Energy Efficiency, Sustainable Design, and Extruded Polystyrene (XPS) Insulation
Relative to a building’s environmental impact, decisions about energy efficiency can be among the most important ones to make. The use of extruded polystyrene (XPS) foam plastic insulation can play an effective and important role in achieving this sort of efficiency, thanks to its ability to maintain insulating power.
Extruded polystyrene (XPS) foam plastic insulation uses highly efficient blowing agents specifically selected for low thermal conductivity and diffusivity—this helps the insulation retain its properties.1 The durability of extruded polystyrene foam plastic insulation is perhaps its most important environmental consideration. The closed-cell structure and lack of voids in extruded polystyrene foam plastic insulation not only impart the material’s durability and strength, but also help the foam resist moisture penetration—without the use of a facer or laminate—better than some other types of insulating materials.
Extruded polystyrene foam plastic insulation is dimensionally stable and products are available in a wide range of compressive strengths (from 103 to 689.5 kPa [15 to 100 psi]) to suit a variety of application requirements, including residential (e.g. foundations, walls, ceilings), commercial (e.g. roofs, belowgrade, waterproofing), and beyond (e.g. soil stabilization, pipe insulation, utility lines).2
As part of a sustainable design process, the effect of material changes in foam formulations should also be analyzed in terms of the resulting thermal performance. Used to insulate commercial buildings and residences, the energy efficiency payback from insulation with high R-values over a long period far exceeds any marginal contribution of ozone-depletion potential (ODP). This analysis was done for estimated emissions until the Montreal Protocol’s phaseout date of 2010.3
Energy efficiency and conservation relative to global climate change (GCC) should also be considered when assessing the environmental impact of materials. In May 1999, technical experts working on both the Montreal and Kyoto Protocols collaborated in Petten, Netherlands, at the Joint Intergovernmental Panel on Climate Change/Technology and Economic Assessment Panel (IPCC/TEAP) Expert Meeting on Options for the Limitation of Emissions of HFCs and PFCs.
Among several conclusions, the report stated the use of foams such as extruded polystyrene foam plastic insulation enabled high levels of energy efficiency. It also noted an average increase in global energy efficiency of one percent in buildings equated to a net annualized reduction of CO2 emissions by some 50,000 to 80,000 tons.
The Alliance for Responsible Atmospheric Policy (ARAP) conducted a study that included a life-cycle climate performance (LCCP) and provided an analysis of insulating sheathing for residential wood-framed walls.5
These results show far more energy is saved than consumed by manufacturing the plastic foam and that far more greenhouse gas emissions due to space condition energy consumption are avoided than are emitted in the manufacture of the plastic foam.
For an accurate environmental assessment, the impact of material changes in plastic foam formulation should be analyzed in terms of their resulting thermal performance.
A critical factor affecting long-term thermal performance is extruded polystyrene foam plastic’s aforementioned ability to resist the intrusion of moisture. Moisture can come in contact with insulation not only during construction, but also throughout the building’s life. To the extent moisture is absorbed by a product, its effect is to drastically reduce thermal efficiency (i.e. R-value).
Extruded polystyrene foam plastic insulation’s ability to resist moisture absorption has been confirmed repeatedly in laboratory tests and validated by actual application use in the field. Extruded polystyrene foam plastic insulation’s manufacturing process forms a natural ‘skin’ surface not conducive to moisture absorbency. Without the need for a facer or laminate, extruded polystyrene foam plastic insulation products only absorb 0.3 percent by weight.6 When installed in walls, extruded polystyrene foam plastic insulation shifts damaging dewpoints, which helps minimize the potential for condensation to occur within. This helps keep the insulating power in the wall and prevent degradation over time due to moisture intrusion—helping keep its energy-efficient properties intact.
Exterior wall sheathing
With a long-term thermal resistance ranging from R-3 (for 13-mm [0.5-in.] thick boards) to R-5 (for 25-mm [1-in.] thick boards), extruded polystyrene foam plastic insulation sheathing products increase the energy efficiency of the entire wall. (The higher the R-value, the greater the insulating power—suppliers can provide fact sheets on R-values.) extruded polystyrene foam plastic insulation sheathing products provide a continuous layer of protection against water moisture infiltration while guarding against thermal bridging. (Thermal bridging occurs due to wood studs and other uninsulated parts of the wall, such as framing, ducts, wiring, and plumbing.)
When properly installed, extruded polystyrene foam plastic insulation sheathing also forms a continuous air barrier that minimizes convection currents and air infiltration, the leading cause of energy loss. When moisture gets into a wall assembly, it compromises components made from traditional materials and can then reduce the overall R-value of the building envelope.
Cold storage applications
In 1997, the U.S. Army Corps of Engineers (USACE) Cold Regions Research and Engineering Laboratories (CRREL) conducted a survey of the moisture content in the roofing systems of existing cold storage buildings for an extruded polystyrene foam plastic insulation manufacturer.7 As discussed in the report issued by CRREL, rooftop nighttime and indoor daytime infrared (IR) moisture surveys were performed. Areas of wet insulation (various product types, including both traditional and plastic materials) were noted in eight of the 10 roofs evaluated.
Core sampling of the membranes and insulation were collected for laboratory evaluation. The specimens were evaluated for dry density, moisture content, and thermal resistance (both as sampled and after drying). The conclusions reached by CRREL suggest the intense vapor drive, air infiltration, and propensity of the cold storage roofs to exhibit water infiltration meant extruded polystyrene foam plastic insulation is among the most suitable roof insulation for freezers and coolers.
Frost-protected shallow foundations
Extruded polystyrene foam plastic insulation is a code-approved product for use in horizontal configurations in code-compliant frost-protected shallow foundation (FPSF) applications.8 The concept of FPSF involves the placement of rigid foam insulation in a way that raises the frost penetration depth around a building. This permits foundation footing depths as shallow as 406 mm (16 in.), even in cold climates.
According to the Department of Housing and Urban Development’s (HUD’s) FPSF Design Guide, the technology not only improves energy efficiency for completed projects, but it also allows a reduction in material use and earth excavation during construction, cutting down on energy consumption.
Although relatively new in the United States, FPSF has been prevalent in Scandinavia for more than 40 years. FPSF is commonly used in monolithic slab-on-grade, independent slab and stem wall, and permanent wood foundation applications. Moisture resistance is extremely important in FPSF due to the insulation’s placement in potentially wet soil and because of the possibility of freeze-thaw cycles.
A protected membrane roof assembly (PMRA) differs from a conventional roof design in that the membrane is placed under the insulation layer, helping to maximize membrane life by protecting it from temperature extremes, freeze-thaw cycles, ultraviolet (UV) ray degradation, and traffic wear. A PMRA begins with the application of the ethylene propylene diene monomer (EPDM) membrane, followed by the extruded polystyrene foam plastic insulation boards, the protective scrim, and finally, the ballast.
Extruded polystyrene foam roofing boards are the only type of insulation recommended for use and approved by many building codes in PMRA systems. Again, this is because extruded polystyrene foam plastic insulation resists moisture absorption and crushing from foot or equipment traffic so thoroughly. The end result of a successful PMRA system is a great shield against unwanted airflow, and further reduction in the heat escaping from the building, which translates into lowered energy consumption.
The current initiative toward green building is manifesting itself throughout the built environment, as design teams seek ways to keep their projects as energy-efficient as possible. One method for helping achieve adequate thermal protection is the specification of insulation in appropriate applications. At several locations within the building, extruded polystyrene foam plastic insulation can offer these energy-efficient benefits.
About the Author
Susan Herrenbruck is the executive director of the Extruded Polystyrene Foam Association (XPSA), a trade association representing manufacturers of XPS insulation products and its raw material suppliers.
1 Due to this gas movement, the overall thermal resistance of an insulation product may change over time. This phenomenon is typically called ‘aging.’ Foam aging is not new and has been discussed in numerous papers over the years. Recent data on extruded polystyrene foam plastic insulation products and long-term performance demonstrate the excellent long-term thermal performance of extruded polystyrene foam plastic insulation products in the laboratory. See Chau Vo and Andrew Paquet’s “An Evaluation of the Thermal Conductivity for Extruded Polystyrene Foam Blown with HFC 134a or HCFC 142b” in the 2004 edition of Journal of Cellular Plastics.
2 For more on XPS applications, visit the XPSA Web site at www.xpsa.com.
6 This information takes into account the following ASTM International standards: ASTM C 578-06, Standard Specification for Rigid, Cellular Polystyrene Thermal Insulation; ASTM 1289-06, Standard Specification for Faced Rigid Cellular Polyisocyanurate Thermal Insulation Board; and ASTM C 1029-05a, Standard Specification for Spray-applied Rigid Cellular Polyurethane Thermal Insulation.
7 See “Development of Experimental Data on Extruded Polystyrene Roofing Insulation under Simulated Winter Exposure Conditions” (Report #SPI-6443, Energy Materials Testing Laboratory). See also “U.S. Army Cold Regions Research and Engineering Laboratories Report: Moisture in the Roofs of Cold Storage Buildings,” by Wayne Tobiasson and Alan Greatorex.