Volume 8, Issue 10 (November 2011)
Dual Barrier Protected Membrane Roof Assemblies
Durable roof design is accomplished through the selection of proven high performance materials that are detailed to accepted standards and assembled by skilled tradespeople. Leak prevention is a high concern of building owners that can be addressed by protected membrane dual barrier roofs. The case studies herein illustrate the improvement of the membrane and insulation. Typically, roofing membranes are generally the only barrier for managing moisture (rain, vapor pressure, air movement) through a building’s roof. The position of the insulation is also an important consideration because insulation on the membrane’s exterior protects it from thermal shock and generally extends the roof membrane’s service life. Largely for this reason, protected roof membranes have been recognized for their superior performance over that of conventional roofs because the insulation protects the primary roofing membrane from wide temperature fluctuations, solar radiation, and foot traffic over the roof membrane. The concept of dual barrier roofing simply includes (a) designing a new roof assembly to include at least one “extra” layer of moisture protection or (b) simply designing the standard existing roof components to resist the elements in order to further improve the performance of a protected roof assembly. Case studies are presented that summarize the physical attributes for dual barrier roofing, both in new construction and for roof replacement/renewal projects. Measured performance comparisons indicate that a dual barrier protected membrane roof can be on the order of 10 % to 20 % more thermally efficient that a typical protected membrane [Tobiasson, W., and Richard, J., “Moisture Gain and Its Thermal Consequence for Common Roof Insulations,” NRCA Fifth Conference on Roofing Technology, US Army Cold Regions Research & Engineering Laboratory, Gaithersburg, Maryland, April 1979; Beech, J. C., and Saunders, G. K., “The Performance of Lightweight Inverted Flat Roofs, Building Research Establishment,” NRCA Second International Symposium on Roofing Technology, Princes Risborough Laboratory, Buckingshamshire, United Kingdom, September 1985]. Additional performance and service life benefits are also summarized.