What is a Cool Roof?
A cool roof is one that reflects the sun’s heat and emits absorbed heat back into
the atmosphere. The roof literally stays cooler and reduces the amount of heat
transferred to the building below, keeping the building cooler at a more constant
temperature. Imagine wearing a white or a black T-shirt on a sunny hot day. By
wearing the white T-shirt, you will remain cooler than if you wore a black T-shirt
because it reflects more sunlight and absorbs less heat. Cool roofs are like a
white T-shirt, keeping the internal temperature of the building cooler.
The two basic characteristics that determine the ‘coolness’ of a roof are solar
reflectance (SR) and thermal emittance (TE). Both properties are rated on a scale
from 0 to 1, where 1 is the most reflective or emissive.
Solar Reflectance is sunlight that is reflected and not absorbed as heat. For
example, a surface that reflects 55% of sunlight has a solar reflectance of 0.55.
Most dark roof materials reflect 5 to 20% of incoming sunlight, while light-colored
roof materials typically reflect 55 to 90%. Solar reflectance has the biggest effect
on keeping your roof cool in the sun.
Thermal Emittance describes how efficiently a surface cools itself by emitting
thermal radiation. Thermal emittance is measured on a scale of 0 to 1, where a
value of 1 indicates a perfectly efficient emitter.
There are numerous benefits in having a cool roof:
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Reducing your utility bills associated with air conditioning
Increasing occupant comfort and avoid installing an air conditioner
where there isn’t already one
Decreasing the size and prolong the life of your air conditioning
system
Lowering roof maintenance costs and extend roof life, avoiding
reroofing costs and reducing solid waste
Receive utility rebates as reductions in installation costs.
In a nutshell, a cool roof can significantly reduce your cooling energy costs and
increase your comfort level by reducing temperature fluctuations inside your
building. Average energy savings range from 15% to 35% of total cooling costs.
Why Use Cool Roofs
Cool roofs are roofs that are designed to maintain a lower roof temperature than
traditional roofs while the sun is shining. Sunlight is the primary factor that causes
roofs to become very hot.
Since most dark roofs absorb 90% or more of the incoming solar energy, the roof
can reach temperatures higher than 150°F (66ºC) when it’s warm and sunny.
Higher roof temperatures increase the heat flow into the building, causing the air
conditioning system to work harder and use more energy in summertime. In
contrast, light-colored roofs absorb less than 50% of the solar energy, reducing
the roof temperature and decreasing air conditioning energy use.
Heat Gains, Heat Losses, and Thermal Insulation
Heat flows naturally from a warmer space to a colder space. Heat that flows into
the building is called heat gain, while heat that flows out of the building is called
heat loss. When too much heat gain (loss) occurs, your air conditioning system
(heating system) operates to keep the space comfortable. A large amount of heat
can be gained (or lost) through a building’s roof.
Cool roofs reduce heat gains throughout the year.
How Much Will You Save
Cool roofs can save money in several ways, including energy savings, rebates and
incentives, HVAC equipment downsizing, and extended roof lifetime.
Energy Savings from reducing cooling loads are achieved each year, reducing
building operating costs. Climate, roof reflectance, insulation levels, utility rates,
and HVAC equipment efficiency all affect the expected savings.
Rebates and Incentives for cool roofs are offered by some utilities and agencies.
To find out if there are any rebate programs in your location check with your local
power provider. Contact the rebate program at your electricity power supplier’s
website to determine the probable savings associated with a cool roof rebate.
HVAC Equipment Savings may be achieved when a cool roof reduces peak cooling
loads significantly enough to reduce the air conditioning capacity needed. The
associated savings can be realized immediately.
Extended Roof Lifetime is the largest advantage of using cool roofs. Roofs failure
is linked to temperature. Higher temperatures speed up material degradation.
Cool roofs maintain a lower average temperature, so this slows heat-related
degradation. Furthermore cooler roof temperatures slow color fading. In cases
where heat-related degradation is the main reason for roof failure, a cool roof will
be more durable and outlast a similar dark roof.
Cool Roof Selection and Applications
The decision to make your existing roof a cool roof usually means deciding to coat
the roof or replace the roof. If your roof is in good condition; has relatively few,
easy-to-repair leaks, a cool coating may be the best option.
The main reason for coating a roof is to extend its service life, and the energy
savings alone will normally provide sufficient financial reason to coat a roof to
make it cool. If your roof is in poor condition, or is approaching the end of its
service life, additional repairs may be needed before the coating project
proceeds.
Roof Coatings can be rolled, brushed, or sprayed onto most kinds of roofs. Any
leaks or problems with the roof membrane must be repaired before coating the
roof. Regardless of how the roof is installed or replaced, here are some notes
about cool roof options.
Single-Ply Membranes come in several types. Older TPO and PVC membranes are
usually light-gray. EPDM membranes are black and they should not be used in the
lower states as the heat gain can take the roof over 166 degrees which is then
translated into much higher energy costs to cool the building.
The types of roof membranes that are attached with chemical adhesives, melted
together, or mechanically attached are subject to the changes from daytime very
hot, to cooler temperatures at night. This expansion and reduction in
temperatures is translated into seams coming apart from the movement. These
types of roofs typically have a no-problem expectancy of 10 years in the lower
states although the manufactures typically state the material warranty can be as
high as 50 years.
Built-Up Roofs are some of the best roofs and have provided some of the most
cost efficient building protection. The problem with these tar & gravel roof
systems is that the insurance industry will not insure the contractors any more.
The insurance pay-outs have been astronomical to the injured . Also, once the tar
roof catches fire it is almost impossible to stop, and the entire building is lost. The
insurance rate is usually higher for these types of roofs.
Modified Bitumen roofs should have protective coatings to provide heat
resistance, ultraviolet resistance, and fire resistance. The problem with these
types of roof is that when there is even a small amount of water penetration the
membrane expands and bubbles of various sizes form, which further pulls and
degrades the seams, and the water penetration problem accelerates. This
process can take as little as two years to soak the roofing insulation making it
non-effective.
Metal Roofs that are unpainted tend to have a high reflectance but a low thermal
emittance, which prevents them from being considered cool in low slope
applications. Applying a cool roof coating will substantially lower the temperature
and seals off the oxygen which stops rusting.
Spray Polyurethane Foam roofs must be coated to protect the foam from UV rays
and water damage. Spray polyurethane foam deteriorates rapidly upon exposure
to sunlight and all insulation properties are lost quickly.
Asphalt Shingles are not typically coated in the field and doing so may void the
manufacturer’s warranty. Other shingles, like those made from wood, polymer, or
metal, can be coated to achieve cool roof status.
Tile Roofs can be coated easily and custom colors can keep the architectural
appearance intact.
Ponding Water occurs when water, typically from rain, accumulates in pools on
the roof for more than 48 hours. This happens when a roof has insufficient slope
(caused by poor design or damage) or drain blockage. It takes longer for ponded
water to evaporate from a cool roof due to its lower temperature. If your cool
roof cannot tolerate ponding, it may be necessary to inspect the roof more
frequently to prevent damage or leaks. There is only one product on the market
that does not automatically void the warranty if there is ponding water for longer
than 48 hours.
When it comes to roofing and the costs considerations, there have been advances
in coatings technology that make selection of what roofing material to use a much
easier process. Since the development and commercial application of pure white
liquid silicone as the roofing material, the roofing industry has undergone dramatic
changes. No longer is it necessary to tear off the old roof and then install the new
roof. If the roof substrate is sound and in relatively good condition the silicone
coatings that have been developed have dramatically reduced the cost of replacing
a roof.
Liquid silicone coating was initially developed to line the inside of drinking water
tanks. The coating was and is applied to the inside of the water tank to keep it from
rusting from the inside, out. These silicone lined water tanks are still in service and
show little degradation after twenty-five years. The silicone that was applied to
the inside of the tanks has been under water for all of that time.
That technology is now applied to roofing because of one other important but very
large factor. When the liquid silicone cures, it is not affected by the outside
elements, the weather. Which includes the fact that there is almost no broad
spectrum Ultra Violate A or Ultra Violate B degradation by the rays of the sun.
Therefore, we have a product that can be under water for extended periods of
time. Coupled with the fact that liquid silicone product experiences little or no
degradation when exposed to all exterior elements. However, there is the third
characteristic of liquid silicone that qualifies it as a unique roofing material. The
flow and bridging characteristics make liquid silicone an extremely practical
product to use in a roofing environment.
The flow characteristics allow the liquid silicone to settle down into all of the
microscopic depressions wherein the liquid silicone attaches to more surface,
howbeit, microscopic surface. This attaches the liquid silicone better to the roof
surface. The liquid silicone is not sitting on the top of the old roof just bridging
across gaps. Another part of the flow characteristics is that the liquid silicone is
self-leveling. This characteristic contributes to a roof that drains properly.
The bridging characteristics of liquid silicone has to do with the cross-linked
molecules that make the cured silicone roof a one-piece monolithic roof. The
manufacture’s specifications mandate that the liquid silicone be applied up all
parapet walls and that the inside half of parapet wall cap also be coated. From the
top of the parapet wall cap, down the inside of the parapet wall, covering all of the
roof and ending at the coated roof drain, the use of liquid silicone is the ultimate
roof for fifteen years at a cost that makes the consideration of any other type of
roofing material unimaginable.
Brian and Tom Dieter, the owners of The Texas Professional Roof Coating
Corporation, are dedicated professionals who will assist you in choosing the right
roofing material for your roof. Whether it is a flat roof, barrel or flat tile, or a
conventional roof, they will provide a free inspection and quote for the ten (10) or
fifteen (15) year warranty liquid silicone that their company installs.
Life Cycle Cost Effects (LCC)
Life cycle costing is an excellent vehicle for demonstrating that low first cost, i.e. a cheap roof, may
actually be more expensive than a premium system when studied over the life of the roof. A simple
analogy exists with an automobile requiring routine maintenance. The premium automobile will
ideally require fewer repairs during its service life than a cheaper model.
The use of coatings to prolong the roof has been studied and shown to reduce the life cycle costs for
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the roof. Obviously, the longer a roof can remain in place without the need for costly tear off,
disposal and the disruption to the occupants during the re-roofing, the lower the long term cost.
Coating and subsequent recoating offer several additional benefits from a LCC standpoint.
Summary
Reflective roof coatings have proven to be a useful and complimentary tool in maintaining and
sustaining roofs. From the early use of asphalt cutback resaturants used to infuse light asphalt
fractions to today’s high performance elastomeric coatings which not only protect but also provide
air conditioning energy savings through solar reflectivity, these coatings have demonstrated
significant “Value-Add” to the building owner. Today’s high performance coatings are actually more
weather resistant than some roof membranes they protect. Ideally, in the future, only the original roof
will be required and merely coated periodically, just as floors or walls are painted and repainted.
“Sustainable Roofing” exists today with Reflective roof coatings.