Extruded polystyrene insulation (XPS) has unique properties that differentiate it from other types of foam plastic insulation and therefore make it ideal for a number of roof designs.
Sometimes considered “the other roof insulation,” XPS is generally specified when higher compressive strength, higher water resistance and higher thermal resistance in the presence of water are needed. Those characteristics make XPS the preferred choice in green-planted roof systems, protected or inverted membrane plaza deck systems, white or “cool” single-ply roofing and metal roofing systems.
When considering sustainable, sturdy roof designs it’s important to start with how water resistant an insulation material is. Extruded polystyrene is manufactured to comply with ASTM C578, “Standard Specification for Rigid, Cellular Polystyrene Thermal Insulation.” ASTM C578 is also the primary reference standard used for specifying polystyrene insulations, meaning it defines physical properties for both extruded polystyrene (XPS) and expanded polystyrene (EPS).
There are fundamental differences between the properties of XPS and EPS, which are critical to understanding why XPS is ideal for applications that require high resistance to moisture intrusion (see Table 1). Water absorption by total immersion for XPS is listed at 0.3 percent maximum by volume, compared to 2.0 to 4.0 percent for EPS, depending on its density. The amount of water absorption is a function of the composition (or morphology) of each material.
The homogeneous and hydrophobic closed-cell structure of XPS makes it resistant to water intrusion (see Figure 1). This is a major contrast when compared to the makeup of an EPS board (see Figure 2). Although EPS beads are closed-cell and hydrophobic, the beads are also surrounded by voids. These voids are responsible for the higher water absorption volume found in the finished EPS board. The blowing agent used in EPS is quickly replaced by air and paired together with these voids the result is an EPS insulation product with lower thermal resistance characteristics when compared to XPS. As EPS density increases, these voids decrease, yielding less potential for water absorption and an increase in thermal resistance.
Polyisocyanurate (polyiso) rigid insulation is manufactured to comply with ASTM C1289, “Standard Specification for Faced Rigid Cellular Polyisocyanurate Thermal Insulation Board.” Polyiso chemistry involves monomers that are hydrophilic (or have an affinity for water). The cell structure tends to be irregular in size and shape and less defined compared to XPS (see Figure 3). Because of these differences, polyiso has higher water absorption than XPS - 1 to 2 percent by volume, depending on the type and classification, which compares to XPS at 0.3 percent. Also, it is important to note that the polyiso standard requires a test method that measures water absorption after a two-hour immersion in water and 10-minute drain time (ASTM C209), versus the XPS test method that uses a 24 hour immersion and requires no drain time (ASTM C272).
Truly “green” roofs (planted or vegetative) are becoming more common, particularly in dense urban areas. There are two basic types of green roofs - extensive and intensive. Extensive and intensive roofs differ in cost, depth of growing media and the choice of plants.
Extensive roof cover media varies in depth between 2 inches and 6 inches with a weight increase of between 16 pounds per square foot to 35 pounds per square foot when saturated. Intensive green roof cover media varies in depth between 8 inches and 24 inches with a weight increase of between 60 pounds per square foot to 200 pounds per square foot when saturated. Green roofs provide a thermal mass effect, which in turn saves energy and provides reduced heating and cooling costs. Other benefits include reduced water runoff, extended useful life of the roof, and added beauty and useable space.
Foremost among the many sustainability objectives of a green roof are to retain water and reduce storm water discharge, and to conserve energy through the cooling and shading properties of soil and plantings that can significantly reduce heat flow into a building, thus lowering the load placed on air conditioning equipment.
The highly water resistant characteristic of XPS roof insulation actually “enables” green roof construction. Green roofs are typically constructed in an inverted configuration, meaning that the insulation is on top of the waterproofing membrane rather than under it as in traditional roof systems. A lifetime of exposure to retained water makes it imperative that the insulation used retains R-value, possesses compressive strength and other critical properties while exposed to water. For that reason, XPS is almost exclusively used to insulate garden roofs. When specifying, one can choose from XPS ASTM C578 Type VI (40 psi), VII (60 psi) or V (100 psi) to best fit the design requirements of that roof.
Cool Roof Designs
LEED (Leadership in Energy and Environmental Design) defines “cool roofs” as roofing surfaces or systems that reduce heat islands. A heat island is a developed area that has a significant temperature difference compared to adjacent undeveloped areas. The LEED-NC 2.2 cool roof definition includes roofs that are either green/vegetated, as already discussed, or roof surfaces with a Solar Reflectance Index (SRI) equal to or greater than 78.
Common cool roof single-ply membranes include thermoplastic olefin (TPO) and polyvinyl chloride (PVC). Both materials are readily available in white, which makes them a very cost effective option for use with XPS roof insulation in both new and reroof applications.
XPS is frequently used under all types and colors of single-ply roofing membranes but it must be protected from solar heat by a cover board when used under non-white membranes in new construction and reroofing with total tear-off. In those applications, a cover board must be placed over the XPS to separate it from the membrane. Dark-membrane-to-XPS interface temperatures can approach 190 to 200 degrees Fahrenheit, which exceeds the recommended service temperature of 165 degrees Fahrenheit for XPS. Therefore a cover board is needed to separate the dark membrane from XPS. Common cover boards used under dark membranes include high-density wood fiber or water resistant gypsum board.
Mechanically attached TPO and PVC membranes require only an inexpensive slip-sheet separation from XPS. The slip-sheet is needed to achieve a Class A (ASTM E108) fire rating with TPO, or to chemically separate PVC from the XPS (see Figure 5).
XPS roof insulation types X (15 psi), IV (25 psi) and V (100 psi) are commonly used with mechanically attached cool white roofing membranes and a simple slip-sheet. XPS and the white membrane, installed directly to the steel roof deck, combine to create a highly sustainable roofing system. High SRI is achieved, while material use and installation labor is minimized as cover boards and underlying thermal barrier layers are eliminated.
Beyond those immediate benefits, life-cycle cost is minimized with XPS that has the water resistance to endure the inevitable roof leaks that occur as roofs age. XPS will maintain its properties in the presence of water in the event that leaks develop over the life of the building, also making it reusable after leaks are repaired. Water resistant insulation means the roofing system will retain its R-value and energy saving power even when exposed to moisture from leaks. Water resistance also means retention of compressive strength and retention of tensile load on roof system fasteners, holding the roof secure until repairs are completed. Reusable insulation means reduced tear-off materials taken to landfills.
Metal roofing systems rely on XPS insulation to provide high compressive strength for a stable substrate to properly seat the base of clips used to attach the roof, and for water resistance to provide a secondary water barrier under the roofing panels.
ASTM C578 Type IV (25 psi) XPS insulation has excellent compressive strength for mounting standing-seam metal roof panels. Compressive resistance establishes initial fastener load and minimizes long-term creep to maintain fastener load retention over time. Yet the XPS surface is flexible enough to allow the clips to seat properly in the surface of the insulation, and with no adhered facing material there is no need to trim around the clip for proper seating, saving labor and cost. A recent examination of an XPS insulated metal roof, after 10 years in service under metal roofing clips revealed clip stability and continued fastener load retention (see Figure 6).
The National Roofing Contractors Association (NRCA) recommends a layer of asphalt-saturated felt between the insulation layer and standing seam metal roof to provide a drainage plane in buildings with high potential for condensation. The NRCA Roofing and Waterproofing Manual recommends using a vapor retarder with a permeation rating of 0.5 perms or less in certain climate zones. When installed with sealed (taped) joints, highly water-resistant XPS insulation eliminates the need for these additional layers (see Figure 7).
The water resistance of XPS also enables more flexible job staging. It is not uncommon for various elements of the roof assembly to have delayed delivery. XPS insulation can be installed continuously and sealed followed by the metal roof as the schedule permits. Manufacturers of XPS do not require that the insulation board be covered or protected as it is installed due to its resistance to job site water absorption. It is common practice for XPS insulation, with sealed joints, to serve as a temporary roof when delivery of the standing-seam roof is delayed. After 30 to 60 days, depending on the intensity of UV exposure, XPS insulation board may begin to fade in color. And while exposed XPS makes a good temporary water resistant covering, it is not intended to remain exposed for extended periods of time.
Hourly Rated Assemblies (ASTM E119)
ASTM E119 is the test method that determines the timed structural fire resistance of a roof assembly. Steel roof deck assemblies insulated with any type of foam plastic insulation generally require some type of fire resistant layer to achieve an hourly rating. When XPS insulation is used in an hourly rated assembly, a layer of 5/8-inch type X gypsum board must be installed on top of the steel deck in accordance with the particular UL assembly specification. Alternate protection layers in lieu of, or in addition to, gypsum board may include sprayed cementitious or fibrous coatings applied to the bottom side of the steel deck. Concrete roof decks are inherently more fire resistant than steel and have a different set of criteria for rating. See specific assembly listings for the form and amount of protection required.
Many moisture-sensitive roof insulations have specific storage requirements that must be followed at the jobsite. Warranties are often voided if water protected storage requirements are not followed. XPS insulation manufacturers generally have no special storage requirements because the inherent moisture resistance of XPS enables it to be stored outdoors without concern for water absorption. XPS stored outdoors and uncovered for a long period may experience color fade due to UV exposure and potential surface “dusting” if exposed to UV for extended periods - generally beyond six months depending on the severity of sun exposure in a given locale.
All XPS manufacturers provide at least 15-year thermal performance warranties per specific ASTM C578 testing criteria. Thermal performance warranties generally claim retention of 90 percent of the R-5-per-inch-of-thickness insulating capacity for 15 years with no restrictions on moisture exposure or content. For specific applications, some XPS manufacturers offer longer (20- to 30-year) warranties on listed ASTM C578 properties. These longer warranties are not typically available in other types of foam plastic insulations and should be examined carefully as specifications are being written. Check with the manufacturers for more specific information about their warranties.
XPS insulation products have a profound impact on the environment in energy savings. According to the U.S. Department of Energy, buildings consume 40 percent of our nation’s energy and account for 43 percent of the country’s greenhouse gas emissions – more than industry and more than transportation. According to a 2007 McKinsey & Company report in The McKinsey Quarterly 2007, insulation is “the single most cost-effective greenhouse gas abatement measure.”
The global warming potential (GWP) reductions associated with the energy savings due to the use of XPS are more than sufficient to offset the impacts of the manufacturing process and the small releases of blowing agent over the installed life of XPS.
Due to the thermoplastic nature of XPS insulation, virtually 100 percent of all in-plant scrap is recycled and reused in the primary extrusion process. In addition, the XPS production process uses post-consumer and post-industrial recycled and/or recovered polystyrene foam. Generally, XPS manufacturers use approximately 15 percent to 30 percent recycled polystyrene in the production of XPS. Remarkably durable and water resistant, XPS insulation can be reused in many situations. In commercial roofing applications, XPS insulation is often reused when a new roofing membrane is installed, saving both the cost of replacement insulation and the cost to haul wet insulation to the landfill. Specifying reusable products is one of the building blocks of sustainable design. As a result, specifying XPS insulation may help achieve credits in the U.S. Green Building Council’s LEED Program.