Rooftop surface choices significantly impact building temperature and energy efficiency. The solar reflectance of the outermost roof layer, exposed to the sun, determines its coolness. Selecting a cool roof covering is crucial for energy conservation, and fortunately, cool options are available for nearly all roof types.
Smart Paths to a Cooler Roof
Opting for cool roofing products doesn’t necessarily mean higher upfront costs compared to conventional options. The most economical approach to achieving a cool roof is by choosing a cool covering during new construction or when replacing an existing roof. While retrofitting existing roofs with cool coatings is possible, it does involve additional expenses for materials and labor.
The Power of Color
White roofing materials excel at reflecting sunlight, bouncing back approximately 60–90% of solar radiation and staying remarkably cool. However, even darker materials can be optimized for coolness. By incorporating special pigments that reflect invisible “near-infrared” radiation, the solar reflectance of darker shades can be significantly increased. These “cool colored” products typically reflect 30–60% of sunlight, offering a cooler alternative to standard colored materials, albeit not as cool as white roofs.
Cool Roofing Solutions for Steep Roofs (Pitch > 2:12)
Asphalt shingles: These common roofing elements consist of a fiberglass web coated with asphalt or modified asphalt, finished with colored, ceramic-coated rock granules. Their overlapping installation facilitates water shedding and allows for breathability, preventing moisture buildup.
Cool option: Choose asphalt shingles pre-surfaced with light-colored or cool-colored granules for enhanced reflectivity. Applying field coatings to existing asphalt shingle roofs for cooling purposes is generally discouraged due to potential moisture retention issues and warranty invalidation.
Wood shingles and shakes: Naturally “cool colored,” wood shingles (machine-sawn) and wood shakes (hand-split) are overlapped to create a roof covering that inherently reflects heat.
Cool option: Wood materials are naturally a cooler choice.
Polymer shingles: Synthetic or composite shingles made from polymer are designed to mimic the appearance of various materials like asphalt, wood, slate, or tile.
Cool option: Select polymer shingles that are factory-colored with light or cool pigments to maximize solar reflectance.
Clay tiles: Available in unglazed terra cotta or glazed varieties, clay tiles offer both aesthetic appeal and functional benefits.
Cool options: Terra cotta clay tiles are naturally cool-colored. For glazed tiles, opt for factory-fired light or cool-colored glazes. Field-applied light or cool-colored polymer coatings can also be used during installation for glazed tiles.
Concrete tiles: Typically made from gray-cement concrete, concrete tiles can be integrally colored or surface-treated with slurry or polymer coatings.
Cool options: Choose concrete tiles with factory-applied light or cool-colored slurry or polymer coatings. Field-applied coatings are also a viable retrofit option for existing concrete tile roofs.
Slate tiles: While durable and aesthetically pleasing, slate tiles are generally not considered a cool roofing option.
Cool options: Slate tiles are not typically cool.
Metal shingles or tiles: Metal roofing pieces, often shaped to resemble other roofing materials, can be unpainted, factory-painted, or mineral-granule surfaced.
Cool options: Opt for metal shingles or tiles with factory-applied light or cool-colored paints or reflective mineral granules. Unpainted metals, while reflective, tend to retain heat and are not ideal cool roof choices. Factory-applied paints are more durable and cost-effective than field applications.
Cool Roofing Solutions for Low-Sloped Roofs (Pitch ≤ 2:12)
Single-ply membranes: Prefabricated sheets rolled onto the roof and secured using various methods, single-ply membranes offer a versatile low-slope roofing solution.
Cool option: Choose membranes factory-colored with light or cool pigments or utilize light-colored ballast for enhanced reflectivity.
Built-up roofs: Constructed with base sheets, reinforcing fabric layers, and a protective surface layer (often cap sheets or ballast), built-up roofs can be optimized for coolness.
Cool options:
- Select cap sheets with factory-applied reflective mineral surfaces or coatings.
- Use reflective marble chips or gray slag instead of dark gravel in asphalt flood coats.
- Apply field-applied reflective coatings to existing built-up roofs.
Modified bitumen sheet membranes: Featuring plastic or rubber layers with reinforcing fabrics and mineral granule or smooth finishes, these membranes can also top built-up roofs for a hybrid system.
Cool options: Choose cap sheets with factory-applied reflective mineral surfaces or coatings.
Spray polyurethane foam roofs: Formed by reacting and expanding liquid chemicals, these roofs adhere as a solid piece and require protective coatings due to their vulnerability to damage.
Cool option: Protective coatings for spray polyurethane foam roofs are typically reflective, providing inherent cool roof performance.
Versatile Solutions for Steep and Low-Sloped Roofs
Standing-seam metal roofs: Formed by interlocking metal panels with raised vertical seams, standing-seam metal roofs can be unpainted, factory-painted, or mineral-granule surfaced.
Cool options: Refer to options for “metal shingles or tiles” for cool choices.
Green (Vegetative) Roofs: A Natural Cooling System
Green roofs, characterized by vegetation cover, offer a unique approach to rooftop cooling. Ideal for urban buildings with low-sloped or shallow-pitch roofs, they range from simple plant cover to elaborate gardens. Beyond cooling, green roofs excel at stormwater management and creating appealing rooftop spaces.
Green roofs are “inverted” systems with the waterproofing membrane situated beneath the plants, shielded from UV degradation. However, leak detection and repair can be complex. They are heavier and more costly than conventional roofs, requiring structural reinforcement and ongoing plant maintenance. Thoroughly assess building suitability, climate, and consult professionals before considering a green roof.
Green roofs cool primarily through evapotranspiration from plant surfaces, supplemented by the insulation and thermal mass of the soil layer.
Deciding on and Timing Cool Roof Installation
Evaluate the energy savings and benefits against any potential cost premiums when deciding on a cool roof. Savings depend on climate, insulation levels, roof type, and HVAC system efficiency. New constructions offer the best opportunity to integrate cool roofing from the outset. For existing roofs, consider these options for conversion:
- Install a cool roofing product at the end of the existing roof’s lifespan.
- Avoid wasteful early replacement by considering a cool roof coating application if suitable for the existing roof type.
- Install a cool roofing product prematurely which might be wasteful.
Cost Considerations and Energy Savings
Cool roofs don’t inherently cost more than standard roofs, particularly in new installations or replacements. However, retrofitting a functional standard roof can be a significant investment.
Climate and Environmental Factors
Climate is a key determinant in cool roof effectiveness. Hot climates yield the greatest cooling savings, while colder climates might see increased heating costs that could offset cooling benefits.
Moisture Management
In humid, warm climates prone to algae or mold, light-colored cool roofs may show dark growths more visibly. Some coatings incorporate anti-algae and anti-mold chemicals for short-term protection.
In colder climates, condensation-related moisture accumulation is a concern, and cool roofs might be more susceptible if not properly designed with moisture control in mind.
Finding Cool Roofing Products
Cool roof standards vary by program, location, building type, and roof slope, lacking a universal definition. The Cool Roof Rating Council’s Rated Products Directory provides solar reflectance and thermal emittance data for numerous roofing products, aiding in informed selection.
