Ballasted Roof Systems

Ballasted roofing systems are fairly common and consist of a membrane that is usually loose laid on the roof deck and held in place by the weight of a natural stone or precast concrete pavers.

The weight of the stone or pavers employs the force of gravity to hold the roof membrane in place and counter the uplift forces of wind.

While not technically an “ENERGY STAR” roof, recent research by Oak Ridge National Laboratory (ORNL) indicates that ballasted roof systems can save energy. To learn more about this, please visit: http://www.ornl.gov/sci/roofs+walls/staff/papers/21.pdf  

“Green” Roofs

A green roof, or rooftop garden, is a vegetative layer grown on a rooftop. Green roofs provide shade and remove heat from the air through evapotranspiration, reducing temperatures of the roof surface and the surrounding air.

Green roofs absorb heat and act as insulators for buildings, reducing energy needed to provide cooling and heating. For more information on green roofs, please visit: http://www.epa.gov/heatisland/mitigation/greenroofs.htm

What is Emissivity?

The emittance of a material refers to its ability to release absorbed heat. Scientists use a number between 0 and 1, or 0% and 100%, to express emittance. With the exception of a metallic surface, most roofing materials can have emittance values above 0.85 (85%).

One example is a metal wrench left in the sun, which is hot to the touch because it has a low emissivity value.

Link Between Energy Savings and Emissivity

Solar reflectance is the most important characteristic of a roof product in terms of yielding the highest energy savings during warmer months.

The higher the solar reflective value the more efficient the product is in reflecting sunlight and heat away from the building and reducing roof temperature. This is particularly important in areas of the U.S. where peak load is a concern.

Emissivity can also contribute to a cool roof. In warm and sunny climates highly emissive roof products can help reduce the cooling load on the building by releasing the remaining heat absorbed from the sun.

However, there is also evidence that low emissivity may benefit those buildings located in colder climates by retaining heat and reducing the heating load.

Research on the benefits of emissivity is ongoing. Discuss reflectance and emissivity with your roofing contractor to determine what characteristics matter most given your unique climate.

ENERGY STAR Requirements

ENERGY STAR qualified roof products must meet minimum initial and aged solar reflectance values. Emissivity is not currently a requirement for ENERGY STAR qualification.

However, starting December 31, 2007, EPA will post emissivity values for all products on the ENERGY STAR Qualified Products List to assist consumers in their purchasing decision.

Longer term, EPA plans to revisit the possibility of adding an emissivity component to the ENERGY STAR specification.

Q: What is a cool roof?

A: The ideal “cool” roof is one whose surface is minimally heated by the sun, such as a bright white roof. However, sometimes the term “cool” is applied to a roofing product whose surface is warmer than that of a bright white material, but still cooler than that of a comparable standard product.

For example, the afternoon surface temperature of a specially designed “cool” red roof is higher than that of a bright white roof, but lower than that of a standard red roof.

An example of a “hot” roof is one with a standard black surface, which grows very warm in the sun.

Q: What makes a roof cool?

A: The most important feature of an ideal cool roof is that its surface strongly reflects sunlight. The surface of an ideal cool roof should also efficiently cool itself by emitting thermal radiation.

Thus, a cool roof should have both high “solar reflectance” (ability to reflect sunlight, measured on a scale of 0 to 1) and high “thermal emittance” (ability to emit thermal radiation, also measured on a scale of 0 to 1).

The solar reflectance and thermal emittance of a surface are called its “radiative” properties because they describe its abilities to reflect solar radiation and emit thermal radiation.

An easy way to judge the coolness of a roof is to compare its surface temperature on a sunny afternoon to that of a reference black roof and that of a reference white roof.

The “solar reflectance index” (SRI) assigns a coolness of 0 to the reference black roof (solar reflectance R = 0.05, thermal emittance E = 0.90) and a coolness of 100 to the reference white roof (R = 0.80, E = 0.90).

Most roofing materials have an SRI (coolness rating) between 0 and 100, though values can be below 0 (hotter than reference black) or above 100 (cooler than reference white). The higher the SRI, the cooler the surface.

Q: What happens to sunlight reflected from a cool roof?

A: On a clear day about 80% of sunlight reflected from a horizontal roof will pass into space without warming the atmosphere or returning to Earth.

Q: What is a “cool color”?

A: About half of all sunlight arrives in the invisible “near-infrared” spectrum. Standard light-colored surfaces strongly reflect both visible and near-infrared sunlight, while standard dark colored surfaces reflect modestly in both spectra.

Special dark and medium-colored surfaces that strongly reflect near-infrared sunlight are called “cool colors.” The solar reflectance of a cool dark color can exceed that of a standard dark color by about 0.4.

Q: What are some examples of a cool roof?

A: A roof with a clean, smooth bright white surface can reflect about 85% of incident sunlight (R = 0.85) and emit thermal radiation with 90% efficiency (E = 0.90). This surface has an SRI of 107 and will be only 9°F [5 K] warmer than the outside air on a typical summer afternoon.

For comparison, the surface of a standard gray roof that reflects only about 20% of incident sunlight (R = 0.20, E = 0.90) has an SRI of just 19, and a surface temperature elevation (surface temperature – outside air temperature) ΔT of 69°F [38 K].

A roof with a clean, smooth “cool color” surface, such as a cool red tile, can reflect about 35% of incident sunlight (R = 0.35) and emit thermal radiation with 90% efficiency (E = 0.90).

This surface has an SRI of 38 and a ΔT of 56°F [31 K]. The cool red tile is much warmer than the bright white roof, but still cooler than a standard red tile (R = 0.10, E = 0.90, SRI = 6, ΔT = 78°F [44 K]).

A roof with a clean, bare zincalume steel surface can reflect about 75% of incident sunlight (R = 0.75) but emits thermal radiation with only 5% efficiency (E = 0.05). This surface has an SRI of 68 and a ΔT of 36°F [20 K].

The surface of the bare zincalume steel is warmer than that of the bright white roof (SRI = 107), but still cooler than that of the cool red tile (SRI = 38) and the standard red tile (SRI = 6).

Figure 1 shows the initial solar reflectance, solar reflectance index, and surface temperature elevation (surface temperature minus outside air temperature on a summer afternoon) of cool products for low-sloped roofs (pitch ≤ 2:12), such as single-ply membranes, field applied coatings, and modified bitumen.

Figure 2 does the same for cool materials for steep-sloped roofs (pitch > 2:12), including white and cool color tile, shingle, and metal products.
 
Q: Do cool roofs stay cool?

A: Over time, the deposition of airborne particles, the growth of microorganisms, and/or the oxidation of bare metal can reduce the solar reflectance of a cool roof. These events can also increase the thermal emittance of certain materials, especially those with bare metal surfaces.

Eventually an equilibrium is reached between processes that soil the surface, such as particle deposition, and processes that clean the surface, such as wind and rain.

Since the solar reflectance and thermal emittance of a outdoor surface typically reach steady values within one to three years, the “aged” solar reflectance and thermal emittance of a roofing product are measured after three years of natural exposure.

Figure 3 shows the aged solar reflectance, solar reflectance index, and surface temperature elevation (surface temperature minus outside air temperature on a summer afternoon) of cool products for low-sloped roofs.

Three years of exposure reduces the solar reflectance of an initially bright-white low-sloped roof to 0.65 from 0.85, and leaves the thermal emittance unchanged at 0.90. This lowers its SRI to 79 from 107 and increases its ΔT to 28°F [16 K] from 9°F [5 K].

Exposure also reduces the solar reflectance of a bare zincalume steel roof to 0.50 from 0.75, and increases its thermal emittance to 0.20 from 0.05. This lowers its SRI to 26 from 68, and increases its ΔT to 65°F [36 K] from 36°F [20 K]. Exposure has no net effect on a standard gray roof, which retains R = 0.20, E = 0.90, SRI = 19 and ΔT = 69°F [38 K].

Because they are newer to market, these is less information currently available about the effects of exposure on the solar reflectances and thermal emittances of cool materials for steep-sloped roofs, such as white and cool color tile, shingle, and metal products. 

However, preliminary data indicate that the radiative properties of cool color products for steep-sloped roofs are minimally changed by exposure.

Q: Are green roofs cool? What about thermally massive, super-insulated, and/or ventilated roofs?

A: There are a variety of roofing assembly technologies that can reduce the flow of heat into a building, including but not limited to solar-reflective, thermally emissive surfaces; vegetative cover (green roofing); thermally massive construction; super-insulation; and ventilation.

We reserve the term “cool roof” to refer to one that stays cool in the sun by virtue of high solar reflectance and (preferably) high thermal emittance.