Solar Reflectance<\/h2>\n\n\n\n
The first part of SRI is made up from Solar Reflectance<\/strong> or reflectivity<\/strong>. Solar reflectance <\/strong>(sometimes called albedo \u2013 Latin: albedo<\/em>, meaning \u201cwhiteness\u201d) is about light and not the total sum of solar heat blocking. As CRRC states above “the fraction of solar energy that is reflected by the surface”<\/em>. White paint has around 80% reflectivity, which means it reflects 80% of visual light (less if dirty) which is 44% of the total solar heat. Meaning standard shiny white paint reflects approximately 35%<\/strong> (80% of 44) of the total solar heat (if 80% reflective – worse if dirty). Infrared heat makes up 53% of the solar heat and UV only 3%. Even claims by scientists that the “whitest” of shiny white paints can reflect 98% (43% of the total heat) of sunlight (Sparkes, 2021<\/a>) yet they don’t mention what sunlight or solar heat is made of. They mean visual light heat, not infrared. Solar thermal heat comprises of:<\/p>\n\n\n\n The total nanowaves of sunlight are infrared, visual and UV and they all carry heat. Infrared carries the most and this is converted as watts of energy that enters the building or any surface as conductive energy.<\/p>\n\n\n For SRI’s to work the CRRC stated “the <\/em>fraction<\/em><\/strong> of solar energy that is reflected by the surface”, <\/em>and the UNSW stated “the <\/em>solar reflectance defines<\/em><\/strong> how much the incident solar radiation is <\/em>absorbed<\/em><\/strong> by the surface”<\/em>. Therefore shiny white due to it’s solar reflectance<\/strong> should reflect most of the heat…and be better than solar absorption<\/strong> as a measurement of heat, definitely not! Solar reflectance<\/strong> doesn’t affect the total incident solar radiation absorbed by the surface, reflectance doesn’t affect infrared which is absorbed.<\/strong><\/p>\n\n\n\n Here’s the catch with shiny white paint. As an example on a 35\u00b0C day, the bonnet of a SHINY WHITE car is 50\u00b0C (42% hotter than ambient) and the DARK car is 64\u00b0C (82% hotter than ambient). If the SHINY WHITE paint blocked 100% of the solar heat absorption, the temperature of the SHINY WHITE car would be near ambient or near 35\u00b0C and not 50\u00b0C. The shiny white paint doesn\u2019t block infrared heat…just visual heat.<\/strong><\/p>\n\n\n\n Once the conductive solar heat from infrared passes the white pigments it is loaded into the metal’s substrate. No longer is SRI effective. <\/strong>That’s why when you touch a SHINY WHITE car on a warm day the surface is still very warm to hot and can burn your skin. The misnomer that shiny white paint is a better solar reflector isn’t factual over coatings that also block the infrared heat and reduce heat load transmittance.<\/p>\n\n\n\n I am the first to say the BCA should include emissivity. Anyhow, the SRI uses solar absorbance (300-2500 nm, thus whole solar spectrum) and thermal emittance (whole thermal IR spectrum), not just colour in the Vis range. What it doesn’t count is the transmission through the envelope, so products that provide some thermal insulation would need to be added the additional thermal resistance in the energy balance calculations.<\/strong> The reason why the SRI is like that is because otherwise one wouldn’t know where to stop counting the layers and some might come with a black (SR 0.05) membrane coupled to a 200 mm insulation claiming it blocks the total solar heat gains. <\/em><\/p>\n\n\n\n Also, SRI doesn’t work for walls or green roofs. For some situations (e.g., green roofs) an SRI equivalent might be meaningful. There’s no consensus on the metric though. It’s not perfect and I don’t use the SRI in research papers, but it’s a scale and it goes beyond colour. <\/strong>The most accurate metric preferred by researchers doesn’t always get used in practice, and we cannot expect everybody to run climate modelling to make a project choice. Then, performance solutions give quite some flexibility. I acknowledge there is margin for improvement in the codes there.<\/em><\/p>\n\n\n\n Essentially Riccardo’s comments on SRI still don’t address the solar transmission and important of absorbing infrared heat into the building. SRI is not about measuring heat load or transmittance, this is about emissivity or heat being released. This is why SRI isn’t a valuable scale of thermal performance, in the end we need to know how much heat is being absorbed.<\/p>\n\n\n\n\t<\/div>\n\n\n\n\n Regulation makers and the construction industry have favoured the use of parameters that allow the forecasting of the interaction between different material properties without the need for complex analyses. Among these, the solar reflectance index (SRI) takes into account solar reflectance and thermal emittance to predict the thermal behaviour of a surface subjected to solar radiation<\/strong> through a physically rigorous mathematical procedure that considers assigned air and sky temperatures, peak solar irradiance, and wind velocity<\/strong> (Muscio, 2018<\/a>). You can download<\/a> an Excel document for SRI here<\/a>.<\/p>\n\n\n\n SRI is based on the hypothesis of an adiabatic irradiated surface and does not consider the insulation or the inertia of the materials below<\/strong>, nonetheless it can again work well as an indicator of the heat transmitted by convection to the external near ground air <\/strong>because the heat flow rate conducted to the ground or through a roof can be lower by one or two orders of magnitude than solar irradiance (Muscio, 2018<\/a>). In the end it’s all about what’s above the surface and not underneath and there’s assumptions made as both hypothesis and indicators only.<\/p>\n\n\n <\/p>\n\n\n\n (Image above: Understanding the Solar Reflectance Index from CRRC) ASTM E1980-11(2019)<\/a> as specified above by the CRRC is based on Standard Practice for Calculating Solar Reflectance Index of Horizontal and Low-Sloped Opaque Surfaces.<\/strong><\/p>\n\n\n\n 1. Scope<\/em><\/p>\n\n\n\n 1.1 This practice covers the calculation of the Solar Reflectance Index (SRI) of horizontal and low-sloped opaque surfaces at standard conditions. The method is intended to calculate SRI for surfaces with emissivity greater than 0.1.<\/em><\/p>\n\n\n\n 1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.<\/em><\/p>\n\n\n\n Even in the description from the CRRC doesn’t give any thermal value or solar absorption value from the SRI definition. How this standard is adopted with zero actual thermal benefit is peculiar to say the least and the whole world is caught up in it.<\/p>\n\n\n This is perfectly reflected in the OTM Accredited Laboratory (OTM Solutions, 2021<\/a>) above as recommended for white paint shows their calculation based on solar reflectance of .80 and emittance of .90 has an SRI of 100. Interestingly the surface temperature they calculated is 49.3\u00b0C on a low wind day, identical to our white car example but what is the ambient temperature? 35\u00b0C or 41% hotter than ambient?<\/p>\n\n\n\n According to Berkeley Labs (Berkeley Labs, n.d.<\/a>): This index is calculated following ASTM Standard E1980-11<\/a> using a roof product’s solar reflectance and thermal emittance values. The SRI calculation can also be used to estimate the surface temperature of the roof product under prescribed conditions. The key here is to create a steady-state surface temperature (Ts)\u2014While you can go to the ASTM Standard for the formula, under point 5. Significance and Use it states:<\/p>\n\n\n\n Solar reflectance and thermal emittance are important factors <\/em>affecting surface and near-surface ambient air temperature<\/em><\/strong>. Surfaces with low solar reflectance, absorb a high fraction of the incoming solar energy. A fraction of this absorbed energy is conducted into ground and buildings, a fraction is convected to air (leading to higher air temperatures), and a fraction is radiated to the sky. For equivalent conditions, the lower the emissivity of a surface the higher its steady-state temperature.<\/em><\/p>\n\n\n\n So again the ASTM standard focuses on surface and air temperatures and not heat load. Interesting it discusses low solar reflectance therefore higher temperatures. The method described above gives the SRI of surfaces based on measured solar reflectances and thermal emissivities of the surfaces therefore not the U value or any allowance for infrared heat transmittance into the building. SRI matches the need of a single performance parameter as it allows to easy comparison of the performance of different solar reflective solutions (Muscio, 2018<\/a>). Yet still no value for the heat absorption into the building!<\/p>\n\n\n\n Reflectivity test equipment cannot pick up \u201cheat\u201d in light which is the main discussion point. It picks up certain nanowaves of the light spectrum with \u201cshiny\u201d or gloss white getting a lot of air play \u2013 which has little to do with heat. So, given the above, reflectivity does not show how heat is managed or transferred.<\/strong><\/p>\n\n\n\n Image: <\/strong>On-site albedo measurement. Thermacs Engineering (Themacs Engineering, n.d.<\/a>)<\/em><\/p>\n\n\n\n\n
![\"\"](\"https:\/\/neotechcoatings.com\/wp-content\/uploads\/2023\/05\/Forms-of-solar-heat-1024x574.jpeg\")
<\/figure>\n<\/div>\n\n\n\n![\"\"](\"https:\/\/neotechcoatings.com\/wp-content\/uploads\/2023\/05\/nanowaves-of-radiation-chart.png\")
<\/figure>\n<\/div>\n<\/div>\n\n\n\n![\"\"](\"https:\/\/neotechcoatings.com\/wp-content\/uploads\/2023\/05\/CRRC.jpeg\")
<\/figure>\n<\/div>\n\n\n![\"\"](\"https:\/\/neotechcoatings.com\/wp-content\/uploads\/2023\/05\/Dark-white-and-super-therm-surface-1024x613.jpeg\")
<\/figure>\n<\/div>\n\n\nRiccardo Paolini<\/a> – Associate Professor at UNSW<\/h3>\n\n\n
![\"\"](\"https:\/\/neotechcoatings.com\/wp-content\/uploads\/2024\/03\/Riccardo-Paolini.jpeg\")
<\/figure>\n<\/div>\n\n\n
\n\n\n\nHow is SRI determined?<\/h3>\n\n\n\n
![\"\"](\"https:\/\/neotechcoatings.com\/wp-content\/uploads\/2023\/05\/Determine-SRI-value-1024x757.png\")
<\/figure>\n<\/div>\n\n\n![\"\"](\"https:\/\/neotechcoatings.com\/wp-content\/uploads\/2023\/05\/OTM-laboratory-1024x384.png\")
<\/figure>\n<\/div>\n\n\n![\"\"](\"https:\/\/neotechcoatings.com\/wp-content\/uploads\/2023\/05\/themacs-engineering-measue-de-alnedo.jpeg\")
<\/figure>\n<\/div>\n\n\n