Frequently Asked Questions

FAQs » TESTING

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

Answer:

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.

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

Answer:

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.

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

Answer:

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.

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

Answer:

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.

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

Answer:

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.

Question: Does embossing complicate reflectivity/emissivity measurements?

Answer:

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.

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

Answer:

(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.