By James R. Kirby, AIA
Published in the February 2011 issue of Today’s Facility Manager
As facilities management (FM) professionals head into the next decade, the biggest trends in the roofing industry—which are inherently connected to the direction of the entire construction industry—are related to energy efficiency and sustainability. This push towards energy efficiency begins with a well insulated building.
Over the past decade, higher and higher levels of insulation have been mandated with each subsequent edition of building and energy codes. The result is an increased thickness of overall roof insulation, which certainly is beneficial to a building’s energy efficiency. However, there are some fire related concerns with increased insulation thicknesses.
Testing One, Two, Three
Roof systems are required by building codes to have minimum fire and wind resistance appropriate for the building type and location. Testing agencies, like Underwriters Laboratories and FM Approvals, perform fire and wind resistance tests and classify roof systems.
The classifications are used to verify building code compliance, but they can include certain limitations. For example, a roof system’s fire resistance classifications often stipulate a maximum thickness of insulation between the roof deck and membrane. The design of a roof system’s insulation needs to take into account code required minimums as well as classification maximums. In addition to insulation placed between the roof deck and membrane (the most typical approach), insulation above a roof membrane or below a roof deck may be needed to satisfy these minimum and maximum requirements.
So how does this impact the energy efficiency trend? In order to achieve the maximum insulating benefit from roof systems, it is best to use at least two layers of insulation and stagger the board joints. This approach provides a more stable substrate for the roof membrane and minimizes the likelihood of unwanted air movement within a roof system.
A single layer of roof insulation means there is a high possibility of air movement from the deck to the underside of the membrane. And because air movement accounts for a significant portion of a building’s heat loss, staggered board joints are quite beneficial to the overall R-value of a roof system.
Additionally, for proper installation of multi-layer insulation, all layers of insulation should be adhered to each other (in order to account for wind resistance). If mechanical fasteners are used, they should not secure the entire thickness of insulation, but rather, they should be used to fasten only the bottom layer(s) of insulation. Mechanical fasteners through the entire layer of insulation will be thermal bridges, reducing the R-value of the roof system.
Installation And Energy
Proper design and installation of a roof system’s insulation layer are critical to the accuracy of the energy modeling of a building. If facility managers (fms) assume a roof system has a design R-value of 30 but the insulation board joints align vertically and mechanical fasteners are used to secure the entire thickness of insulation, this could present a problem.
Studies have shown that this configuration can create a loss of R-value of over 15%. In fact, the in place R-value would be closer to 25.
This “loss” due to an improper design can significantly alter the energy modeling assumptions. In the end, this can change outcomes of anticipated energy efficiencies and savings.
Additionally, the National Roofing Contractors Association (NRCA) now recommends the per inch design R-value for rigid board polyisocyanurate insulation be 5.0 for heating conditions and 5.6 for cooling conditions. (More information on this topic can be found in The NRCA Roofing Manual: Membrane Roof Systems—2011.)
The rationale to be conservative with insulation R-value is much more appropriate than to overestimate the R-value. Otherwise, fms could end up with incorrect energy modeling and analysis of their buildings.
Energy: Save It Or Create It?
While there are merits of insulation, the sexier trends regarding energy efficiency and sustainability of roofing systems involve energy production (e.g., photovoltaics), vegetation and gardens, and highly reflective surfaces.
Roof mounted photovoltaic (PV) systems are trending upward because of the 30% federal tax credit. There are also many state and local rebate and incentive programs. (More information about tax credits, rebates, and incentives can be found online.)
The American Recovery and Reinvestment Act of 2009 (ARRA) has incentivized the PV industry, and the industry is continuing its use of rooftops as platforms for energy production. Roof mounted PV installations can be better understood if fms are familiar with the two most common types: rack mounted PV panels and adhered PV panels.
Rack mounted PV panels are supported by a rack which is placed on top of the roof surface and held in place with ballasts or penetrates the roof system and is attached to the structure or roof deck. The trend is definitely moving towards ballasted, non-penetrating rack systems. However, from a roof system point of view, the ballasted, non-penetrating option may not be the best long-term solution. The roofing industry has learned that items that are not secured in place tend to move and cause damage to roof membranes due to extreme weather events. Sometimes, many small weather events can be equally harmful.
Experts in the roofing industry also have concerns about wind resistance of ballasted, non-penetrating rack-mounted PV panels. Cooperation between the PV and roofing industries is needed to ensure roof mounted PV systems do not reduce the weatherproofing or service life of a roof system. A professional roofing contractor is essential to ensuring a weatherproof installation.
Additionally, there may be issues with the potential reduction of fire resistance when rack mounted PV panels are installed on a roof system. Solar America Board for Codes and Standards (Solar ABCs) recently undertook a research project to determine how a roof system’s fire rating could be affected when rack mounted PV panels are installed. The initial work did not provide comforting results, as many of the roof systems’ fire ratings were reduced when tested with a rack mounted PV panel placed on top of it. [This report can be found at the Solar ABCs website.]
Meanwhile, vegetative roof systems continue their growth (no pun intended). This trend is also continuing in large part because of local mandates and incentives. However, the trend seems to be for 50% rooftop coverage rather than 100% rooftop coverage.
The benefits of a vegetative roof are more ideally achieved when this approach is used over an entire roof area. Benefits may include storm water management, rooftop temperature reduction, and extended service life. NRCA recommends using a durable waterproofing membrane under a vegetative roof system as a best practice for the design and installation of these systems.
White and light colored, highly reflective roof surfaces also are trending up and have been for some time. The benefit of roof surface reflectivity should be looked at from two perspectives: the building and the overall environment.
Low rise buildings with large roof areas (relative to the entire building envelope) are likely to be more energy efficient during periods of air conditioning operation when a highly reflective roof surface is used to reduce solar gain into the building. On the other hand, a highly reflective roof will reflect the sun’s heat energy, subsequently reducing rooftop temperatures. This is overall beneficial for the environment regardless of building size, proportions, and location. Understanding the different perspectives about the benefits of using a reflective roof surface is important when determining if a building will or will not glean reduced energy consumption.
The most recent trend in roof system design, as well as building envelope design, is the use of air barriers. The use of air barriers is trending up relatively quickly and may significantly alter the way roof systems are designed, installed, and maintained.
Air barrier requirements have been included in standards specific to high performance buildings. Recently, they have been included in standards specific to all buildings as well. Mandates for air barriers definitely should be on the radar screen for fms trying to stay on top of the roofing game.
A weatherproof roof system starts with proper design, quality materials, proper installation, and regular maintenance. This should be the starting point or minimum for every roof system.
The trends—efficiency and sustainability through energy production, vegetation, and reflectivity—are noble and considerate; however, they are secondary benefits that can be achieved only after a quality roof system is in place. Fms should not forget the primary function of a roof—to make a building’s top surface weatherproof!
Kirby is associate executive director, technical communications for the National Roofing Contractors Association. For more information on NRCA, visit the NRCA Bookstore (for publications) and NRCA University (for educational offerings) online, or call (800) 323-9545.
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Topic Tags: TFM-Feb-2011