Cost/Benefit Analysis for Cool Roofs

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Guest Post by Joel West, Professor of Innovation and Entrepreneurship, San José State University

We’re doing a remodeling project which includes installing a new roof. Here in California, we get a lot of sun, so the impact of solar irradiance on solar heat gain is a major concern — either for A/C costs (and thus peak summer energy loads) or on comfort (for those of us who don’t have A/C).

Thus, I’ve been looking into solar reflectivity and what has been called the “cool roofs“ movement. There is the Cool Roof Rating Council, “created in 1998 to develop accurate and credible methods for evaluating and labeling the solar reflectance and thermal emittance (radiative properties) of roofing products and to disseminate the information to all interested parties.”

There are also cool roof pages from the California Energy Commission (both consumer and business oriented pages) and at the EPA. (Unlike my smug friends praising German solar initiatives, cool roofs seems to be one place where the Europeans are copying the US).

A March 2009 presentation by Sheila Blake of the City of Houston summarizes the issues and their public policy implications — demonstrating how important a factor this is for energy efficiency, especially in the Southwest and Southeast. There’s also a March 2009 article at the McGraw-Hill continuing education site for the construction industry.

Normally one would assume that it‘s just a matter of lighter colors, but it’s not that simple.

Standard roofing materials have been developed to last a decade or more, and many of these materials (e.g. slate) are natural materials that come in specific colors. Lighter colored dyes, coatings and other treatments will fade or weather over time, reducing or eliminating the benefits of such treatment. (Apparently 19th century tin roofs provided durability and superior reflectivity.)

Another issue is that reflection isn’t enough. A roof also needs to emit heat (via infrared radiation) or it will raise the temperature of the roof and thus the house. The Metal Building Manufacturers Association has explains why this is important.

The solution to combined solar reflectance and emissivity is the SRI. Here is a succinct explanation from Astec Paints of Australia:

Total Solar Reflectance (T.S.R.) figures are expressed as a percentage falling between 0% and 100% dependant on a product’s Total Solar Reflectance as tested to ASTM C-1549 or ASTM E-903.

Emissivity or (Infrared emittance), is a measure of the ability of a surface to shed some of it’s heat in the form of infrared radiation away from the surface. The results from tests conducted to ASTM C-1371, express the emittance value as a percentage falling between 0% and 100% depending on the product’s performance.

Solar reflectance index (SRI), combines both the T.S.R.% reflectivity value and emittance value as a measure of a coating’s overall ability to reject solar heat.

The SRI is normalized to be in the range of [0,100] based on nominal “black” and “white” values. Lawrence Berkeley National Laboratory has an Excel spreadsheet that turns TSR and IE values into an SRI value, while Oakridge National Laboratory has a calculator to convert the ratings into energy savings. (These are among several calculators listed on the CCRC website).

Not everyone wants a white roof, particularly for a peaked roof (as is common here) seen by all your neighbors. The Cool Colors Project of LBNL and ORNL is trying to address this question. (It is also raising questions about the revising measurement standards for reflectivity, but let’s set that aside for now).

Starting in 2005, California’s Title 24 began requiring Cool Roofs for business buildings — perhaps because of their impact on daytime A/C and peak energy consumption — but I’d bet money that residential buildings will be covered in the next go-round. The California standards are being copied by other states.

What strikes me is how little of this is filtering down to the homeowner, perhaps because it’s not (yet) mandated. I’m painfully familiar with the state’s energy conservation regulations — Title 24 — that are either praised as the ultimate either in environmental leadership or bureaucratic micromanagement

However, I’ve heard nothing about roofing colors from my builder or roofing contractor. The only reason I investigated this is due to a stupid decision 20 years ago on re-roofing my home — replacing a light gray asphalt shingle with a medium gray — that raised the summer temperature 20°F until I added roof vents.

All the roofing bidders recommended a specific brand of 4.5mm thick APP modified bitumen membrane as the roofing material. After reading about it, it appears this material (guaranteed for 20 years) is now the preferred solution for low-slope roofs, replacing asphalt, hot mop and other solutions.

I couldn’t find the suggested roofing material in the CCRC database or the LBNL database. The manufacturer (Johns Manville) doesn’t provide data online, but a phone call provided numbers for the four lightest colors. In addition, a competitor (CertainTeed) has created a “CoolStar” variant of its competing APP product which is designed for Cool Roof initiatives. Clearly the latter has much better performance

Mfr. Product Solar Reflectivity Thermal Emissivity SRI
JM White .33 .84 35
JM Tan .26 .84 26
JM Desert Blend .21 .85 19
JM Grey .16 .84 13
CT GTA .74 .88 91†
CT GTA-FR .75 .92 93†

† SRI estimated using LBNL calculator

I ran the numbers using the ORNL calculator, with several California locations. Given some reasonable assumptions, the lightest color roof provided huge savings for hot inland locations (Bakersfield, Fresno, Sacramento), while coastal locations in Central and Northern California (Santa Maria, San Francisco, Arcata) with low cooling needs actually increased costs due to reduced winter sun. Southern California locations (San Diego, Long Beach, Los Angeles) showed more modest gains. (The results were magnified or attenuated based on the thermal resistance of the roofing assembly).

Even in the Southernmost clime, the difference between the white roof and the medium colored roof was about $40/year in air conditioning. Since we won’t have air conditioning, I need to figure out what this means for the interior temperature on hot days, but the data suggests that rooftop temperature will be at least 20°F cooler with the white roof.

© 2009, Joel_West. All rights reserved. Do not republish.

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Author: Joel_West (3 Articles)

Joel West is professor of innovation and entrepreneurship at San José State University's College of Business. His research and industry experience focus on profiting from technological innovation. He is a chronic blogger, including at his Cleantech Business blog, which focuses on the economics of renewable energy and energy efficiency from a Silicon Valley Perspective

  • Jim

    Interesting article explaining the immediate benefit from a cool roof – but it makes me wonder!

    Part of the equation regarding the greenhouse effect and therefore climate change is that Co2 traps the reflected sun rays within the atmosphere preventing they reflection back into space. Doesn’t reflective roofing material contribute to this effect versus green roofs which absorb the suns ray?

    I’d appreciate seeing some scientific data on any comparisons that might have been made.

  • Sandra

    Jim,

    You comment is interesting. Don’t have a response other than to think about the differences in temperature come back from white concrete verse tarred roads.

    What I am wondering is has the author considered metal roofing with thin film solar generation or alternately what innovations are going on with cooling the roof at the same time as heating water? The later may be imagined to be similiar to under floor heating, but the opposite. “Grand Design,” a British TV show (Channel 4, Google it), that looks at innovations in housing design did a show on a bungalow with this very concept applied with a concrete roof. My husband and I were wondering if it could be applied in California and how effective it would be? It looked like it would be quite effective if you could determine a low cost method of applying it.

  • http://cleantechbiz.blogspot.com Joel West

    I’m installing conduit to bring solar energy down from the roof, but I’m not installing solar panels right now due to the high capital cost.

  • Jerry Toman

    While installing a “white roof” may make a lot of sense for a new roof or roof replacement, many of us are stuck, at least for awhile, with an “dark roof” which absorbs a lot of heat, at least in inland California locations.

    An alternative for this situation would be to improve attic ventilation with a (solar?) powered ventilator. Even with passive devices such as wind turbines, attic temperatures can reach 150 F or more, giving a temperature difference of 75 F between the attic and desired room temperature of 75 F. A ventilator could easily cut this difference by up to 2/3 resulting in only a 25 F difference. Assuming 50% of the A/C load was due to heat transmission through the ceiling insulation into the living space, a powered ventilator would cut the overall A/C load by about one-third, which could amount to several hundred dollars per season. Estimated payout in two seasons, unless, of course, power costs go up (surprise!) which would result in more savings (based on 1500 cfm ventilation rate and 2000 sq. ft. of ceiling).

    If you install one, be sure that there is enough intake area to allow replacement of the hot air expelled (soffits, gable vents, etc.) and avoid depressurization.

  • Joel West

    @Jerry Toman – You raise an important point. For one house, I stupidly said “a darker roof would look nice” and went from a light gray to a medium gray. To counteract the effect, I had to add ceiling insulation (it was a 40 year old house) and passive wind turbines.

    A/C? Oh, you mean that luxury I have at work. (It’s supposed to hit 90+ today and all we have is lots of insulation and a whole house fan.)

  • Jerry Toman

    Hi Joel–I’m assuming that, like most, your whole house fan vents into the attic, bringing fresh air in through the windows. That limits it to use in the evening after the outside temperature has dropped below 75 F and the early mornings (4-7 AM) when you can make use of the 60 F outside temperatures to “pre-cool” the entire structure, which, with the presence of good insulation, can keep the inside temperature in the comfort zone until well into the afternoon.

    During the day you appear to be relying on the turbine ventilators to keep the attic temperatures from becoming excessive. If there is little wind as is the case in San Jose, these may not be very effective. When the whole house fan is turned off, a small, solar-powered fan may be of assistance in removing heat from the attic and keeping the living space a bit cooler in the 2-9 PM time frame.

    Here in Antioch, where we do have some wind, I had envisioned building a “wind collector” that would look something like a VAWT, but with only static vanes that could force some cooler air into the attic (say by converting one of the wind turbines to inflow) while warmed air exited through the other turbine or existing vents. I don’t have the time or energy to put a prototype together, but would be willing to work with a student, if one from the ME or some other department is interested in building one.