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Green Building News

Researchers Find New Way to Keep Roofs Cool

Scientists learn that ruby-red pigments can stay just as cool as a white surface, pointing the way to cooler buildings and vehicles

Ruby-red roofs as cool as a white roof? Researchers at the Lawrence Berkeley National Laboratory think they've found a way to keep dark-colored surfaces like this just as cool as more reflective white and pastel colored surfaces.
Image Credit: Michael Coghlan / CC BY SA 2.0 / Flickr

Research at the Lawrence Berkeley National Laboratory holds the potential for a new type of dark reflective surface that will keep both buildings and vehicles cooler.

Scientists experimenting with synthesized ruby crystals and pigments have shown that a dark roof coating can reflect heat as effectively as a white roof, the lab said in a news release. The research holds promise for reducing cooling loads in buildings and vehicles and mitigating the urban heat island effect, the collective heat generated by dark surfaces in a dense, urban environment.

Researchers tested the idea by coloring cool roof coverings with ruby red, which the lab described as aluminum oxide doped with chromium. Then they synthesized ruby pigment to mix into coatings, and found that white surfaces treated with these paints stayed as cool as white materials.

The results enhanced previous work to make darker surfaces more reflective of heat.

“Over the past 15 years, Heat Island Group researchers have used special pigments that strongly reflect invisible ‘near-infrared’ light to make dark surfaces that stay cooler in the sun than conventional dark surfaces, though still not as cool as white surfaces,” the lab said. “This new work shows that fluorescent cooling can boost the performance of these pigments by re-emitting at longer wavelengths some of the visible light that the surface must absorb to appear dark.”

The lab hopes the work, if commercialized, could be applied to coatings for a number of products and materials. Synthetic ruby crystals were available online and were “surprisingly inexpensive.”

Manufacturers such as CertainTeed already produce roofing products with higher solar reflectance than conventional roofing, such as its Presidential Solaris brand. But Berkeley Lab researchers said the use of fluorescence for cool materials is a new concept.

Their findings were published in the journal Solar Energy Materials & Solar Cells.


  1. charlie_sullivan | | #1

    Great concept
    This is a great concept--fun science as well as a way to make cool roofs accessible across more aesthetic choices. I am hoping they will also make "cool feet" ruby slippers for vacationing during hot summers in Kansas.

  2. user-2310254 | | #2

    "and found that white
    "and found that white surfaces treated with these paints stayed as cool as white materials." Did you mean treated dark surfaces stayed as cool as white surfaces?

    "saving energy at a "surprisingly inexpensive."" Saving energy is surprisingly inexpensive?

  3. GBA Editor
    Scott Gibson | | #3

    Thanks, Steve
    Thanks for catching that butchered sentence, which I have corrected.

    On your first point, here's how the Lab described the research:

    "[Paul] Berdahl and Berkeley Lab research associate Sharon Chen then synthesized ruby powder, or aluminum oxide, doped with varying amounts of chromium to create different shades of red pigment. They prepared ruby paint from the powders, and applied these paints over bright white substrates. When exposed to sunlight, the ruby paint samples stayed as cool as white materials."

  4. AlanB4 | | #4

    This is very impressive
    I am curious the cost if it makes it to mass production, and does anyone else think that roof likes like one continuous piece. Not sure if its just the photo or thats how it actually looks. Some will be turned off by that, but it could also be the new in style (and i'm sure that can be changed anyways)

    Edit: It took about 6 captchas to get this posted, glad its not posted 6 times.

  5. Dana1 | | #5

    Coloring agents can be used in any number of roofing types.
    There is nothing about aluminum oxide that makes it suitable only for metal roofing (as in the picture). It could as easily be applied to asphalt shingles, or even a mop-on coating for torch-down roofing. It has long been used as an abrasive in sandpaper & fingernail files.

  6. Jon_R | | #6

    Note that white materials
    Note that white materials have a wide range, up to 3:1, in temperature gain.

  7. GBA Editor
    Patrick Mccombe | | #7

    I wonder how long the
    I wonder how long the pigments will stay ruby red. I've seen red roofs in Northern New England turn pink after a few years.

  8. Expert Member
    Dana Dorsett | | #8

    A mineral pigment won't fade to pink. (@ Patrick)
    Chromium-doped aluminum oxide isn't going to fade on you- it can't/won't break down over time. Surface contamination from grit, algae, lichens and moss can change the color of any roof, but the coloring agent itself is extremely stable stuff here, even at VERY high temperatures.

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