Potential benefits of cool walls on residential and commercial buildings across California and the United States: Conserving energy, saving money, and reducing emission of greenhouse gases and air pollutants

Abstract

Solar-reflective "cool" walls reduce absorption of sunlight by the building envelope, which 13 may decrease cooling load in warm weather and increase heating load in cool weather. Changes 14 to annual heating, ventilation, and air conditioning (HVAC) energy use depend on climate, wall 15 construction, wall orientation, building geometry, HVAC efficiency, and operating schedule. 16 Changes to annual energy cost and energy-related emissions further vary with local energy prices and emission factors. We used EnergyPlus to perform over 100,000 whole-building energy simulations, spanning 10 different building categories, three building vintages, 16 California climate zones, and 15 United States (U.S.) climate zones. Cool walls yielded annual source energy, energy cost, and emission savings in all California climate zones and in warm U.S. (ASHRAE) climate zones. In California, annual whole-building 22 HVAC energy cost savings were 4.0-27% in single-family homes, 0.5-3.8% in medium 23 offices, and 0.0-8.5% in stand-alone retail stores. In warm U.S. climates-zones 1A (Miami, 24 FL) through 4B (Albuquerque, NM)-annual HVAC energy cost savings were 1.8-8.3% in 25 single-family homes, 0.3-4.6% in medium offices, and 0.5-11% in stand-alone retail stores. 26 California and U.S. fractional source energy and emission savings were comparable to fractional 27 energy cost savings. Per unit surface area modified, cool-wall savings often exceeded cool-roof savings because building codes typically prescribe much less wall insulation than roof insulation.

Empirically Modeling the Energy Implications of Cool Roof Retrofits

“…Perhaps the most difficult of these assumptions is the expected heating penalty, modeled as a percent reduction. Previous works estimate ranges in heating penalties around 2% to 14% (Rosado and Levinson, 2019) and 10% to 30% (New et al, 2016). The latter estimated a cool roof heating penalty intensity of 1.8 MJ per m 2 of roof area.…”

“…Simulations were aggregated by month, and then months were randomly sampled into a yearly record for each household. Heating penalty (Levinson et al, 2019;Rosado and Levinson, 2019) % increase Uniform 2%-5%. Estimated using theoretical models for homes of varied vintages (pre-1980, 1989, and 2012) Normally distributed variables were truncated at zero to avoid negative values.…”

“…The U.S. Energy Star program promulgated its first consumer label for cool roof products in 1999, which has since been revised twice (Energy Star, 2017). Researchers have previously recommended changes to California's building energy code on the basis of theoretical energy reductions from cool roofs (Levinson et al, 2005;Rosado and Levinson, 2019).…”

Design, characterization, and fabrication of solar-retroreflective cool-wall materials

“…It can also reduce atmospheric temperature via negative radiative forcing, abating global warming [3][4][5][6]. "Cool" horizontal surfaces with high solar reflectance, such as white roofs and light-colored pavements, are ideal solutions for a low-density city with short buildings, in which most of the sunlight is intercepted by roofs and pavements [7][8][9][10][11][12][13]. However, in a densely populated urban area with many tall buildings, more sunlight may fall on walls than on roofs or pavements.…”

Effects of soiling and weathering on the albedo of building envelope materials: Lessons learned from natural exposure in two European cities and tuning of a laboratory simulation practice

“…Cool roofs can also offset CO2, and a reduce the building cooling needs [2,3], which are increasing due to global climate change, growing urbanization, and a deepened market penetration of air conditioning [4]. Per unit surface area, cool walls savings can equal or exceed cool roof savings as often energy codes prescribe less wall than roof insulation [5], and at pedestrian height can produce peak air temperature reductions of ~ 0.08 °C per 0.10 increase in wall albedo [6]. Moreover, the risk of thermal shocks decreases with increasing wall albedo [7].…”