.5-in.-foil-face polyisocyanurate board insulation
• Diagonal metal strapping
• 2 × 6 advanced framed wall
• 1.5-in.-closed-cell spray polyurethane foam in each stud bay
• 3-in.-cellulose insulation
• 0.5-in.-gypsum with latex paint finish

BSC expects that the proposed hybrid wall systems will be suitable for deployment in whole house prototype demonstrations as early as 2014. Further testing will be required during the 2012 Building America program. Further testing would determine the structural capacities of various assemblies and variations on the hybrid walls analyzed in this report.

1 Problem Statement

1.1 Introduction

Based on past experience in the Building America program, Building Science Corporation (BSC) has found that combinations of materials and approaches—in other words, systems—usually provide optimum performance. No single manufacturer typically provides all of the components for an assembly, or has the specific understanding of all the individual components necessary for optimum performance. For example, most manufacturers of cavity insulation do not manufacture housewrap and sheathing, or sealant and cladding. Integration is necessary and is
the reason for the teaming approach that has been taken with this research project.

1.2 Background

One of the most promising technologies for high performance residential wall assemblies in the mass built production house arena will have the following configuration:

• Advanced wood frame (24 in. 2 × 6, single plates, stack framed, single headers) (Lstiburek and Grin 2010)
• Drained cladding and window openings (rain control)
• Insulating sheathing (rigid insulation)
• Spray-foam cavity insulation (SPF as a critical seal) (Straube and Smegal 2009)
• Cellulose or spray fibrous cavity insulation (for suppression of convection)
• Gypsum board interior.

This wall provides an approximate nominal thermal resistance of R-35. It does not need a housewrap and it’s believed that it will not need structural sheathing, such as oriented strand board (OSB), when the composite action of the high-density spray polyurethane foam is used to transfer the shear capacity of the insulating sheathing, the interior gypsum lining, and a diagonal metal strap to the advanced frame wood wall. Athough other researchers have examined the composite effect of high density spray polyurethane foam in light wood frame wall assemblies in conjunction with structural sheathing (Parasin and Nagy 1991), BSC has completed a preliminary investigation into the option of removing the structural sheathing with the proposed walls.

1.3 Relevance to Building America’s Goals

Overall, the goal of the U.S. Department of Energy’s (DOE) Building America program is to “reduce home energy use by 30%-50% (compared to 2009 energy codes for new homes and preretrofit energy use for existing homes).” To this end, research has been conducted to “develop market-ready energy solutions that improve efficiency of new and existing homes in each U.S. climate zone, while increasing comfort, safety, and durability.”¹

The proposed hybrid wall systems have the ability to reduce the heat loss through the wall portion of the enclosure by more than 50% from 2009 code practices. The specific composition of the hybrid walls can also provide hygric buffering, significantly reduced condensation potential, and improved airtightness. These factors will reduce embodied energy through improved durability and lifespan and will greatly reduce energy use for space conditioning. The proposed hybrid walls can be recommended for each climate zone. When implemented in conjunction with other BSC recommended high R-value systems, these hybrid wall systems can significantly reduce the space conditioning energy consumption of residential homes.

1.4 Cost Effectiveness

Each proposed hybrid wall assembly will be assigned a cost relative to standard construction. These costs will be developed in partnership with BSC’s prototype and community builders and verified with construction cost databases. It is important to note that although a system may cost more initially, the more expensive wall (as specified in this project) will be more energy efficient, and this energy cost savings must be taken into account over time. It cannot be ignored that a system that is slightly more expensive initially may have to be implemented to save a significant amount of energy over the entire life of the structure, which is often much longer than a standard mortgage. Research has shown that walls exceeding an R-value of 35 can financially pay back during the life of the initial mortgage through energy savings while reducing greenhouse gas emissions (Grin 2008). Because the building enclosure is designed to use less energy, the energy and greenhouse gas emissions savings extend for the life of the building and not just for the duration of the initial mortgage.

Improving the moisture tolerance and durability of an assembly will also figure into the equation of life cost. The longer the assembly lasts, the more energy it will use over its lifetime and the more the initial energy efficiency savings could have an impact. Proper detailing of the assembly is also important to ensure that over the life of the assembly, as components such as windows and doors require replacement, the assembly easily allows these replacements without risking damage.

The additional cost for this wall system should be fairly low due to cost trading of materials (the exterior wood sheathing is replaced with insulated sheathing, etc). BSC estimates that the new wall systems will be sold at a premium of approximately $1.75–$2.25/ft² of wall surface. This would be for a 1.5-in.-foil-faced polyisocyanurate (FFPIC) exterior insulated advanced framed 2 × 6 wall with spray foam and cellulose insulation within the cavity. Higher R-value systems may cost more.

1.5 Tradeoffs and Other Benefits

Each proposed hybrid wall will have the following characteristics compared to a code wall:

• Higher R-value
• Increased airtightness
• Improved occupant comfort
• Enhanced durability and enclosure lifespan.

Each of these components is interlinked. The increased R-value and airtightness improves energy efficiency and occupant comfort through reducing drafts and improving surface temperatures. The added durability of the system reduces maintenance requirements, increases the lifespan of the structure, and increases its tolerance to the possible operating conditions within the home.

Energy modeling has been completed in past Building America research using EnergyGauge to show that the use of exterior insulation improves energy efficiency. As part of this project, Building Energy Optimization (BEopt) software will be used to verify the energy efficiency of the hybrid wall systems.

1.6 Integration Opportunities

The information developed from this research will help enable BSC to implement hybrid wall systems on a set of prototype homes and eventually in communities. It is anticipated that the first prototype homes could be built as early as 2012. Adoption by community builders could occur by 2015. Based on the success of these implementations, hybrid wall systems may be common place with certain builders by 2018.

1.7 Location of experiment

The experimental location for this project will be the Building Science Consulting, Inc. laboratory. The address for the laboratory is as follows:

167 Lexington Court, Units 4/5/6
Waterloo, Ontario, Canada
N2J 4R9

Industrial partner laboratory spaces may be used as necessary.

1.8 Contact Information

The following BSC Industry Team members will be involved in this project:

Table 1: Industry Team Member Contact Information

2 Experiment

2.1 Research Questions

The following research questions were answered by this project and are discussed throughout the Results and Analysis section of this report.

• What are the likely hybrid assemblies that use currently available materials?
• What are the expected energy saving and other benefits of hybrid wall assemblies?
• What is the required thickness of high-density spray polyurethane foam to achieve structural performance?
• What are the likely solutions for cladding attachment over insulating sheathing in hybrid wall assemblies?

2.2 Technical Approach

This research project evaluated the thermal, hygrothermal, and structural properties of the proposed assemblies using computer modeling and laboratory testing. BEopt was used to calculate the associated energy savings of implementing the hybrid assemblies on an average home. Therm5 was used to determine the thermal properties of each assembly. Wärme und Feuchte instationär (WUFI) was used to measure the hygrothermal properties. Dow, as an industry partner, provided the equipment, laboratory space, laboratory technicians, and materials to complete the structural testing of the hybrid assemblies.

3 Results and Analysis

A number of hybrid wall systems were proposed and compared to a common standard wall. The hybrid walls do not need a housewrap and they do not need structural sheathing (such as OSB) when the composite action of the high-density spray polyurethane foam is used to transfer the shear capacity of the insulating sheathing and a diagonal metal strap to the advanced frame wood wall. This is believed to be one of the most promising technologies for high performance residential wall assemblies in the mass built production house arena. Table 2 summarizes the walls that were investigated. The main variables for this analysis were the use of two different exterior sheathing products and two different fibrous insulations to fill the gap remaining after the installation of 1.5 in. of closed-cell spray polyurethane foam (ccSPF). The thickness of the ccSPF was not varied. It was chosen as 1.5 in. because it was determined that this is likely the thinnest it can be reliably installed in a single pass to create both an air barrier and to transfer the structural loads from the wood frame to the insulating sheathing. . .

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Footnotes:

1. http://www1.eere.energy.gov/buildings/building_america/program_goals.html