Imagine a roof design you are specifying, but the roof is adjacent to a taller building which could be offices or a hotel. You check the orientation and find that the mid-day sun will be reflected off the lower roof you’re designing and provide the office or hotel occupants with a bright glare. Maybe you just don’t like the aesthetics of having a white lower roof that’s part of a larger complex. Do these concerns automatically rule out thermoplastic roof membranes such as TPO or PVC? Fortunately no, there are lower reflectance thermoplastic membranes that will still provide some improvements in energy efficiency versus a dark roof and the security of welded seams.
The following is an analysis of the impact on energy efficiency of selecting a gray or tan TPO membrane versus a dark membrane such as EPDM, and a highly reflective membrane such as white TPO. There can be code requirements that drive reflectivity choices and these are briefly discussed at the end of this blog.
Description of Building – The building used for this study was a single story big-box type, less than 35 feet in height, with a roof area of 125,000 ft2 in a rectangular configuration of approximately 290 × 431 ft. The roof was assumed to be a new installation, i.e., new construction or a total roof replacement.
Roof Assembly – Three thermoplastic polyolefin membranes, TPO, were evaluated with three year aged solar reflectance, emissivity, and solar reflectance index values shown below:
Typical three-year solar reflectance, emissivity, and solar reflectance index values for the TPO membranes evaluated.
|Color||Solar Reflectance||Emissivity||Solar Reflectance Index||Designation|
These three-year values represent long-term roof performance and data for most membranes can be found in the Cool Roof Rating Council’s directory of rated products.
An insulation thermal resistance of R-30 was used because this value is representative for most US locations per the 2015 International Energy Conservation Code. Therefore, results and conclusions of this study would also be applicable to new construction and reroofing of existing buildings.
Building Locations and Energy Costs – For this study, thirteen cities were considered and their energy costs are shown below. The gas and electric costs for commercial customers were obtained from the US Energy Information Administration and averaged over 2017:
|City, State||Electric Cost||Gas Cost|
|Fort Worth, TX||0.1115||7.71||0.771|
It was assumed that heating was by natural gas and all buildings were air conditioned.
Electric demand charges were assumed to be $15 / kW, in line with a 2017 National Renewable Energy Labs survey.
Converting from a Dark to a Highly Reflective Roof
Comparing modeled energy costs for a white highly reflective roof membrane, such as GAF EverGuard TPO, versus those for a dark, absorptive membrane such as EPDM, the following annual savings are projected, for the 125,000 sq.ft. building.
Converting from a Dark to a Mid Reflectance Roof (e.g. Tan TPO)
Comparing modeled energy costs for a mid-reflectance roof membrane, such as GAF EverGuard Tan TPO, versus those for a dark membrane such as EPDM, the following savings are projected, for the 125,000 sq.ft. building.
Converting from a Dark to a Low Reflectance Roof (e.g. Gray TPO)
Comparing modeled energy costs for a mid-reflectance roof membrane, such as GAF EverGuard Gray TPO, versus those for a dark membrane such as EPDM, the following savings are projected, for the 125,000 sq.ft. building.
The results clearly show that energy savings are reduced for the lower reflectance membranes such as tan or gray versus white. However, savings are still projected to occur for the non-white (gray and tan) membranes versus an EPDM or similar dark colored roof.
This means that for those who want the advantages of welded seams, but want reduced reflectivity, then membranes such as tan or gray can be used while still giving some improvement in energy efficiency.
Colored TPO and Energy Codes
There are national and local programs or policies related to cool roofing, each with their own unique criteria and definitions of cool roofing.
What to do if you don’t want white, but you still want to be LEED compliant?
It could be that for aesthetic reasons you don’t want to use a bright white membrane, but you want to meet the LEED standard for the roof. There are two GAF membranes that you could consider for such an application; GAF EverGuard EnergyTan™ and EnergyGray™. The 3 year CRRC data for these are shown below.
|Membrane||Solar Reflectance||Emittance||Solar Reflectance Index|
|GAF EverGuard EnergyGray™ TPO||0.66||0.89||80|
|GAF EverGuard EnergyTan™ TPO||0.67||0.90||82|
These two membranes meet the requirements for “cool” roofing, but are slightly off-white and could offer aesthetic advantages.
Want to know more? The study shown here is based on a larger article published in Buildings which was peer reviewed by independent experts.