A two-degree temperature drop in room temperature may not sound like a lot, but it can make a huge difference in a nation’s aggregate energy costs and investments. This is particularly true in a warm climate nation with many ceramic tile rooftops and a developing economy, such as Thailand. The (relatively) easy solution is to use a white TiO2-based pigment, which has a high near-infrared reflectance (87%). The problem is that not everyone likes white roofs. White isn’t a huge problem for flat roof that can’t be seen, but getting widespread adoption of white tiles for pitched roofs visible from below can be a problem for aesthetic and cultural reasons.
But, as reported on in ACerS’ International Journal of Applied Ceramic Technology (doi:10.1111/j.1744-7402.2010.02599.x), there is now a green color alternative to white for roof tiles. A new paper in ACT discusses the discovery by a trio of Thai ceramic reseachers investigating “cool materials” (a generic term for building materials that make use of pigments that have high near-infrared solar reflectance or low NIR solar absorptance) of a green-hued, heat-blocking alternative to white-glazed tiles.
The group, working out of King Mongkut’s University of Technology and Suratthani Rajabhat University, started with a chromium oxide pigment base, which by itself has a medium-high NIR reflectance (50-57%). In Thailand, where steep, tinted roofs are preferred, Cr2O3 is of interest not only because of its reflectance but also ceramic glazes containing it can provide shades of green, which, tend to be dull and dark.
To see if some brighter colors and more reflective compositions are possible, the researchers added varying amounts of TiO2, Al2O3 and V2O5 to the Cr2O3 and synthesized a test group of 39 different compositions. These compositions were then calcined and formed into test disks, whereupon their reflectance was measured using a UV–Vis–NIR spectrophotometer.
As it turned out, one particular sample — “S9” — composed of (by weight percent) 80% Cr2O3, 4% TiO2, 14% Al2O3 and 2% V2O5 stands out because it has a NIR solar reflectance of 82.8%, i.e., close to the reflectance of TiO2.
To test the S9 sample under more realistic conditions, the group constructed two identical test “houses” built so that heat could only penetrate through the roof tops. One house was covered with 50 pieces of S9-glazed ceramic roof tile. For control purposes, the other house was roofed similarly, but with tiles glazed with a commercially available green pigment. Reflectance and temperature measurements were then recorded over a five-day summer period.
As expected, they found the NIR reflectance of S9-coated tiles was relatively high, 76.3%, compared to the control-coated tiles (65.7%).
In regard to the thermal tests, temperatures were taken at three locations within each house. They found that the house with the S9-coated tiles stayed cooler than the control house by about 2°C across the three measured positions.
The group’s work was supported by several institutions and governmental agencies, including the Energy Policy and Planning Office, Ministry of Energy; the Commission on Higher Education under the Strategic Scholarships for Frontier Research Networks for Thai Doctoral Degree Program; and National Research University.
By the way, for Oak Ridge National Lab’s Building Envelopes Program (a.k.a., “Roofs and Walls”) has many resources and calculators for researchers and consumers related to “developing technologies that improve energy efficiency and environmental compatibility of residential and commercial buildings.”
As a final note, it should be pointed out that while the benefits of cool roof technologies vary depending on geographic location among other things, they are not necessarily only of interest to nations and people living in topical or semitropical regions. While the addition of insulation may be more cost effective in many cooler climes, even New York City seems to have a fairly robust effort called NYC°CoolRoofs (and is part of the “100 Cool Cities Global Initiative”).