Extruded polystyrene in protected membrane roof assemblies

Photo courtesy of DuPont

By John Woestman

In roofing assemblies for low-slope concrete roof decks, protected membrane roofs set the bar for cost-savings, labour-efficiency, and roof life span among other advantages. These advantages may appear counterintuitive: Common sense suggests the roof’s waterproofing covering needs to be on top, protecting all the other components of the assembly from the elements of nature, including the insulation. Nonetheless, the primary moisture barrier (or waterproof roof covering) itself tends to be the weak link when exposed to environmental stresses.

The first ‘insulated roof membrane’ or ‘protected membrane roof’ assemblies were asphalt built-up roofs (BURs) with extruded polystyrene placed on top of the asphalt flood coat. This arrangement protected the asphalt roof. Additionally, placing the insulation on top of the asphalt BUR reduced thermal cycling of the roof membrane.

Roof membranes benefit from being protected from weathering and mechanical damage. Ultraviolet (UV) radiation and heat cycling in the presence of moisture, oxygen, and contaminants can chemically degrade a roofing membrane, shortening its useful life. Also, when the waterproof membrane is on top of insulation, the relative expansion and contraction of the roofing assembly due to thermal cycling can be detrimental to roof longevity. Conversely, on a ‘typical’ roof, hailstorms and maintenance foot traffic both bear potential of mechanically damaging an exposed roof membrane. Additionally, the waterproofing membrane on top of the insulation can act as a vapour barrier, allowing moisture to accumulate in and around the insulation beneath the membrane, causing a whole different set of problems.

This ‘upside-down’ arrangement with the insulation on top of the waterproof membrane may be desirable as the waterproof membrane can act as a vapour barrier preventing migration of interior water vapour into the insulation.

For these and many other reasons, protected membrane roofing assemblies are beneficial and desirable in modern building and construction.

Changing paradigms in roofing

The use of protected membrane roof assemblies (PMRAs) received a boost with the parallel developments of robust membranes that could support the weight of the assembly and water-resistant insulating materials. Fibreglass or polyester-reinforced modified bituminous roofing membranes provide a seamless monolithic roof covering membrane that could last for many decades—especially when shielded from UV radiation and punctures. Coupled with boards of extruded polystyrene (XPS) insulation placed on top of such extremely durable, continuous, moisture-resistant roof coverings, the use of PMRAs gained acceptance among architects and construction specifiers.

According to a comprehensive account by Watts, in the early years of PMRAs, these roofs consisted of XPS insulation embedded in the flood coat of an asphalt built-up roof. This technology was quite successful in extending the service life of built-up roofs. By the 1980s, much was known about the performance of these ‘upside-down roofs’ which had the waterproofing membrane underneath the insulation rather than on top. Roofing materials were also changing. Modified bituminous roofing membranes were gaining in popularity. A new generation of these commercial roofing systems began entering the market under the PMRA designation (see Figure 1).

A PMRA is sometimes referred to as a ‘upside down’ roof assembly because XPS insulation boards are placed on top of the roofing membrane. Here, a conventional single-ply roof assembly on a metal deck (bottom) is compared with a PMRA (top) including vegetative roof layers. Illustration courtesy Extruded Polystyrene Foam Association

A versatile roof

Aside from the obvious advantage of extended service life compared to traditional low-slope commercial roofing systems, PMRAs offer several other attractive features and benefits. Once the waterproofing membrane is installed, the building is sealed, and the building interior work can begin immediately. Concurrently, complex assemblies can be constructed on top of the building, tailoring the roof to the needs of the locality, as in the following examples:

∞ If the environ around the building behaves as an urban heat island, then the roof assembly can be designed to mitigate this effect. The urban heat island effect can be countered with a vegetative or landscaped roof or with the use of reflective materials for the top layers of the roof assembly;

∞ If water runoff from sudden downpours is a problem in the building environ then a blue roof design may be desirable. The roof itself can serve as a water detention container;

∞ A hybrid ‘green & blue’ stormwater management system allows some water to be captured by the roof and reused as non-potable ‘grey water’ for various suitable building needs, such as flushing toilets and watering lawns. This roof design reduces the demand from the water utility by repurposing water usage in areas where water conservation is important;

∞ Occupiable roofs are also flourishing. There are countless designs for occupiable roofs with innovation abounding. Architects are only limited by their imagination. Occupiable roofs can be used for anything from rooftop dining and social gatherings to urban farming and recreational activities. High compressive strength extruded polystyrene (XPS) insulation competently supports occupiable roofs without compromising the performance of the roof assembly; and

∞ Photovoltaic (PV) panel assemblies are readily installed on top of buildings with PMRAs. The structural components above the waterproofing membrane provide a foundation for the framework that supports the solar assembly. Penetrations of the membrane and deck can be avoided using a PMRA as the firm foundation for PV panels.

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