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Reflective Roofing Systems |
introduction
We have all felt the effects of dark versus light clothing
on a sunny summer day. Because dark colors reflect less solar energy,
dark is hot and light is cool. The same principles govern roof temperatures.
Light-colored roof materials make for a cool roof by reflecting more
of the sun's rays. Because over 50 percent of the sun's energy received
is in the visible range, the color of a roof system is often a good indicator
as to what percentage of the incident solar energy is reflected by the surface.
Light-colored roof materials have a high solar reflectance (as high as 85
percent), whereas dark-colored roof materials have a reflectance of less than
10 percent.
The lower temperatures of light-colored roofs reduce the
air-conditioning energy needed to maintain comfort cooling. Homeowners
want the roofs on their homes to protect the underlying structure for a long
time at an affordable cost. Since the roof is generally visible from
ground level, appearance is important. It is a common perception that
dark roofs look better than light roofs. Also, the attic under many
pitched roofs shields ceilings from hot roof temperatures. However,
in commercial businesses, the roofs are predominately low-slope and are not
visible from ground level. Reduced air-conditioning costs and other
potential savings from light-colored roofs can be weighed directly against
cost premiums for the cool roofs.
issues
Buildings with light-colored roofing
materials better reflect sunlight and significantly reduce the peak roof temperatures.
The lower roof temperature in turn improves the durability and life of the roof.
Cool roof coatings and membranes can therefore be a critical component of a
proactive roof maintenance program that results in lower lifetime roof temperatures during
sunny periods. The lower temperatures, in turn, reduce the air conditioning loads of
the building and potentially lengthen the service life of the roofing system. If the
building is located where cooling loads predominate, peak load reductions and net annual
energy savings are also realized.
Our experience with cool roof membranes, and especially coatings
that have high initial solar reflectance (over 0.75), shows that they age
under field conditions. The reflectance of a roofing system changes
with time as the weather degrades the surface or as dirt accumulates on the
surface. Typical results are that reflectance is reduced by about 0.1
to 0.3 after the first several years and then levels out. As the solar
reflectance changes, so do the potential energy savings. Washing helps;
however, the membranes we have tested in the past have not maintained their
high level of initial reflectivity even after washing. The roofing industry
recognizes that a record of long-term performance is needed in order to develop
realistic, defensible claims about annualized energy savings and affordable
cost premiums. Testing is needed to document the long-term performance
of reflective roofing systems, since the data accumulated to date show significant
variation.
objectives
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Quantify the long-term energy and durability benefits of reflective roofing materials to the roofing industry |
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Develop and promote fair and credible procedures to help consumers choose a coating or membrane for specific applications |
research
For the past ten years, research has been focused on applying
white radiation control coatings to existing low-slope roofs with non-reflecting
membranes. The project has evolved into a three-year study of 24 different
white and aluminum coatings. This research, begun in the summer of 1997,
is conducted in partnership with the Roof Coating Manufacturers Association
and several coating manufacturers.
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| Preparing the base uncoated roof test section | Laying out the test areas for different coatings | Applying individual coatings by roller or brush | A completed test section with five coated areas and one uncoated area |
This project involves periodic measurement of the solar reflectance
and infrared emittance of each coating and continuous measurement of the thermal
performance of the coatings on instrumented test sections. Temperatures
throughout insulated test sections (especially the temperature of the external
surface) are measured as a function of weather conditions. As heat flows
through the test section, it is monitored continuously to help calibrate models
of each test section's thermal behavior. Calibrated models help to predict
the behavior of similarly coated roofs with varying levels of insulation in
different climates. Measurements of the mechanical performance of the
coatings are possible with separate samples of the same coatings on uninstrumented
weathering panels. Results from the test panels will be analyzed and
compared with concurrent results from uncoated substrates and with results
from other ORNL on-site tests. Long-term monitoring is needed to capture
the effects of weathering on the coatings, especially on their solar reflectance.
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| Measuring the solar reflectance after coatings have weathered | Layout of low-slope roofing systems on half of test building for SPRI project; metal roof project will take place on other half of roof | A typical metal roof; metal roof project will test over ten different configurations |
A similar test procedure is being followed in a 3-year study
of 18 different low-slope roofing systems. This project began in the
summer of 1998 and cooperates with several members of SPRI (sheet membrane
and component suppliers to the commercial roofing industry) to study the energy
efficiency and service life of single-ply low-slope roofing systems.
A third project, started in the summer of 1999, involves five U.S. steel roof
manufacturers who are keenly interested in documenting whether both high-
and low-slope bare and painted metal roofs can reduce building heating / cooling
loads.
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