FAQs

If you have a question that has not yet been answered please contact us on the Ask the Experts and an industry expert will answer your question.

Q?

Can you provide contact information for speakers and panelists?

A.

Requests for speakers and panelists should be forwarded to Mr. Mark Thimmons, Executive Director of the Cool Metal Roofing Coalition, at mthimmons@steel.org

Q?

What literature do you consider to be helpful?

A.

Literature on cool roofing and urban heat island effects can be found on the websites of Lawrence Berkeley National Laboratory, Oak Ridge National Laboratory and the Florida Solar Energy Center. Some specific papers are also shown below:

1) Wilkes, Kenneth E., et. al., "Roof Heating and Cooling Loads in Various Climates for the Range of Solar Reflectances and Infrared Emittances Observed for Weathered Coatings," Oak Ridge National Laboratory, no date (available on DOE’s Cool Roof Calculator CD).

2) Konopacki, S., et. al., Cooling Energy Savings Potential of Light-Colored Roofs for Residential and Commercial Buildings in 11 U.S. Metropolitan Areas, Lawrence Berkeley National Laboratory, May 1997 [LBNL-39433].

3) Akbari, H. and Bretz, S., "Cool Systems for Hot Cities," Professional Roofing, October 1998 [Also LBNL-43814].

4) Parker, D.S., et. al., "Comparative Evaluation of the Impact of Roofing Systems on Residential Cooling Energy Demand," Contract Report FSEC-CR-1220-00, Florida Solar Energy Center, Cocoa, FL, November 2000.

Q?

A quick scan of HDD and CDD for cities in the US shows that the HDD are much higher. What opportunities do you see for addressing this side of the equation especially for metal?

A.

Not surprisingly, the greatest benefits to "cool roofs" are to be found in cooling-dominated regions. A study of non-metal roofs with various coatings concluded, "Regardless of surface coating, energy savings were economically significant only in cooling-dominated climates with poorly insulated roofs." [Petri et. al., "Effect of Solar Radiation Control on Energy Costs A Radiation Control Fact Sheet for Low-Slope Roofs", Oak Ridge National Laboratory, no date found] However, the potential benefits of using metal roofs with dark paints that absorb energy in heating-dominated regions have received very little favorable attention.

Q?

Is the Cool Metal Roofing Coalition available for local CSI or AIA educational presentations?

A.

Contact Mr. Mark Thimons (mthimons@steel.org) regarding your presentation request with specific objectives, time frame and allotted time, and we will do our best to accommodate your event.

Q?

What determines a “Heat Island”? Size”?, Location”?

A.

"Urban areas tend to have higher air temperatures than their rural surroundings as a result of gradual surface modifications 4 that include replacing the natural vegetation with buildings and roads. The term ‘Urban Heat Island’ describes this phenomenon." [LBNL –49638] No references can be found that quantified the potential impacts of the changing urban fabric (the relative amounts of buildings, trees, grass, roads, roofs, etc.); the size and climate of the urban area; and the normal, heat-producing activities of a large and dense population (transportation, running domestic and industrial equipment, etc.).

Indeed, it is sometimes difficult to determine whether temperature changes can be attributed to a "Heat Island Effect" at all. One study of the Los Angeles area concluded, "These increases [in yearly temperature maxima] suggest that the daytime warming is becoming more important over time at [three of the eleven stations studied] during the summer months. However, whether this rise in the daily maxima is caused by local-scale heat island effects or simply by changes in microclimate below canopy height is uncertain." The situation was clearer in another city: "Data from stations in the center of Phoenix showed clear and significant nighttime and daytime temperature differences of 1 -- 2OK (3 -- 4OF). These temperature increases may be attributable to a local-scale heat island." [LBNL-41973]

Q?

What is the acrylic coating thickness and % of solids [on 55% Al-Zn Coated Steel]?

A.

The thickness is approximately 1 micrometer (0.039 mils) but may vary among suppliers. To put this in perspective, a standard paint system is often about 25 micrometers thick. The percent solids may also vary among coating suppliers but is nominally 25%.

Q?

Do $/unit values correlate to CDD? To other weather data?

A.

In the Oak Ridge National Laboratory model for low slope roofs, cooling energy loads and heating energy loads were considered separately. Cooling loads were correlated to a "cooling index", which was the product of the average solar insolation and cooling degree days, divided by a constant to normalize the data. In other words, the amount of cooling energy required is related to both the amount of solar radiation and the number of cooling degree days. However, this relationship between cooling load and cooling index was not linear. Heating loads were correlated to a "heating index", which was the number of heating degree days divided by a constant to normalize the data. The correlation, though not linear, was significant without considering solar insolation in this case. [Petrie, T. W., et. al., "Effect of Solar Radiation Control on Energy Costs " A Radiation Control Fact Sheet for Low-Slope Roofs," Oak Ridge National Laboratory, no date, available on DOE’s Cool Roof Calculator.]

Q?

In the LBNL studies for a given city, the total savings, going from Low to High reflectance is the product of $/unit x no. of units. ($/ft2 x no. of ft2). What are the $/unit values?

A.

Data are available in LBNL-39433, Table EX-2 in $/1000 ft2. Direct savings are:

Location: Residential Commercial Combined
Atlanta 10 11 10
Chicago 8 11 9
Los Angeles 16 30 20
Dallas / Ft Worth 11 13 11
Houston 17 20 18
Miami / Ft Lauderdale 21 19 21
New Orleans 14 26 17
New York 9 21 17
Philadelphia (-2) 20 4
Phoenix 34 35 34
DC / Baltimore 5 10 5

 

Q?

Do you see coated metal requiring a note detailing compliance with a “cool roof” standard?

A.

The Cool Roof Rating Council and Energy Star programs use a labeling system to indicate compliance with their specifications. However, there are other cool roof programs and standards that may or may not require labeling of the roof product.

Q?

What is being done with ASTM and similar organizations to better standardize measurement of TSR and emissivity?

A.

ASTM has a task force under Committee E06 that develops standards for total solar reflectivity and total emissivity. Two new standards (E1918 and E1980) are the results of the work of the task group.

Q?

With the high TSR (0.70) required by the CEC – Title 24, is there any reason to sign up as a charter member of the Cool Roof Rating Council if you don’t have a painted product that meets 0.70?

A.

Participation as a Charter Member in the Cool Roof Rating Council could provide an opportunity to influence its mandated criteria. As stated in the CRRC information, although you must be a CRRC licensed seller or manufacturer, you do not have to be a member of CRRC to be able to list a product in the CRRC Product Directory.

Title 24 currently offers credits for cool roof systems that are CRRC labeled. The 2013 version of Title 24 will also link cool roofs to the CRRC labeling program. Roof products will not be mandated to meet 0.63 aged reflectance, but those that don’t will affect a building’s energy budget and other adjustments will have to be made, such as smaller windows, more insulation, etc. in order for a building permit to be issued. The proposed language indicates that a roofing product that is not CRRC labeled, regardless of its reflectance, will by default be considered as having a 0.1 reflectance which will significantly impact the building energy budget and threaten the issuance of a building permit.

Q?

Is one of the goals of the CMRC to promote a self-certified /self-policing CMRC approved product to be included in regulatory statutes as opposed to the CRRC program? (i.e., lower cost to producers?)

A.

Yes. These vary by jurisdiction. Earlier Federal legislation contained tax credits for homeowners who installed reflective metal roofing on new or renovated homes. Unfortunately, no such legislation is currently in effect. Incentives in mortgages are often tied to cool roofing, due to the energy cost savings over time. Utility incentives are available in local jurisdictions. Cool Community programs offer many building owners tax or energy credits for adopting cool roofing construction practices. There have been several cool roof rebate programs throughout the country, most notably in California, that offered building owners and contractors financial incentives for using cool roofing. The State Energy Offices (NASEO) and the Department of Energy could serve as good resources for more incentive information.

Q?

Will there be any loss of durability with the new IR reflective pigmented paints?

A.

Because IR-reflective pigments are synthetic minerals, it is anticipated that they will outperform conventional pigments in the long term. Many of the pigments that convey IR-reflectance have been in use for more than three decades. Other more recently developed pigments do not have such lengthy historical data. However, accelerated and short-term weathering results suggest that they will also exhibit remarkable stability. The fact that the IR reflective pigments absorb less heat should actually prolong their lifespan thus resulting in better durability for the products on which they are used.

Q?

For certifying colors, how would coaters certify colors that they make themselves (match colors)? Would this be possible or would there be

A.

This question is best answered by the individual paint manufacturer. There is a complicated dynamic at play:

1) The paint manufacturer may opt for a standard range of reflective paints, or may offer custom colors. Pricing may vary.

2) The number of colors in the reflective range may influence the coaters’ pricing, at least to some extent.

3) The roofing fabricator may choose to offer a limited line of EnergyStar compliant (or otherwise reflective) products in order to maximize production efficiency, even if the paint or the coated stock is available in a wide range of colors.

As for certification, it depends on the regulation. EnergyStar compliance, and for example, is self-certified, and can be done by anyone with the right equipment (spectrophotometer or D&S instrument) and a spreadsheet. CRRC ratings must be certified by an accredited laboratory, in the case of standard colors, or by an accredited paint manufacturer, in the case of custom colors.

Q?

The marketplace is always price conscious. How does a metal roofing fabricator convert inventories (coil) to IR reflective pigmented coatings with minimal cost and efficiency?

A.

This is a question for the roofing manufacturer. There are issues of stocking or otherwise supplying (currently) small volume products, segregation of inventory, self-certification, etc. Today there is enough non-reflective roofing business to mitigate the risks of obsolete inventory. Somewhere down the road, greater attention may have to be paid to manage the stockdown of non-reflective products and the corresponding increase in cool roofing equivalents.

Q?

Besides reflectivity, how do these new IR reflective pigments compare to currently used products?

A.

There are two types of pigments used in coatings: organic and inorganic. Organic pigments, as a class, offer very vivid and bright colors but are generally not as lightfast and opaque as inorganic pigments. Due to their outstanding durability, it is the inorganic pigments that are used for applications that are warranted against color change from sunlight and the elements. For the most demanding applications, such as premium finishes for metal roofing, paint makers utilize a special class of inorganic pigments known as CICP’s (Complex Inorganic Color Pigments). Certain of these pigments exhibit high IR-reflectivity for a given visible color. This allows for the manufacture of colored metal roofs that can now meet the U.S. EPA’s EnergyStar and other regulatory and code requirements.

Q?

Does absorption of the roof and heat capacity of the roof not play a role?

A.

Thermal mass or heat capacity of a roofing system will have an impact on its energy efficiency by moderating the temperature swings of the roofing system. The thermal benefits of a massive roof vary depending on the location of the roofing system and the amount of solar radiation the roof absorbs. The thermal mass benefits will be maximized in locations where the roof surface temperature cycles above and below the interior temperature. This may be an effective strategy for the Chicago area.

Q?

The impact of insulation minimizes the effect of emissivity. All jurisdictions require insulation therefore emissivity should be disregarded because the heat load does not enter the building.

A.

As with all other energy efficiency issues, the greatest energy savings benefit is generated by implementing the first energy efficiency measure. Therefore, adding a roofing surface with a high solar reflectivity or a high emissivity has less benefit on a roofing system that is highly insulative. Emissivity should not be completely ignored for highly insulated roofing systems as it still impacts the surface temperature and therefore the roof’s contribution to the urban heat island phenomena.

Q?

No one mentioned venting. Does this not help emissivity even though the product has absorbed the energy and can be emitted below does this count to these rules?

A.

The energy efficiency of a steep roof assembly is impacted by the insulation level, whether the roof system is ventilated and the ventilation rate, the roof location and the direction it faces (or exposure), the thermal mass of the roofing system, and finally the solar reflectance and infrared emittance. All of these features need to be considered when designing an energy efficient roofing system.

Oak Ridge National Laboratory has been monitoring the energy efficiency of a pair of steep slope roofing systems located in Knoxville, TN where the only variable is whether the attic space is ventilated. We find that ventilation has only a second order effect on the energy efficiency of an attic assembly because the bulk of the heat transfer to the insulation surface is through radiation from the roof deck. Reducing the roof deck temperature (through changes in solar reflectance and infrared emittance) is a much more effective means of improving the energy efficiency of attic assemblies.

Q?

Have traditional roof materials (asphalt, etc,) been compared to cool roof coated metal? How do they compare in similar colors such as light gray, dark gray, brown, black? If good can this be used to sell metal?

A.

Typical 25-year gray asphalt shingles have a reflectivity of 0.09 and emissivity of 0.91, while white prepainted steel has a reflectivity of 0.64 and emissivity of 0.83. Gray standard paint would typically have reflectivity values from about 0.30 for a light gray to about 0.10 for a dark charcoal gray. These values can be increased to around 0.45 and 0.25 respectively by using IR reflective pigments. The emissivity of these metal painted systems would be in the 0.8 to 0.9 range; the same as all painted metal.

Q?

How long can steel roofs last in Florida?

A.

Using unpainted AZ55 Galvalume (AZ 165 g/m2) as the roofing material, the anticipated life of a low slope commercial roof in Florida is >25 years. Based on extensive building inspections carried out by the NamZAC Inspection Team, a roof life of 35 – 40 years is expected.

Q?

Most data presented dealt with coated/uncoated steel. How does aluminum compare?

A.

ORNL has not tested unpainted aluminum roofing materials but has measured the performance of bronze (PVDF resin) prepainted aluminum. Data indicate a reflectivity of 0.07 and emissivity of 0.87, however, with prepainted metal roofing, the substrate has no influence on the exterior surface reflectivity and emissivity.

Q?

What other roofing products meet the most stringent reflectivity (>0.90) and emissivity (>0.80) benchmarks?

A.

No currently commercially available materials have reflectivity >0.90 and emissivity >0.80. Based on the measurements made to date by the various Research Laboratories, white single ply membrane has the highest R (0.85) and E value (0.93).

Q?

How will the concept of higher reflectivity roofing be sold in colder climates?

A.

(Savings in Chicago going worst to best for 1500 ft2 building is approx. $23/year according to LBNL report)

In cold climates like Chicago, both the cooling and heating loads must be considered. ORNL computer modeling for Chicago shows that for year-round energy efficiency, a highly reflective, low emissivity roof may give the lowest energy cost. The negative impact of high reflectivity in the winter months is minimized by the low angle of the sun and short daylight hours. If one were concerned strictly with the urban heat island effect, and disregarded energy efficiency, the choice would be high reflectivity, high emissivity.

Q?

Does embossing complicate reflectivity/emissivity measurements?

A.

Certain tests that are employed for measuring solar reflectivity cannot be used if the surface is highly embossed. Typically, apparatus that use small samples require flat surfaces. Variegated roof surfaces would require ASTM E1918, the pyranometer procedure, as the test. Smooth surfaces can be tested with a wider variety of test methods including ASTM E408 and C1459. We recommend measuring reflectivity/emissivity values on the smooth surface before it is embossed.

Q?

How does the profile of a (smooth) metal roof product impact the reflectivity/emissivity values?

A.

Is it considered in the test procedures?
Answer: Reflectivity and emissivity are properties of the roof product’s surface but are not impacted by the roof profile. Test procedures specify measuring reflectivity and emissivity on flat surfaces only.

Q?

What happens to reflectivity and emissivity after half the lifetime of the roof (meaning after more than 3 years)?

A.

Emissivity of most products will stay about the same for the life of the product. The exception is unpainted metal roofing, which starts off very low, but increases as an oxide layer forms on the panels. This aged value, however, is still much lower than most other materials or painted metal. Reflectivity typically goes down as the material gets dirty and weathered. White membrane roofs get darker and lower in reflectivity in 3 years. Allowing these products to be washed before measuring the aged reflectivity gives unrealistically high aged reflectivity values. Metal panels also lose reflectivity as they get dirty, but they typically don’t get as dirty as other materials and don’t lose reflectivity as fast. Airborne dirt and contaminants are much more easily washed off the smooth metal surfaces by natural rainfall.

Q?

How do you measure the actual (aged) emissivity and reflectivity?

A.

There are standard ASTM procedures for measuring both these properties. These procedures include ASTM C1371 and E408 for emissivity and ASTM E903 and E1918 for reflectivity. Most codes are only asking for initial emissivity but initial and aged reflectivity. The Energy Star program specifically requires aged reflectivity values tested after 3 years. The present requirement specifically states to test 3 roofs of the same color as the labeled product. At least one of those 3 roofs must be within a large urban area. You then can wash the roof and take 3 readings at each of 10 areas on all three roofs. That’s a total of 90 readings to get one aged value for one color. This process is very restrictive and expensive, and is preventing most metal roofing manufacturers from registering their colors. The Cool Metal Roofing Coalition has been campaigning for more easily obtainable test fence values. Additionally, washing the roof before taking the readings will give unrealistically high reflectivity readings for materials that tend to accumulate dirt and mildew. Testing washed surfaces therefore does not indicate the true aged reflectivity under realistic conditions.

Q?

How can we help to promote the use of test fence values for 3-year TSR values?

A.

The word can be spread to code officials and architects. The EPA Energy Star Roof Products Program is studying a proposed protocol that would allow exposure panels to be used for the 3-year TSR values. Charter members need to vote on the protocol before it becomes an alternative procedure. The Cool Roof Rating Council is planning to use weathering farms to obtain 3-year reflectivity and emissivity values for their labeling program. Aged values are much easier to obtain on test panels rather than on actual roofs, so more companies can start labeling products with these values. We all know light colors and/or reflective pigments are better to reduce cooling costs, but until we can label our products, no one is using these superior products. We can all also campaign for more realistic aged values obtained by not washing the aged samples before testing. Metal does much better than most roofing products in the unwashed condition. Since no one ever washes their roof on a regular basis, these unwashed values are more representative of actual aged values.

Q?

Should the tests on roof materials be done on entire roof assemblies rather than on the surface material?

A.

This depends on what one is trying to test. If the interest is in the total energy costs of heating and air conditioning the building, then the entire assembly including insulation, ventilation, etc. should be tested. If the concern is the urban heat island effect, then the exposed surface will have the most significant impact on the results. The reflectivity and the emissivity are the important surface properties. Most codes are trying to separate the insulation requirements from the reflectivity and emissivity requirements for simplicity. The Cool Metal Roofing Coalition believes this could be misleading.

Q?

Can you comment on the disadvantages of non-metal roofing systems compared to prepainted metal roofing?

A.

The success of cool roofs relies on maintaining the reflectivity of the roofing system. Energy savings are not necessarily constant over the lifetime of the roof system, as the solar reflectivity of a roof surface changes. Color changes caused by aging, dust, air-borne pollutants, water ponding, and the effects of biological attack can all reduce the performance of non-metal Cool Roofing (e.g. EPDM, sprayed-on coatings). High levels of maintenance and/or cleaning are critical to maintaining a highly reflective, cool surface for non-metal roofing systems. In contrast, unpainted and prepainted metal roofing systems shed dirt and do not have problems with biological attack. Unpainted and prepainted metal roofs are able to maintain their reflectivity over longer periods of time with minimal maintenance. In addition, metal roofing has a significant recycled content, is recyclable, sustainable, and has a much longer life span compared to most non-metal roofing systems.

Q?

What are emissivity and thermal emittance?

A.

Emissivity is a measure of the thermal emittance of a surface. Emissivity is defined as the ratio of radiant heat flux emitted by a material to that emitted by a blackbody radiator at the same temperature. Emissivity values range between 0 and 1. Emissivity may be thought of as the ability of a material to emit heat (via infrared radiation) to the surrounding atmosphere. Higher numbers indicate faster heat transfer. There are standardized methods to measure the thermal emittance of roofing materials like ASTM C1371 and ASTM E408.

Q?

What are reflectivity and solar reflectance?

A.

Reflectivity is a measure of the solar reflectance of a surface. Reflectivity is defined as the ratio of the reflected solar radiation flux to the incident flux. Reflectivity values range between 0 and 1 with the higher number indicating higher reflectivity. This is also sometimes expressed as a percentage from 0% to 100%. A more reflective roof will reflect the solar radiation away from the surface rather than absorbing it. Absorbed solar radiation will increase the surface temperature. There are standardized methods to measure the solar reflectance of roofing materials like ASTM E903 and ASTM E1918.

Q?

What are some practical examples of roofing materials that are cool, warm and hot in a warm climate?

A.

Table I Roof Surface Temperature in Oak Ridge, TN August 14, 2000

Material Reflectivity Emissivity Max. Surface Temp. (°F)
White prepainted metal roof .64 .83 123 Cool
Unpainted metal roof .64 .08 142 Warm
Asphalt Shingle .09 .91 164 Hot

Remember that emissivity and reflectivity both play a part in the surface temperature of a roof.

Q?

Does metal thickness affect emissivity?

A.

No. This is a property of the roofing surface itself.

Q?

Should emissivity be a part of requirements in such programs as Energy Star?

A.

The EPA does not currently require emissivity as part of their program. This follows the logic of Oak Ridge National Laboratory that the desired radiation properties for a roof should be based on an analysis of the local situation, rather than on the generality that "high reflectivity, high emissivity" is good for everyone, everywhere. With the variations achievable with metal roofing, properties can be "tailored" to make economic, energy-efficient sense for the specific location.