PMRA application examples
The following examples demonstrate XPS insulation solutions for various projects where performance requirements for thermal resistance and structural integrity are solved with XPS insulation. These application examples focus on the insulation layer for PMRAs.
The application examples provide general guidance for specifying various types of PMRAs. However, the details may vary from project to project and for different roof sections on the same building. It is recommended specifiers work closely with the suppliers of the XPS insulation as well as other component suppliers.
Example 1: PMRA with solar-reflective aggregate
The first application example is for a ballasted PMRA assembly on top of a concrete deck with a high solar reflectivity top layer of aggregate ballast. This PMRA is for a federal building in Winchester, Virginia, which has a warm humid climate (ASHRAE 90.1 Zone 4A). The roof area is 19,264 m2 (207,360 sf) (see Figure 3).
An R-value of 5.28 K·m2/W, or R-30 (30 F·sf·h/Btu) was obtained using 152 mm (6 in.) of type VI XPS. The compressive strength of 276 kPa (40 psi) was adequate for the relatively light load on this PMRA thanks to adequate drainage. The bottom layer of the XPS insulation board itself contained drainage channels facilitating water removal through a drainage system.
A fabric material on top of the 152 mm of XPS separates the XPS from the aggregate and helps to prevent displacement of the insulation (see Figures 4 and 5). The only thing holding down the XPS insulation is the aggregate ballast. Hence, installation occurs in stages with the insulation being covered with fabric and aggregate soon after it is laid down on the waterproof roof covering.
The insulation and the reflective ballast greatly reduce the heat load on this building. This is an environmentally friendly solution because the PMRA not only extends the service life of the underlying membrane by protecting it from UV radiation and high temperatures, but also saves on cooling energy costs for this building which is in a warm humid climate.
Example 2: Large multifunctional plaza deck PMRA
Another example of a PMRA roof is the Dickies Arena Plaza Deck in Fort Worth, Texas. The building is in Northern Texas which has a humid subtropical climate with mild winters and hot summers (ASHRAE 90.1 Zone 3A).
The Plaza Deck is constructed atop a concrete slab with an area of 13,006 m2 (140,000 sf). The concrete roof deck is covered with a hot-mopped waterproofing membrane. There are 261 drains at the roof deck layer, directly beneath the XPS insulation.
The Plaza Deck supports the weight of pavers, which serve as ballast and walking surfaces; concrete topping slabs (i.e. split slabs), foot traffic and furnishings; and, in some areas, vegetative roof assemblies (VRAs). Various XPS types support the differing dead and live load requirements. Type V XPS is used at locations that require the highest compressive strengths. Type V provides a minimum compressive strength of 690 kPa (100 psi).
Atop the sloped structural deck, the insulation is reverse tapered to provide a flat surface; about 0.3 m (1 ft) of flat insulation follows; and finally, a third course of tapered insulation, sloping to the linear drains at the top of the aesthetic surface, completes the insulation thickness (see Figure 6). Using insulation to control heights instead of other fills such as concrete reduces the weight across the entire roof.
The layers above the insulation support the various aesthetic and functional requirements of the Plaza Deck. There are layers of filter fabric, a sand bed, pavers, and planters filled with trees. The Plaza Deck is open to visitors who can enjoy cool evenings outdoors during events at the arena. Meanwhile, when there is a downpour, the deck is engineered to efficiently drain the rainwater from this precisely engineered PMRA.
The Dickies Arena serves as an excellent example of a mixed-use PMRA application that allows for foot traffic and VRAs on a flat occupiable roof with good drainage.
