Cool roofs are roofs that are highly effective in reflecting incident solar radiation, while also emitting thermal energy at infrared wavelengths, namely they have a high solar reflectance rating and high thermal emissivity.
They also have a low surface temperature, even in direct sunlight.
There are several advantages to producing a Cool Roof:
- Improved ambient conditions inside the building during summer months;
- Less energy required for air-conditioning to cool rooms inside the building in summer;
- Improved energy efficiency of the whole building envelope;
- Reduced heat island effect (the term heat island is used to describe the phenomenon whereby the temperature generated in urban areas is higher than that recorded in rural areas);
- Greater dimensional stability of roof build-up system and underlying load-bearing structures as the effect of high summer temperatures is mitigated (less movement, expansion, cracking, etc.);
- Longer service life expectancy of waterproofing roofing membranes as the low surface temperature greatly reduces the natural heat-induced aging process.
Solar reflectance is the property of a material/surface to reflect sunlight.
A surface with a high solar reflectance reflects most of the incident solar radiation and hence stays cooler on the surface.
Light colours, especially white, have a high solar reflectance.
Solar reflectance is rated from 0 to 1 or as a %.
Thermal emissivity is a measure of the ability of a material/surface to disperse heat.
A surface with a high thermal emissivity rating has the property of effectively radiating heat to the surrounding environment.
Polymer waterproofing membranes (PVC-P and TPO/FPO), MBDP polymer distilled bitumen membranes and protective coatings for MBDP membranes all have high thermal emissivity.
Emissivity is rated from 0 to 1 or as a %.
E’ un valore basato su un metodo di calcolo per definire e misurare la combinazione tra le proprietà di riflettanza ed emissività di un materiale/superficie.
L’impatto combinato delle caratteristiche di riflettanza solare ed emissività termica viene misurato secondo lo standard ASTM E1980, viene calcolato secondo tre condizioni di ventosità (bassa, media, alta).
Il valore SRI si definisce in %.
Più è alto il valore SRI più bassa sarà la temperatura superficiale sotto irraggiamento. (trattandosi di un metodo di calcolo basato su valori/superfici di riferimento il risultato può essere superiore al 100%).
LEED (Leadership in Energy and Environmental Design) is a rating system for high-performance environmentally sustainable buildings.
LEED certification refers to the building as a whole, certifying its environmental impact and sustainability, and was developed in the US in the early 90s and introduced into Italy by the Green Building Council Italia in 2008.
The certification system is based on a series of credits awarded for different categories. More specifically, cool roofs can earn points towards certification under:
- Category 1 Sustainable sites
- Credit 7.2 Heat island effect - Roof
In order to achieve the credit for reducing the heat island effect, the finished roof should:
- Have an SRI ≥ 78 (for roofs with a slope ≤15%)
- Have an SRI ≥ 29 (for roofs with a slope >15%)