This page has a list of frequently asked questions about cool metal roofs, broken up into specific question topics. Each general topic of questions has its own section. To see a particular section, click on the section title. To see all the questions, scroll down this page. To see the answer to a specific question, click on the question and it will jump to the question and answer. If you would like additional information or have further questions, please contact MBMA by clicking here.
Emissivity
What are emissivity and thermal emittance?
What are reflectivity and solar reflectance?
Give me some practical examples of roofing materials that are cool, warm and hot in a warm climate.
Why does a white prepainted metal roof have a much higher emissivity than an unpainted metal roof?
When a roof panel emits infrared radiation, does it emit only to the sky or does it also emit radiation in the other direction, that is, into the building?
Does metal thickness affect emissivity?
Should emissivity be a part of requirements in such programs as Energy Star?
Can you comment on the disadvantages of non-metal roofing systems compared to prepainted metal roofing?
Should energy codes for roofing be the only governing factor regarding what kinds of roofing material can be installed on a building?
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Testing
Should the tests on roof materials be done on entire roof assemblies rather than on the surface material?
How can we help to promote the use of test fence values for 3-year TSR values?
How do you measure the actual (aged) emissivity and reflectivity?
What happens to reflectivity and emissivity after half the lifetime of the roof (meaning after more than 3 years)?
How does the profile of a (smooth) metal roof product impact the reflectivity/emissivity values?
Is it considered in the test procedures?
Does embossing complicate reflectivity/emissivity measurements?
How will the concept of higher reflectivity roofing be sold in colder climates?
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Comparison to other products
What other roofing products meet the most stringent reflectivity (>0.90) and emissivity (>0.80) benchmarks?
What about comparisons with BUR, single ply, comp shingles including their color offerings?
How long can steel roofs last in Florida?
Have traditional roof materials (asphalt, etc,) been compared to cool roof coated metal in similar colors such as light gray, dark gray, brown, black? How do they compare? If good can this be used to sell metal?
Regarding requirements for Chicago, which roofing materials meet them? Which ones don’t?
In the ORNL study, it would be useful to have comparisons between metal and competitive roofing materials.
Would unpainted metal include metal coated with metallic alloys? (copper, coated copper, zinc-coated, tin coated, stainless and stainless coated)
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Ventilation, Insulation, etc.
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?
Has the DOE evaluated ventilation of attics on cooling loads? What is the impact of ventilation?
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?
Does absorption of the roof and heat capacity of the roof not play a role? Particularly in moderate climates like Chicago?
Have you considered the impact of mass and specific heat of alternative materials on potential energy savings?
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IR Reflective Pigments
Besides reflectivity, how do these new IR reflective pigments compare to currently used products?
What are the costs of reflective paint relative to conventional paint systems?
For certifying colors, how would coaters certify colors that they make themselves (match colors)? Would this be possible or would there be roof colors" set?
Will there be any loss of durability with the new IR reflective pigmented paints?
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Codes, Organizations, Research, etc.
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?
Are there any tax credits from the government (DOE, Fed, state or local.) or incentives from utilities or any other group to buy and install a "cool" roof?
What is being done with ASTM and similar organizations to better standardize measurement of TSR and emissivity?
Do you see coated metal requiring a note detailing compliance with a "cool roof" standard?
In the LBNL studies for a given city, the total savings, going from Low to High reflectance is the product of $/unit x no. of units. ($/ft2x no. of ft2). What are the $/unit values?
Do $/unit values correlate to CDD? To other weather data?
Are complete reports of ORNL studies available?
What is the acrylic coating thickness and % of solids [on 55% Al-Zn Coated Steel]?
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Miscellaneous questions
What determines a "heat island"? Size? Location?
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?
What literature do you consider to be a "must read" for a better understanding of this topic?
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Emissivity Questions
What are emissivity and thermal emittance?
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.
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What are reflectivity and solar reflectance?
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.
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Give me some practical examples of roofing materials that are cool, warm and hot in a warm climate.
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.
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Why does a white prepainted metal roof have a much higher emissivity than an unpainted metal roof?
The physical characteristics of paint made from organic and inorganic materials are much different than the characteristics of a metallic material. White paint is a better emitter of radiation than bare metal. The controlling characteristic is the surface finish and not the underlying material.
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When a roof panel emits infrared radiation, does it emit only to the sky or does it also emit radiation in the other direction, that is, into the building?
The roof panel emits radiation in both directions.
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Does metal thickness affect emissivity?
No. This is a property of the roofing surface itself.
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Should emissivity be a part of requirements in such programs as Energy Star?
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.
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Can you comment on the disadvantages of non-metal roofing systems compared to prepainted metal roofing?
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.
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Should energy codes for roofing be the only governing factor regarding what kinds of roofing material can be installed on a building.
No. There are many other factors that should be considered when developing mandatory codes or incentive programs that influence what types of roofing material can be used for buildings. Recycled content, recyclability, sustainability and durability should be included as part of a balanced approach to any building code or incentive program. Otherwise, one may fill landfills with non-recyclable roofing materials that are "cool," but have short life spans. Metal roofing is recyclable and sustainable. Metal roofing’s life span is much longer compared to most non-metal roofing systems.
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TESTING
Should the tests on roof materials be done on entire roof assemblies rather than on the surface material?
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.
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How can we help to promote the use of test fence values for 3-year TSR values?
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.
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How do you measure the actual (aged) emissivity and reflectivity?
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.
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What happens to reflectivity and emissivity after half the lifetime of the roof (meaning after more than 3 years)?
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.
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How does the profile of a (smooth) metal roof product impact the reflectivity/emissivity values?
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Is it considered in the test procedures?
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.
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Does embossing complicate reflectivity/emissivity measurements?
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.
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How will the concept of higher reflectivity roofing be sold in colder climates?
(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.
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COMPARISON TO OTHER PRODUCTS
What other roofing products meet the most stringent reflectivity (>0.90) and emissivity (>0.80) benchmarks?
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).
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I heard a lot about comparing bare vs. black vs., white metal roofing. What about comparisons with BUR, single ply, comp shingles including their color offerings?
The ORNL study has compared several BUR and membrane roofing materials over a 3-year period. Their report will be published in 2003. Generally speaking, smooth black BUR’s have R of 0.05 and E of 0.90 whereas shiny metals have high R and low E. The Lawrence Berkeley National Laboratory also has a database comparing thermal properties of a variety of roofing products.
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How long can steel roofs last in Florida?
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.
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Have traditional roof materials (asphalt, etc,) been compared to cool roof coated metal in similar colors such as light gray, dark gray, brown, black? How do they compare? If good, can this be used to sell metal?
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.
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Regarding requirements for Chicago, which roofing materials meet them?
Which ones don’t?
Based on the 2002 draft of the cool roof requirements for the City of Chicago, no unpainted metal roofing material can meet the minimum benchmark. Painted metal can meet the requirements since the paint film is considered the exterior surface of that product. In moderate and cold climates, emissivity has a very much-reduced effect on heating and cooling energy loads. The use of roof insulation is much more effective in these climates.
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In the ORNL study, it would be useful to have comparisons between metal and competitive roofing materials.
The Oak Ridge National Laboratory study has done exactly that. Over 50 roofing materials in several colors and material types have been evaluated over a 3-year period. Of the fifty materials, 11 are metal roofing systems; the others are asphalt, smooth BUR, ballasted BUR and several membrane systems. As stated earlier, their report is due in early 2003.
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Would unpainted metal include metal coated with metallic alloys (copper, coated copper, zinc-coated, tin coated, stainless and stainless coated)?
Yes, unpainted metals include pure metals (copper, tin) and alloys. All the above materials have been used as roofing materials. Typically, unpainted AZ55 Galvalume Plus is used for low slope roofing and prepainted AZ50 Galvalume and galvanized are used for steep slope roofing.
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VENTILATION,INSULATION, ETC.
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?
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.
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Has the DOE evaluated ventilation of attics on cooling loads? What is the impact of ventilation?
See answer above.
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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.
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.
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Does absorption of the roof and heat capacity of the roof not play a role?Particularly in moderate climates like Chicago?
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.
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Have you considered the impact of mass and specific heat of alternative materials on potential energy savings?
See answer above.
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IR REFLECTIVE PIGMENTS
Besides reflectivity, how do these new IR reflective pigments compare to currently used products?
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. Additional research has resulted in pigments with even greater reflectivity. Although some of these newly developed chemistries have not undergone long-term weatherability testing, they are of the same chemical class as the CICP’s that have performed successfully for more than three decades in the field.
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What are the costs of reflective paint relative to conventional paint systems?
Many CICP’s used for years already possess reflective characteristics. Others are more recent developments and carry a premium. However, there are a number of other significant cost drivers, such as paint reformulation costs, production and inventory of newly-formulated paints, setup and changeover for small line runs, stocking and shipping short-run coils, etc. Many of these other costs are more significant than the raw material cost differential from the pigment. This is a Question that is best answered by the paint and roof manufacturers.
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For certifying colors, how would coaters certify colors that they make themselves (match colors)? Would this be possible or would there be "roof colors" set?
This question is best answered by the individual paint manufacturer. There is a complicated dynamic at play. The paint manufacturer may opt for a standard range of reflective paints, or may offer custom colors. Pricing may vary. The number of colors in the reflective range may influence the coaters’ pricing, at least to some extent. 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, 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.
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Will there be any loss of durability with the new IR reflective pigmented paints?
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.
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CODES, ORGANIZATIONS, RESEARCH, ETC.
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?
Participation as a Charter Member in the Cool Roof Rating Council could provide an opportunity to influence its mandated criteria. 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 2005 version of Title 24 will also link cool roofs to the CRRC labeling program. Roof products will not be mandated to meet 0.70 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.
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Are there any tax credits from the government (DOE, Fed, state or local.) or incentives from utilities or any other group to buy and install a "cool" roof?
Yes. These vary by jurisdiction. The proposed Federal Government’s Energy Bill of 2002 contains tax credits for homeowners who install reflective metal roofing on new or renovated homes. Unfortunately, the bill has not yet passed in Congress. 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.
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What is being done with ASTM and similar organizations to better standardize measurement of TSR and emissivity?
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.
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Do you see coated metal requiring a note detailing compliance with a "cool roof" standard?
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.
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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?
Data are available in LBNL-39433, Table EX-2 in $/1000 ft2. Direct savings are:
|
Location |
Residential |
Commercial |
Combined |
|
Atlanta |
10 |
11 |
10 |
|
Chicago |
|
