XPS Tech Talk
Extruded Polystyrene (XPS) Insulation Roof Applications:
Direct to Steel Deck (Direct to Deck)
Photo Courtesy of DuPont
A common roof insulation in the commercial roofing market is extruded polystyrene (XPS) insulation because of its outstanding moisture resistance, strength, and resulting long-term insulating value.
Extruded polystyrene (XPS) insulation can be applied directly on a steel roof deck without a thermal barrier when part of an approved single ply roofing system. Also referred to as “direct-to-deck” systems, this application can result in significant savings at the time of installation for building owners.
Building Code Compliance
In general, the International Building Code (IBC) requires that a thermal barrier must be placed between the steel roof deck and the XPS insulation unless specific fire performance requirements are met (2018 IBC Section 2603.4.1.5). In order for XPS insulation to be used directly on steel roof decks, without a thermal barrier, the roof assembly containing the XPS insulation must pass third-party standard tests, either UL 1256 Standard for Fire Test of Roof Deck Constructions or NFPA 276 Standard Method of Fire Test for Determining the Heat Release Rate of Roofing Assemblies with Combustible Above-Deck Roofing Components. Either of these tests can be used to comply with IBC requirements.
Several single ply roof systems containing XPS insulation have been tested to evaluate the system fire performance when XPS insulation is applied directly on steel roof decks. These specific roof systems meet the stringent test requirements of UL 1256 when used in conjunction with single-ply roofing membrane or standing seam metal roofing systems.
UL listings for approved XPS direct-to-deck systems can be found on UL’s website and are listed as UL Product iQ Category Number TGKX; specifically Construction No. 440, Construction No. 457, and Construction No. 659. Note: A (free) online account is required to access UL Product iQ information.
Important note: All roof assembly tests apply to a specific roof assembly (i.e., specific roof deck, specific insulation, fasteners, and specific roof membrane, etc.). Passing the prescribed tests with one specific roof assembly does not cover all combinations of roof systems where XPS insulation is used.
Proven Safety
Numerous fire tests, which most closely simulate real-world conditions and construction practices, demonstrate the safety of extruded polystyrene direct-to-deck applications. Fire testing was completed in accordance with UL 1256, “Fire Test of Roof Deck Construction”, which meets the IBC requirement for this application. A roof assembly with XPS insulation applied directly on the steel roof deck that passes UL 1256 is not required to have a thermal barrier between the top of the steel roof deck and the XPS insulation.
Of course, the strongest safety evidence is real-world success. The outstanding track record of extruded polystyrene insulation in direct-to-deck systems installed in commercial buildings nationally speaks volumes to its safe use. Extruded polystyrene direct-to-deck systems have been installed since 1986 and have performed just as expected, based on the successful UL 1256 fire tests.
Other Benefits
Facilities professionals enjoy the unique benefits that XPS insulation offers, including a competitively-priced and superior long-term R-value performance which plays an important role in the building’s energy efficiency. XPS insulation’s R-value remains stable for a long-period of time, even under harsh, real-world conditions. The longevity of this sustainable R-value translates into lower utility bills.
Extruded polystyrene insulation is durable on the rooftop, as it withstands exposure to foot traffic, freezing and thawing cycles, and effectively resists moisture absorption. Any insulation that is unable to endure these types of real-world application exposures can significantly lose its R-value, translating into higher energy costs and the premature need for re-roofing.
XPS insulation is dimensionally stable and serviceable in real-world applications. These benefits are particularly important to facilities management professionals since the main goal of roofing insulation is to minimize energy costs for the life of the building. Other insulating materials that are dimensionally unstable can affect the roof’s energy performance, and membrane integrity of the roofing system can be severely compromised.
Architects, builders, and occupants can be confident that XPS insulation products are among the most durable, convenient and environmentally responsible products available on the market today.