NetZeroCities

To decrease the heat gains of buildings, reflective or cool materials can be applied on the roof or the facades of buildings (1-3). Reflective materials are characterized by high solar reflectance (SR) combined with a high thermal emittance value (4).

Numerous reflective white or light-colored materials are currently commercially available for buildings presenting solar reflectance values ranging from 0.4 to 0.9, and emissivity values close to 0.9. Reflective materials present a much lower surface temperature than conventional materials of dark color. For example, under solar conditions of about 1000 W/m2, an insulated black surface with solar reflectance of 0.05 and under low wind speed conditions, presents a surface temperature up to 50 °C higher than ambient air temperature, while for a white surface with solar reflectance of 0.8, the temperature rise is about 10 °C (5).

Cool Materials

Figure 1: Visible and infrared images of cool [1,2] and standard [3,4] black coatings applied on concrete tiles, (15)

Reflective coatings can contribute to reducing the surface temperature of a concrete tile by 7.5 °C, and it can be 15 °C cooler than a silver-grey coating [6,7].

Parallel to the development of white reflective materials, a new technology of colored infrared reflective materials has been developed and is commercially available, (8,9). Use of infrared reflective materials increases the solar reflectance of the commercially available dark products from 0.05–0.25 to 0.30–0.45, (9).

Recently, the development of daytime radiative photonic cooling technologies has permitted to decrease the surface temperature of the building materials at sub ambient levels, (10). Photonic materials coolers exhibiting an extraordinary solar reflectance combined with a high value of emissivity in the atmospheric window, can operate at sub ambient surface temperatures, (11). Sub-ambient photonic materials are already available for building applications. A review of the developments and recent achievements in the field of daytime radiative cooling technologies is given in Ref. [12].

Cool Roofs

Figure 5: A cool roof on an industrial building in the Netherlands. Visual and infrared image after the implementation of the cool roof, (17).

Use of reflecting materials contributes to lowering the surface temperature of the building materials since solar radiation is reflected rather than absorbed. As a result, the heat penetrating into the building is considerably decreasing, indoor temperatures are lower and the need for air conditioning is significantly reduced.

The use of reflective materials in vertical facades may create a visual discomfort and an energy surplus to the neighboring buildings as lighting and solar radiation is reflected to them. To overcome this problem, retroreflective (RR) materials have been recently developed as an effective solution for vertical facades, (13).

Retroreflectivity refers to the ability of a specially engineered surface to preferentially reflect incident radiation back towards its source regardless of the direction of incidence, (14). Numerous retroreflective products are commercially available and can be implemented on vertical facades to increase the reflectance of the surfaces without creating any optical and energy burden to the surrounding buildings.

Reflective materials suffer from ageing problems and their optical characteristics worsen as a function of time. Regular cleaning of the roofs is necessary.

The present document aims to provide knowledge, information, and recommendations on the use reflective materials on roofs and facades of buildings.

Retroreflective Materials

Figure 6: A commercial retroreflective material, (14).

Figure 7: A retroreflective façade. Credit (18).

MATURITY:

The technology of cool roofs and cool facades is very mature, almost all products are rated and certified by the European Cool Roof Council, (21), and can be used to provide indoor comfort and decrease the cooling needs in buildings. The use of retroreflective materials is not so common in buildings, however, the existing commercial products are of very high quality.

A.H. Rosenfeld, J.J. Romm, H. Akbari, M. Pomerantz, Cool communities: strategies for heat islands mitigation and smog reduction, Energy and Buildings 28 (1998) 51–62.

A.H. Rosenfeld, H. Akbari, J.J. Romm, M. Pomerantz, Cool communities: strategies for heat island mitigation and smog reduction, Energy and Buildings 28 (1) (1995) 51–62.

H. Akbari, M. Pomerantz, H. Taha, Cool Surfaces and Shade Trees to Reduce Energy Use and Improve Air Quality in Urban Areas, 2001

S. Bretz, H. Akbari, Long-term performance of high albedo roof coatings, Energy and Buildings 25 (1997) 159–167.

A. Synnefa, M. Santamouris and H.Akbari: Estimating the effect of using cool coatings on energy loads and thermal comfort in residential buildings in various climatic conditions, Energy and Buildings, 39,11, 1167-1174, 2007

Synnefa A, M. Santamouris, I. Livada, A study of the thermal performance of reflective coatings for the urban environment, Solar Energy 80 (2006) 968–981.

P. Berdahl, S.E. Bretz, Preliminary survey of the solar reflectance of cool roofing materials, Energy and Buildings 25 (1997) 149–215.

Synnefa, M. Santamouris, K. Apostolakis, On the development, optical properties and thermal performance of cool colored coatings for the urban environment. Solar Energy

H. Akbari, P. Berdahl, A. Desjarlais, N. Jenkins, R. Levinson, W. Miller, A. Rosenfeld, C. Scruton, S. Wiel, Cool colored materials for roofs, in: ACEEE Summer Study on Energy Efficiency in Buildings, Proceedings of American Council for an Energy Efficient Economy, Asilomar Conference Center in Pacific Grove, CA, August, 2004.

Jie Feng, Maria Saliari, Kai Gao, Mattheos Santamouris: On the cooling energy conservation potential of super cool roofs, Energy and Buildings, 264 (2022) 112076

M.Santamouris Geun YoungYun: Recent development and research priorities on cool and super cool materials to mitigate urban heat island, Renewable Energy, 792-807, 2020

M. Santamouris, Jie Feng: Recent progress in daytime radiative cooling: is it the air conditioner of the future? Buildings 8 (2018) 12.

Burgess G, Shortis MR, Scott P. Photographic assessment of retroreflective film properties. ISPRS J Photogram Rem Sens 2011;66:743–50.

Federico Rossi, Beatrice Castellani, Andrea Prosciutti, Elena Morini, Mirko Filipponi, Andrea Nicolini, Matheos Santamouris: Retroreflective façades for urban heat island mitigation: Experimental investigation and energy evaluations, Applied Energy, Volume 145, 1 May 2015, Pages 8-20

M. Santamouris Synnefa, K. Apostolakis, On the development, optical properties and thermal performance of cool colored coatings for the urban environment, Sol. Energy 81 (2007) 488-497.

Synnefa, M. Saliari, M. Santamouris: Experimental and numerical assessment of the impact of increased roof reflectance on a school building in Athens Original Research Article Energy and Buildings, Volume 55, December 2012, Pages 7-15

Elena Mastrapostoli; Theoni Karlessi; Alexandros Pantazaras; Kostas Gobakis; Dionysia Kolokotsa; Mattheos Santamouris : On the cooling potential of cool roofs in cold climates: Use of cool fluorocarbon coatings to enhance the optical properties and the energy performance of industrial buildings, Energy and Buildings, Volume 69, February 2014, Pages 417-425

Retro-reflective-facade-2 - MaterialDistrict

A. Synnefa, M. Santamouris : Advances on technical, policy and market aspects of cool roof technology in Europe: The Cool Roofs project , Energy and Buildings, Volume 55, December 2012, Pages 35-41

Cool Roofing Materials Database | HEATISLAND (lbl.gov)

Rated Products Database – EUROPEAN COOL ROOFS COUNCIL (coolroofcouncil.eu)

Study on the Cool Roofs Mitigation Potential in Australia (unsw.edu.au)

Million Cool Roofs Challenge - Arup

Energy Impact : Millions of applications of cool roofs and cool facades already exist all around the world. The potential energy savings resulting from two types of cool roofs, with reflectance close to 0.6 and 0.85, implemented on a typical residential building, for 27 cities under various climatic conditions, are estimated in (5), Table 2.

Table 1: Calculated cooling loads for the base case (roof solar reﬂectance equal to 0.2) and the two increased albedo cases (DSR1 = 0.4 and DSR2 = 0.65) and the corresponding % decrease in cooling loads between the base case and the two scenarios for a typical residential building with a cool roof. Source (5).

Analysis of the existing theoretical and experimental data show that increasing the roof solar reﬂectance reduces cooling loads by 18–93% and the peak cooling demand in air-conditioned buildings by 11–27%.

In parallel, the potential improvement of the indoor environmental conditions and in particular the decrease of the indoor temperature during the peak period in non-air-conditioned buildings, when cool roofs are implemented in buildings, is estimated in (5), Table 2.

The indoor thermal comfort conditions were improved by decreasing the hours of discomfort by 9–100% and the maximum temperatures in non-air-conditioned residential buildings by 1.2–3.3 °C.

The heating penalty from increasing solar reﬂectance cooling loads were also estimated, Fig. 1, together with the change of the cooling load resulting from an increase in roof solar reﬂectance of 0.65. The possible heating penalty refers to the decrease of the indoor temperature during the winter period because of the increased reflectance.

Table 2: The calculated number of hours with the indoor temperature exceeding 27 and 29 °C, (5).

As shown, the potential energy savings are greater in cooling season dominated climates. Even in the cases where heating is more important than cooling loads, the decrease in cooling loads always exceeded the increase in heating load (except for the case of Mexico City) although in some cases this distinction was small. Therefore, it can be concluded that increasing the solar reﬂectance of a roof is typically more beneﬁcial in hot climates where cooling load dominates most of the year.

Provide Comfort: The use of cool roofs and facades improves indoor comfort conditions and contributes to decrease the indoor peak temperature up to 5 C.

Examples of proposed indicators to measure the impacts if the solution is implemented in cities:

Energy savings compared to reference building (% kWh/m2/y). Calculate the energy consumption before the implementation of the Cool Roof / Cool Facade system, A, in kWh/m2/y and the corresponding energy consumption when Cool Roof / Cool Facade system is implemented, B. Then calculate the Impact Indicator as E=A-B

GHG emissions avoidance (e.g., by removing the need for energy consumption) in %CO2e. Calculate emissions corresponding to the energy consumption A and B, as above, then calculate the difference.

Figure 8: Climate effect on cooling and heating load changes for a change in roof solar reﬂectance of 0.65, (5).

The EU Cool Roof Project: An EU supported project called Cool Roofs has achieved to promote this technology in the EU by the implementation of an Action Plan. The work is built along four axes, i.e. technical development, market, policy and end-users. The Action Plan is steered by the development of the European Cool Roof Council that brings together all related stakeholders. The main results of the project include the creation of the European Cool Roof Council (ECRC), five cool roof pilot studies, a database of cool roofing materials, a handbook and a toolkit to assist the better understanding of the technical aspects of the technology, a market promotion plan, a proposal for a successful strategy to overcome possible policy barriers and engage with key stakeholders who could support and accelerate the creation of an EU policy and regulatory friendly environment to cool roofs, the organization of workshops and seminars and other actions of dissemination (19).

The Cool Roofing Materials Database. The purpose of the Cool Roofing Materials Database is to assist with the selection of reflecting roofing. This Database has been prepared by the Heat Island Project within Berkeley Laboratory's Energy Technologies Area, (20).

European Rated Reflecting Materials Data Base. This database of rated reflective materials for buildings is developed by the European Cool Roof Council, (21).

The Australian Cool Roof Project: A full assessment of the environmental, energy, financial and health impact of cool roofs in Australia is given in (22).

Million Cool Roofs Challenge, London, New York. Sustainable cooling for low-income housing. The aim of this project was to improve indoor environmental conditions and health outcomes for low-income communities in Mexico facing heat stress by implementing cool roofs and increased access to effective, sustainable passive cooling strategies, (23).

ADDITIONAL INFORMATION FROM CASE STUDIES

Use reflective materials with optimum optical characteristics for the specific climatic zone
Clean annually the cool roofs/facades to avoid aging

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