Protected Membrane Roofing: Cool Roofs
Cool Roof Options
The roofing industry at large, architects and government agencies typically view white single-ply or modified bitumen sheet membranes as the only cool roof options available. Protected Membrane Roof designs can also satisfy this requirement while also providing other substantial benefits, must important of all…long term performance.
The solar reflectance and thermal emittance of a surface is referred to as the Solar Reflectance Index number, or SRI. The SRI is defined as an alternative metric for comparing the coolness of roof surfaces. The higher the SRI, the cooler the roof will be in the sun. For example, a clean black roof has an SRI of 0, while a clean white roof has an SRI of 100. A minimum SRI value of 78 is required in order to qualify for LEED points.
Hydrotech has a number of cool roof assemblies available…
Hydrotech’s Protected Membrane Roof (PMR)...with high reflective white aggregate:
There are a number of quarries that produce a very white aggregate with a SRI value greater than 78 in order to qualify for LEED credit. (not available everywhere)
Hydrotech’s PMR Assembly…with extra stone ballast or standard weight pavers:
Testing done by the Oak Ridge National Laboratory (ORNL) and reported in “Evaluating the Energy Performance of Ballasted Roof Systems”, indicated that a ballasted system with a minimum of 17 LB/SF performed at the same or better level of thermal performance compared to Energy Star rated cool roof products, such as a white TPO membrane.
- Based on this study, ASHRAE 90.1-2007 Addendum recognized a ballasted roof as a type of cool roof in addition to the traditional types of cool roofs defined by reflectivity and Solar Reflectance Index. The ASHRE 90.1 2007 Addendum wording in Section 220.127.116.11.2 Cool Roof is as follows: “Exceptions to 18.104.22.168.2: 1. Ballasted Roofs with a minimum stone ballast of 17 LB/SF (83 KG/MS) or 23 LB/SF (117 KG/MS) pavers…”
- The USGBC is also studying this research as a potential option for LEED credit. Reference…ORNL Report Number UF-04-396 referenced above was prepared for the Single Ply Roofing Industry (SPRI), April 2008 Tech Solutions 512.0 Thermal Advantages of Ballasted Assemblies vs. White Membrane Cool Roofs – The Dow Chemical Company 17 LB/SF ballast layer Minimum 23 LB/SF Concrete Paver.
Hydrotech’s PMR Assembly…with high reflective concrete pavers: (with Hanover Glacier White Concrete Paver available through Hydrotech)
The Glacier White paver in the Diamond, Finish 13, Stipple and Tudor finishes provide a SRI of 78 or better.
Hydrotech’s PMR Assembly…with Hydroguard®:
Hydroguard is a lightweight solution for a PMR Assembly, composed of Styrofoam insulation topped with a latex modified concrete, weighing only 4.5 LB/SF. Hydroguard protects the roof membrane from temperature extremes and mechanical abuse. A high reflective white coating (available from the T. Clear Company) can be applied to achieve a SRI of 109.
Hydrotech’s PMR Assembly…with the Garden Roof® Assembly:
Hydrotech’s Garden Roof Assembly should also be a consideration. It helps to mitigate the heat island effect more effectively than a white reflective surface and has many other unique benefits as well. Click here to learn more about the Garden Roof Assemblies