Cool Paint

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Good Practices

Cool paint is applied in-situ on roofs, either brushed, rolled or sprayed on. Usually a primer coat is applied first, followed by two coats of the paint, generally striving for a total coverage of 9.6 to 14.4 square metres per litre²⁵. Before any application, the roof surface has to be cleaned, normally by pressure washing, with all leaks and cracks repaired. It is important to note that cool paint is not meant to seal or repair leaks, neither is it a direct substitute for insulation to reduce the amount of heat penetration into the building.

Cool paint is best applied on low-slope roofs which are in good condition and have a minimum expected service life of five⁸. A low-slope roof is typically defined as a roof with a rise to run ratio of not greater than 2:12 or a pitch of 9.5° or less; essentially almost flat¹⁸. For optimum results, paint application should be done in warm and dry weather, according to the manufacturer’s specifications for temperature and humidity².

Although there are building codes and standards in the United States such as ASHRAE Standard 90.1 that allow for reduced roof insulation (U-factor) when a high-albedo roof is used¹⁵, both insulation and cool paint technologies work very differently and thus their performance cannot be directly compared in this manner⁵. It is still recommended for reflective roofs to be used as a complement to insulation instead of a substitute^14,26.

Holistic Approach to Determine Effectiveness

In order to accurately determine the effectiveness of applying cool paint on a building, a holistic approach has to be adopted. Every building is unique, and it has to be evaluated as a whole system, instead of solely assessing the effects on specific parts such as the roof or the walls. There is more than one source of heat gain through the building envelope⁶, all of which have to be considered to assess the effect of cool paint as a whole. In this way, the performance and effectiveness of cool paint for a particular building can then be accurately evaluated².

Thickness of Cool Paints

Cool coatings and paints need to be applied to a thickness greater than normal paints (sometimes greater than 1mm) as reflectance tends to increase with thickness¹.

Chemical Additives

To maintain effective cooling it is important therefore to keep cool coatings and paints as clean as possible. Some coatings and paints include chemical additives to retard biological growth and resulting staining¹.