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Energy modelling still revolves around conductivity and R-values. That made sense when winter heat loss was the dominant issue. It makes less sense now.
In Australia, overheating, solar loading and peak heat stress are the real performance risks. Yet most models still treat materials as if heat transfer is steady and slow. Solar gain is neither.
If we want buildings that perform in extreme heat, we need to include thermal diffusivity in the modelling conversation.
R-values measure resistance to steady-state conductive heat flow. They do not describe how fast a material responds to sudden heat input.
Solar radiation is dynamic. Roofs and walls experience rapid temperature rise driven primarily by near-infrared radiation, which accounts for roughly 53% of total solar energy. Once absorbed, that energy becomes internal surface heat.
What matters in summer is not just how much heat passes through, but how quickly the material temperature spikes and transmits energy inward.
Standard modelling frameworks such as NatHERS and EnergyPlus primarily rely on conductivity and thermal mass assumptions. They rarely focus on the rate of temperature propagation at the surface layer, where solar loading actually begins.
This is the blind spot.
Thermal diffusivity describes how quickly a material changes temperature when exposed to heat. It combines three properties:
The equation is:
α = k / (ρc)
Where:
α = thermal diffusivity
k = thermal conductivity
ρ = density
c = specific heat
In simple terms, diffusivity tells you how fast heat spreads through a material.
A high-diffusivity surface heats up quickly and transmits temperature changes rapidly.
A low-diffusivity surface changes temperature slowly and moderates heat penetration.
In transient solar conditions, diffusivity often determines interior comfort more than conductivity alone.
Authoritative reference material explaining thermal diffusivity and transient heat transfer can be found via:
These standards acknowledge that dynamic heat response is different from steady-state heat flow.
Overheating is not caused by winter conduction losses. It is driven by:
When roof and wall surfaces absorb infrared radiation, their temperature can rise well above ambient. That heat then conducts inward.
If the external layer has high thermal diffusivity, the temperature spike transfers inward quickly. The internal surface temperature rises earlier in the day. The air conditioner works harder and sooner.
If the external layer has low thermal diffusivity, temperature change is moderated. The internal response is delayed and reduced. Peak load shifts. Energy demand drops.
This is transient performance. Most energy codes are still calibrated to steady-state logic.
Before heat moves through insulation batts or bulk materials, it hits the outer surface.
Surface behaviour determines:
If the surface blocks and moderates heat at the point of impact, the internal load reduces significantly before it ever reaches bulk insulation.
Research into cool roofs and urban heat mitigation consistently shows the importance of surface solar reflectance and infrared emissivity. The US Department of Energy’s cool roof guidance confirms that surface properties directly influence roof temperature and cooling demand.
But reflectance alone is not enough.
A high reflectance coating with high diffusivity can still transmit heat quickly once absorption occurs. That is where diffusivity becomes critical.
Modern building simulation tools are capable of transient analysis. EnergyPlus, for example, models time-dependent heat conduction through layers. However, many practitioners do not emphasise or test surface layer diffusivity in design comparisons.
We test R-values.
We rarely test temperature propagation speed.
In extreme heat climates, especially in lightweight construction such as modular buildings and containers, this oversight is costly.
The modelling conversation should include:
Without this, we are only modelling part of the physics.
This is where advanced ceramic insulation coatings enter the discussion.
Super Therm® is a multi-ceramic insulation coating developed in collaboration with NASA research engineers in the late 1980s and early 1990s. It applies at a dry film thickness of approximately 250 microns and is designed to block solar radiation while maintaining low thermal diffusivity at the surface layer.
Rather than relying solely on thickness, it focuses on surface energy control.
Independent testing shows:
More detailed testing and performance data can be reviewed here:
https://neotechcoatings.com/super-therm-testing-and-results/
The point is not thickness. The point is behaviour.
By moderating heat absorption and slowing temperature propagation, low diffusivity surface coatings reduce peak heat transfer before bulk insulation becomes relevant.
This complements traditional insulation rather than replacing it.
If we continue to judge performance solely on K-values and R-values, we will continue designing for yesterday’s climate problem.
Future-ready modelling should account for:
Heat does not politely move in steady-state lines. It arrives in waves.
Modelling must reflect that reality.
Including thermal diffusivity in energy modelling is not academic theory. It is a practical adjustment to reflect how buildings actually experience heat.
In high solar climates, surface control determines resilience.
Conductivity tells you how much.
Diffusivity tells you how fast.
If we ignore the second, we misjudge the first.
That is the gap.
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