Which is Better – a Cool Roof or Green Roof?

When selecting a roof, there are many different options for roofing contractors, including thermoplastic, EPDM, metal, shingles, greens roofs and more. While each of these possesses its own advantages and disadvantages, cool roofs and green roofs present facility owners with great options. Cool roofs reflect sunlight and heat back into the atmosphere while green roofs help absorb the sunlight and heat before it can enter the building. After selecting to use a green roof or cool roof, the roofing contractor must look at the pros and cons of each in order to decide which would be best for the property.

Cool roofs are a viable option and made of materials that help reflect the sun’s energy through light-colored paints, roof tiles, coatings and shingles. Another option is a vegetative roof that holds plant life and reduces reflective heat. Green and cool roofs help buildings in highly-populated areas stay cool by elevating the “heat island effect.” Streets, parking lots and rooftops contribute to the heat island effect and can bring roof surface temperatures up to 70 degrees hotter than the ambient temperature. Green roofs help a building stabilize air above the roof to near ambient temperature.

History

Buildings in hot climates around the world have been whitewashed for centuries. Green roofs are not a new concept either and are one of man’s earliest roofs. Ancient roof gardens were installed as far back as 2020 BC in Mesopotamia, and have been widely used in Europe for more than 70 years.

Advantages

One of the largest advantages to having a cool roof or a green roof when compared to other options is the aesthetics the roof system produces. These roofs have been known to reduce energy bills by decreasing air conditioning needs, improve indoor comfort for spaces that are not air conditioned and decrease roof temperature, which can extend service life. Another advantage to having a cool roof or green roof is the environmental impact. These roofing options reduce local air temperature and lower peak electricity demands that can help prevent power outages. When a cool roof is used on a power plant, emissions can be reduced, including carbon dioxide, sulfur dioxide, nitrous oxides and mercury.

Green roofs can reduce noise infiltration and offer hail protection. They can also extend the lifecycle of the roof system by eliminating the UV rays from directly hitting the roof membrane. Another advantage is the design to delay, filter and reduce the volume of storm water runoff. Green roofs typically hold between 50 and 60 percent of storm water, which lessens demand on storm drains. Lastly, green or garden roofs provide an aesthetically pleasing sitting or walking area to building tenants. Tenants are not the only ones that can benefit from a green or garden roof. Having a green or garden roof can provide tenants with the option of creating a habitat for birds and insects. 

Disadvantages/Considerations

While there are many advantages to having a cool roof or a green roof, there are also disadvantages and common considerations that come along with these roof systems. One of the most prominent disadvantages to a green roof is that it will need an irrigation system in certain climates to water the plants, which can be costly and roof gardens are not always maintenance free. Depending on the vegetation, weeding and fertilization may also be required.

Consider the geographical impact that having a cool or green roof installed may pose. There are many restrictions to take into consideration when installing a cool roof in certain areas of the United States. Areas in the northern half of the country require more heating cost than cooling cost; in this case there is no advantage to a cool roof. Facilities must consider the possibility of a green or cool roof’s modestly increasing heating costs in winter. The small increase in heating costs can be attributed to a few reasons. Winter months bring less hours of sunlight causing the sun’s natural heat to reflect off of the roof rather than be absorbed by it. A rise in costs during the winter should be taken into consideration when choosing to install a cool roof or green roof. Cool roofs achieve the greatest cooling savings in hot climates.

Finally, there are also potential aesthetic disadvantages. One of the largest aesthetic disadvantages is that cool, reflective roofs may cause an unwanted glare. Unwanted glare can be a nuisance to neighbors or other businesses that are close by. Another disadvantage is that cool roofs tend to be varying shades of white, making dirt and grime more visible. Green roofs tend to be very heavy; some up to 25 lbs. per square foot when full of water. Many structures simply can’t handle that load.

Cost and Energy Savings

One of the most important things to consider when weighing the options between a green roof and a cool roof is the long-term cost and energy savings. The install cost of a cool roof is not necessarily more expensive than a non-cool roof, which is a common misconception among building owners. Cool roofs achieve the greatest cooling savings in hot climates. The costs tend to enter the equation when a standard roof in good condition is converted into a cool roof. The transformation process can get fairly expensive and should be evaluated closely. When adding a new roof system, building owners can save money by having the roofing contractor add more insulation to the roof system. Green roofs may also be eligible for  city, state, and federal rebates, grants and other incentives to help offset their cost. 

When installing a green roof it’s important to install a “beefy” roof membrane. If there’s a leak, the vegetation and growth media need to be removed to make the repair. In most cases, the cost to remove the green roof is handled by the building owner.

Climate and Environment

The final thing to consider when a contractor is weighing the options of installing a cool roof is moisture control. A business located in warm, moist climates with a cool roof can be more susceptible to algae and mold growth. However, there are options to control moisture. Special coatings and chemicals that prevent mold or algae from growing are available. In cold climates, cool roofs can accumulate moisture through condensation if not designed properly. Cool roofs in northern locations can lower the dew point to where it’s in the roof system and cause condensation and moisture to migrate into the roof. In these cases, a vapor barrier needs to be used, adding to the overall cost. Condensation can be avoided using proper design techniques.

Conclusion

The idea of installing a cool or green roof is a decision that contractors need to evaluate properly. A few things to consider when selecting a roof system include geography, occupancy and insulation thickness. There are many appealing aspects to a cool or green roof with aesthetics, reduction of local air temperatures and energy savings. But while these might be appealing, there are many disadvantages and considerations to a cool or green roof. There are also times when it makes sense to go with another type of roofing system. The best way to save energy in a roof system is to add more insulation. Please contact a designer professional to help determine which roof system makes sense for the property.

About the Author

Sonny McKellar is a business development manager at RSS Roofing Services & Solutions, a nationally recognized design-build roofing contractor covering projects of all sizes for the commercial, industrial and institutional markets. RSS Roofing Services & Solutions, a subsidiary of MHS Legacy Group, is headquartered in St. Louis and has locations in Florida, Indiana and Tennessee, along with a special projects division. For more information, call (812) 465-2440 or visit www.roofingsands.com.