Certain building projects and conditions require higher insulation performance and long-term resilience. The unique closed-cell composition of extruded polystyrene foam insulation (XPS) provides greater density than other types of rigid foam polystyrene insulation, making XPS superior at moderating extreme temperatures and deterring moisture permeation and thereby minimizing the performance degradation related to water absorption. Two applications where these attributes are critical are frost protected shallow foundations and protected membrane roof assemblies. In both of these building techniques, XPS outperforms other forms of rigid foam polystyrene insulation.
Let’s take a closer look, starting from the ground up. A frost protected shallow foundation is an energy-efficient and cost-saving building technology, making it attractive to both builders and property owners. These foundations consist of a monolithic slab with rigid foam insulation installed vertically on the exterior of the foundation and additional, strategically placed insulation “wings” extending horizontally off the bottom of the foundation. In climates where there is seasonal ground freezing, the placement of rigid foam insulation around the foundation prevents frost from developing below the wall and footer, protecting the building from potential damage caused by freeze-thaw cycling and/or frost heave.
There’s a triad of science related to the thermal interaction of building foundations, insulation, and the surrounding ground that makes a frost protected shallow foundation so effective. First, the technique recognizes that proper insulation around the exterior sides of the foundation retains heat. Next, the insulation wings off the foundation reduce heat transfer from the surrounding earth, effectively increasing the ground temperature and preventing frost from forming under the building. Third, an insulated frost protected shallow foundation helps to conserve natural geothermal heating below a building.
Just as the depth of footings are key for the structural integrity of typical foundations in areas that experience ground freezing, with a frost protected shallow foundation, the long-term thermal performance of the insulation is equally critical. The foundation must be constructed using insulation that resists moisture permeation and maintains long term R-value performance.
By definition, ASTM C578 requires extruded polystyrene foam insulation to absorb no more than 0.3 percent water by volume. In contrast, expanded polystyrene insulation must allow no more than two to four percent water absorption by volume—that’s six to 13 times more than XPS. Because water conducts heat well, the more water that insulation absorbs, the greater the loss in R-value, and the corresponding reduction in energy efficiency.
The American Society of Civil Engineers (ASCE) 32-01 standard: Design and Construction of Frost-Protected Shallow Foundations established the industry accepted method for frost protected shallow foundation construction and in establishing those standards, found that XPS had lower moisture absorption and greater R-value retention than EPS. The ASCE Standards Committee completed an objective analysis of the in-service performance of XPS and EPS insulation in below grade applications, in both vertical and horizontal orientations that would take into account varying moisture exposure in a frost protected shallow foundation assembly. Their conclusion, as per ASCE 32-01, is that in below-grade applications, after long-term exposure, XPS insulation retained 90 percent of its R-value in vertical installation and 80-81 percent of its R-value in horizontal orientation, surpassing the retained R-value of EPS at 80 percent and 65-67 percent, in the respective below grade orientations. Determining the minimum R-value of the insulation needed for a frost protected shallow foundation depends upon the air freezing index at the building location.
An additional consideration when selecting insulation for a frost protected shallow foundation or any below grade installation is the product’s compressive strength. Here too, XPS has shown to offer superior performance.
Moving up from the foundation we find another construction application in which XPS excels: the protected membrane roof, also called an inverted roof membrane assembly. In this type of installation, the rigid foam XPS insulation and ballast are placed on top of the roof membrane, providing protection to the membrane from environmental elements or other physical damage.
A protected membrane roof assembly is commonly used on vegetative or green roof installations, which may be totally or partially covered by plants. Vegetative roofs offer economic, environmental and aesthetic benefits. These roofs have higher thermal mass than traditional flat roofs and therefore can contribute to reduced heating and cooling costs. The vegetative covering reduces the membrane exposure to harmful UV light, thus extending the useful life of the roof. While adding beauty, green roofs also retain water and reduce storm water discharge.
In order to function effectively and provide these economic and environmental benefits, vegetative roofs require insulation that resists moisture absorption, retains long-term R-value and offers significant compressive strength. XPS meets all of these criteria and for this reason, is the recommended product for vegetative roofs in which the insulation is installed above the membrane.
One crowning example of XPS in a protected roof membrane assembly for a vegetative roof is at the U.S. Coast Guard headquarters in Washington, DC, which opened in 2013.
This 12-million-square-foot, 11-story office building is covered with an extensive configuration of more than 300,000 square feet of vegetative roofs, designed and built by Clark Construction Group. The green roofs are part of a storm water retention system in which the runoff is routed to a basin and then recycled for irrigation on the property.
Extruded polystyrene foam insulation was laid directly over a membrane of hot rubberized asphalt to provide the moisture resistance and structural integrity that were essential to this project.
Both vegetative roofs and below grade foundation applications are challenging environments which require insulation that can provide superior thermal performance, moisture resistance and compressive strength. Understanding the distinct properties of specific rigid foam insulation products enables contractors to choose the best product for job, whether it’s protecting the structural integrity of a foundation or ensuring the durability of a green roof. ■
About the Author: John Woestman is director of codes and standards for the Extruded Polystyrene Foam Association (XPSA). He has more than 25 years of experience in the construction and building products industry with various responsibilities in construction, manufacturing, human resources, marketing, and codes, standards, and regulations. Woestman has a diploma in building trades, a degree in mechanical engineering, and an MBA. He is a member of the International Code Council (ICC), ASTM International, and the National Fire Prevention Association (NFPA).
Modern Contractor Solutions – October 2016
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