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For decades, building performance has revolved around insulation thickness and R-values.
Important, yes. Complete, no.
The dominant force driving overheating is not air temperature. It is solar radiation, particularly near-infrared energy. If you do not control radiation at the surface, you are always managing heat after it has already entered the system.
Solar energy that strikes a building is roughly:
The majority of heat loading sits in the near-infrared band. This is not visible to the eye, but it carries most of the thermal energy that drives surface temperature rise.
NASA’s overview of the solar spectrum outlines this distribution clearly:
When sunlight hits a roof:
That sequence is physics, not opinion.
R-values measure resistance to conductive heat flow. They do not measure how much solar radiation is absorbed at the surface.
By the time bulk insulation starts working, radiant energy has already been converted to heat.
Standards such as AS/NZS 4859.1 focus on thermal insulation materials and R-value measurement under steady-state conditions:
That testing does not evaluate:
Which means a roof can comply on paper and still overheat in practice.
The cool roof movement has correctly highlighted the importance of solar reflectance and thermal emittance. The US Department of Energy explains this framework here:
However, many coatings focus primarily on visible reflectance. A surface can look bright and still absorb significant near-infrared energy.
The key metric that often gets overlooked is spectral reflectance across the full solar spectrum, not just brightness.
ASTM E903 and ASTM C1549 measure solar reflectance. ASTM E1980 calculates Solar Reflectance Index (SRI). These help, but SRI still does not directly address thermal diffusivity or surface density effects.
Real performance starts at the outer fraction of a millimetre of a building.
Three material properties matter at the surface:
High reflectance in the near-infrared band directly reduces heat loading.
Thermal diffusivity defines how quickly heat moves through a material. ISO 22007-2 outlines measurement of thermal diffusivity for plastics and coatings:
Low diffusivity slows inward heat flow even if some energy is absorbed.
High density materials store more energy. Lower density ceramic matrices can reduce the ability of the surface to load and retain heat.
These are measurable physical properties, yet they are rarely prioritised in mainstream compliance conversations.
Urban materials absorb and re-emit infrared energy, heating surrounding buildings even after direct solar exposure reduces.
The US Environmental Protection Agency provides a detailed explanation of urban heat island mechanisms:
Hard surfaces such as asphalt and metal roofs become secondary radiant heat sources. Buildings are not just heated by the sun but by surrounding surfaces that re-radiate broadband infrared energy.
If those surfaces are not engineered to manage radiation, entire precincts become thermal amplifiers.
Recent research into passive daytime radiative cooling focuses on emitting heat through the atmospheric transmission window.
A review published in Nature Sustainability explains this emerging field:
Raman et al., “Passive radiative cooling below ambient air temperature under direct sunlight,” Nature 2014
While promising in certain climates, strong radiative emission can also increase night-time heat loss in winter. Without careful surface density and diffusivity control, buildings may experience seasonal instability.
It is not simply about reflecting or emitting. It is about controlled surface thermal behaviour.
A 35°C day does not overheat a roof by air contact. Infrared radiation drives surface temperatures far beyond ambient.
Surface temperatures of 70°C or more are common on dark roofs under full sun.
Once that happens:
All because the initial radiant load was not controlled.
If building science continues to measure only R-value, we will continue designing for conduction instead of radiation.
Future performance evaluation should include:
Solar radiation is the primary driver of envelope heat load.
Control it at the surface and you stabilise the system.
Ignore it and you are permanently reacting downstream.
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