Cool roofs are characterised by materials having: high solar
reflectance (SR) and high thermal emittance (TE). The former
expresses the ability of the materials of reflecting most of the incident
solar radiation during daytime, keeping their surfaces cooler
respect to conventional construction materials. The high thermal
emittance allows the materials to radiate away the heat stored in
the structure, mainly during night time. This thermal behaviour
allows the roof to reduce the heat transfer to the built environment.
Roofs characterised by low emittance values tend to not dissipate
the stored heat at night and can be considered cool only if they
have a very high solar reflectance. White mortars and plaster were
widely used in ancient massive Mediterranean dwellings, in order
to create a more comfortable built environment during the hot season.
The coastal villages of Greece, Italy and Spain still witness
this construction technique, which emerged again as an efficient
solution during the recent years.