NC 2009 SSc7.2: Heat Island Effect—Roof

Shriram Bhide replied Jan 18 2010

Bill,
Thank you so much for your response.

Mara Baum replied Sustainability Coordinator, Anshen + Allen Architects Jan 22 2010

This is a great question, and one that has been on my mind as well. SRIThe solar reflectance index (SRI) is a measure of a material's ability to reject solar heat, as shown by a small temperature rise. Standard black (reflectance 0.05, emittance 0.90) is 0 and standard white (reflectance 0.80, emittance 0.90) is 100. For example, a standard black surface has a temperature rise of 90_F (50_C) in full sun, and a standard white surface has a temperature rise of 14.6_F (8.1_C). Once the maximum temperature rise of a given material has been computed, the SRI can be calculated by interpolating between the values for white and black. Materials with the highest SRI values are the coolest choices for paving. Because of the way SRI is defined, particularly hot materials can even take slightly negative values, and particularly cool materials can even exceed 100. is calculated using solar reflectance (albedoAlbedo is synonymous with solar reflectance.) and emissivityEmissivity is the ratio of the radiation emitted by a surface to the radiation emitted by a black body at the same temperature. (emittance). If you have these two values, you can use a calculator like the one at http://www.arkema-inc.com/index.cfm?pag=986. If you don't have these values, you're in for some big excitement (so to speak).

Although I've never had to measure this myself, I have heard that it's a big pain -- and now I know why. I took a look at a slide show on this topic at the Lawrence Berkeley National Labs Cool Roofs website http://eetd.lbl.gov/HeatIsland/CoolRoofs/Overview/. This LBL group developed much of the leading research and research methods on this topic. To summarize, you need a combination of the following instrumentation, depending on the situation: UV-VIS-NIR Spectrometer with integrating sphere, a solar spectrum reflectometer, and a Fourier-Transform Infra-Red (FTIR) spectral emissometer and/or an emissometer. The protocols must comply with specific ASTMVoluntary standards development organization which creates source technical standards for materials, products, systems, and services standards for each type of measurement.

If your project is local and presents a scientific question that might pique LBL interest, perhaps you should contact them for advice or assistance. (A note to other readers, Lauren is lucky to be practically in LBL's back yard.) Paul Berdahl is the primary contact for SRI info, see http://eetd.lbl.gov/HeatIsland/STAFF/.

Although I didn't see a specific discussion on the question of roof vs. hardscape measurement procedures, it makes sense that they would be the same.

Have any other readers undertaken these measurements themselves?

Dave Intner replied Firmitas Architecture & Planning Jan 25 2010

It may be more cost effective to send a sample to a lab, instead of bringing the equipment on-site. We researched this for a project where we were filling the cells of pervious pavers with gravel; we had manufacturer SRIThe solar reflectance index (SRI) is a measure of a material's ability to reject solar heat, as shown by a small temperature rise. Standard black (reflectance 0.05, emittance 0.90) is 0 and standard white (reflectance 0.80, emittance 0.90) is 100. For example, a standard black surface has a temperature rise of 90_F (50_C) in full sun, and a standard white surface has a temperature rise of 14.6_F (8.1_C). Once the maximum temperature rise of a given material has been computed, the SRI can be calculated by interpolating between the values for white and black. Materials with the highest SRI values are the coolest choices for paving. Because of the way SRI is defined, particularly hot materials can even take slightly negative values, and particularly cool materials can even exceed 100. values for the pavers themselves, but not the gravel. It ended up not being necessary because the USGBC reviewers accepted the paver value for 60% of the area (excluding the 40% voids filled with gravel), which put us over the 50% area threshold. The lab we received quotes from for testing was Atlas Material Testing in Phoenix, www.atlaswsg.com.

Mara Baum replied Sustainability Coordinator, Anshen + Allen Architects Jan 27 2010

Great idea, Dave, thanks. That approach is a little tougher for an existing roof, though.

Lauren Glasscock replied Green Building Consultant, KEMA Services, Inc. Jan 28 2010

Thanks Dave for your comments