Reflective Roof 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
- Quantify the long-term energy and durability benefits of reflective roofing materials to the roofing industry
- 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. 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. 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.
