What is Urban Heat Island Effect (UHI)?

The UHI occurs when urban or developed land is hotter than surrounding rural and undeveloped land. Sunlight energy is absorbed by rooftops, pavements, structures, and other surfaces and then converted and released as heat energy into the air. Since the spacing of man-made structures is much more dense in urban areas, more heat is released causing the outside temperature in a city to be sometimes 6-8 degrees (F) hotter than rural areas.

Invisible Structures’ Product Line can help mitigate the effects of Urban Heat Island (UHI). Cool pavements, more trees, green roofs, and unique development options make ISI’s product line a perfect choice to help fight UHI in you site plans. Grasspave2 and Gravelpave2 address many components of Urban Heat Island mitigation including: thermal conductivity, heat storage capacity, density, albedo (see chart to right), and emissivity. Grasspave2 has the added component of “air-conditioning effect.”

SRI and Emissivity

Click here for the Solar Reflectance Index information on Gravelpave2’s fill in pdf

Click here for the Solor Reflectance Index and Emissivity values of Grasspave2 in pdf

Cool Pavements


Product SRI Number
New Aslphalt* 
Gravelpave2 Dark Gravel*
Gravelpave2 Light Gravel*
Gravelpave2 White Gravel*
*The Lawrence Berkley National Laboratory (LBNL)
*** Calculated from 0.25 SR and Emissivity of 0.91


Product Albedo Number
New Asphalt* 
Aged Asphalt
Draincore2 Green Roof
Slopetame2 Grass Filled
 Gravelpave2 with Light Gravel  .16
*Grass has an effective albedo of 0.4, including a reflectance of 0.25 + 0.15 “effective reflectance” to account for evapotranspiration of well-watered grass. **Larger is better1


“Thermography and visible light photographs show gravel and grass surfaces can be cooler than dense paved surfaces” – images reprinted with permission from The National Center of Excellence on SMART Innovations for Urban Climate and Energy Arizona State University.


Gravel surfaces can substantially cool down faster at night time than adjacent dense pavements as seen through this thermographic image. – images reprinted with permission from The National Center of Excellence on SMART Innovations for Urban Climate and Energy, Arizona State University.


This chart compares the temperatures below vehicles on different Permeable Paver Types. Only Grasspave2 and Gravelpave2 measured below the ambient outside temperature – They are the “Best in Class.” Click to view larger graph. Test was conducted by Sasaki Design Firm.