by Susan Herrenbruck
Demand for energy conservation has been a catalyst for code change in many parts of the country. Concerns for the environment have led to calls for the use of more sustainable roofing systems—from original application to re-roofing and disposal. For the design professional, the specification of these systems requires careful consideration about which building products should become a part of the larger roof assembly.
For example, when it comes to insulation materials, there are many types of products to choose among for re-roofing an existing structure. The function of roof insulation is to protect the building against heat flow (both in and out) and to provide a smooth, dry, clean, and firmly-attached substrate for the membrane. Proper insulation is necessary for energy efficiency, condensation control, and, in some systems, structural integrity against uplift pressure. One aspect of extruded polystyrene (XPS) is its long-term durability that can often make it reusable in re-roofing situations.
All roofs leak over time and, as such, facilities need to be re-roofed two or three times during their service lives. When roofs leak, the intruding water often damages the insulation below the membrane. However, this is rarely the case with extruded polystyrene (XPS) due to its strength and ability to resist moisture. Extruded polystyrene (XPS) retains its R-value, compressive strength, dryness, and integrity when the time comes for re-roofing.1 If the originally-installed insulation maintains these critical properties, it is highly likely it will not need to be removed, transported to a landfill, and replaced. The existing roofing membrane can be stripped and substituted with a new membrane, leaving the original extruded polystyrene (XPS) insulation in place. This translates into cost savings for the building owner in terms of both labor and new materials, and can also divert materials from the wastestream, helping to preserve the environment.
Extruded polystyrene (XPS) is also a good choice for‘re-cover’ applications, which is another form of re-roofing. In this situation, the existing membrane and insulation are left in place and are simply re-covered with new insulation and new membrane. By re-covering old roofs, the existing materials are left in place to dry, recouping some of their initial R-value. Also, the cost to remove the old system is avoided—this expense involves both the labor to remove the old system and the price of transporting the removed materials to a landfill.
Tear-offs can become extremely costly when hazardous materials are discovered in the old assembly. Sometimes covering in place is the best option, rather than disturbing, containing, and disposing hazardous materials from older buildings. Contractors assigned to re-cover buildings should have the roof deck examined for structural and fastener integrity, the presence of a vapor retarder, and other aspects of drying potential, as well as required fire classification.
Based on the inspection, if re-covering is a good solution, special extruded polystyrene (XPS) products designed for these applications can be a good choice for insulation. Extruded polystyrene (XPS) re-cover products are able to stand up to the moisture left in place in existing roofs since extruded polystyrene (XPS) ’ R-value and structural integrity are not compromised by the presence of moisture. Eventually, the existing roof, left in place, should dry out and leave a sound new layer of extruded polystyrene (XPS) insulation and a new membrane.
1 As insulation formulation may vary from manufacturer to manufacturer, design professionals should consult the suppliers’ specification sheets to understand the exact properties over time, including the actual R-values. Factors affecting the R-value include thickness of application (i.e. the thicker the foam, the better the aged R-value), the substrate, and the covering systems used (i.e. the lower the perm-rated covering and substrate, the higher the aged R-value).
Susan Herrenbruck is the executive director of the Extruded PolystyreneFoam Association (XPSA), a trade association representing manufacturers of XPS insulation products and its raw material suppliers. She can becontacted via e-mail at email@example.com.
Extruded polystyrene (XPS) ’ Extended Flight at DFW Airport
When the time came to replace membranes in insulated and ballasted roofing assemblies on four terminal buildings at the Dallas/Fort Worth (DFW) International Airport, everything old was new again. The project team looked for insulation under the rock ballast already in-place and found perfectly re-usable
extruded polystyrene (XPS) insulation made more than 17 years ago. After sending samples of this extruded polystyrene (XPS) insulation to a laboratory to verify a lack of moisture and the appropriate compressive strength and R-value (the material retained 90 percent of the latter), the hired roofing consultant designed the projects to reuse as much of the existing extruded polystyrene (XPS) insulation as possible—nearly 90 percent of the old material.
This efficient use of resources allowed the airport to save thousands of dollars that would have been spent taking the old material to a landfill and replacing it with all new insulation. Labor for getting the old insulation off the roof and the price of new materials (including transportation) would have otherwise added thousands of dollars to the project. There are also environmental benefits to being able to reuse the extruded polystyrene (XPS) . The strategy meant reusable material was not being sent to the landfill, and fossil fuel was not being consumed in manufacturing new insulation.